[PATCH 0/6] refs: excessive hook execution with packed refs

[PATCH 0/6] refs: excessive hook execution with packed refs

[Patrick Steinhardt]

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Hi,

as reported by Waleed in [1], the reference-transaction hook is being
executed when packing refs. Given that the hook ideally ought to track
logical updates to refs instead of leaking low-level implementation
details of how the files backend works, this is understandably leading
to some confusion.

This patch series aims to fix that by improving how the tandom of loose
and packed refs backends interact such that we skip executing the hook
when the loose backend:

    - repacks references.
    - needs to delete packed refs when deleting a loose ref would
      uncover that packed ref.

Patrick

[1]: <CAKjfCeBcuYC3OXRVtxxDGWRGOxC38Fb7CNuSh_dMmxpGVip_9Q@mail.gmail.com>

Patrick Steinhardt (6):
  refs: open-code deletion of packed refs
  refs: allow passing flags when beginning transactions
  refs: allow skipping the reference-transaction hook
  refs: demonstrate excessive execution of the reference-transaction
    hook
  refs: do not execute reference-transaction hook on packing refs
  refs: skip hooks when deleting uncovered packed refs

 refs.c                           | 11 +++++--
 refs.h                           |  8 ++++-
 refs/files-backend.c             | 25 +++++++++++-----
 refs/packed-backend.c            | 30 ++++++++++++++-----
 refs/packed-backend.h            |  6 ++++
 refs/refs-internal.h             |  1 +
 sequencer.c                      |  2 +-
 t/t1416-ref-transaction-hooks.sh | 50 ++++++++++++++++++++++++++++++++
 8 files changed, 113 insertions(+), 20 deletions(-)

-- 
2.34.1


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[PATCH 1/6] refs: open-code deletion of packed refs

[Patrick Steinhardt]

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When deleting loose refs, then we also have to delete the refs in the
packed backend. This is done by calling `refs_delete_refs()`, which
then uses the packed-backend's logic to delete refs. This doesn't allow
us to exercise any control over the reference transaction which is being
created in the packed backend, which is required in a subsequent commit.

Extract a new function `packed_refs_delete_refs()`, which hosts most of
the logic to delete refs except for creating the transaction itself.
Like this, we can easily create the transactionion in the files backend
and thus exert more control over it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c  | 12 +++++++++---
 refs/packed-backend.c | 28 +++++++++++++++++++++-------
 refs/packed-backend.h |  6 ++++++
 3 files changed, 36 insertions(+), 10 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 4b14f30d48..453adc38ea 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1250,6 +1250,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct ref_transaction *transaction = NULL;
 	struct strbuf err = STRBUF_INIT;
 	int i, result = 0;
 
@@ -1259,10 +1260,14 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
-		packed_refs_unlock(refs->packed_ref_store);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	if (!transaction)
+		goto error;
+
+	result = packed_refs_delete_refs(refs->packed_ref_store,
+					 transaction, msg, refnames, flags);
+	if (result)
 		goto error;
-	}
 
 	packed_refs_unlock(refs->packed_ref_store);
 
@@ -1289,6 +1294,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	else
 		error(_("could not delete references: %s"), err.buf);
 
+	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return -1;
 }
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 9da932a540..cb19b5291e 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1522,15 +1522,10 @@ static int packed_initial_transaction_commit(struct ref_store *ref_store,
 static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			     struct string_list *refnames, unsigned int flags)
 {
-	struct packed_ref_store *refs =
-		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
-	struct string_list_item *item;
 	int ret;
 
-	(void)refs; /* We need the check above, but don't use the variable */
-
 	if (!refnames->nr)
 		return 0;
 
@@ -1544,6 +1539,27 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (!transaction)
 		return -1;
 
+	ret = packed_refs_delete_refs(ref_store, transaction,
+				      msg, refnames, flags);
+
+	ref_transaction_free(transaction);
+	return ret;
+}
+
+int packed_refs_delete_refs(struct ref_store *ref_store,
+			    struct ref_transaction *transaction,
+			    const char *msg,
+			    struct string_list *refnames,
+			    unsigned int flags)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct strbuf err = STRBUF_INIT;
+	struct string_list_item *item;
+	int ret;
+
+	(void)(refs); /* We need the check above, but don't use the variable */
+
 	for_each_string_list_item(item, refnames) {
 		if (ref_transaction_delete(transaction, item->string, NULL,
 					   flags, msg, &err)) {
@@ -1554,7 +1570,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	}
 
 	ret = ref_transaction_commit(transaction, &err);
-
 	if (ret) {
 		if (refnames->nr == 1)
 			error(_("could not delete reference %s: %s"),
@@ -1563,7 +1578,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			error(_("could not delete references: %s"), err.buf);
 	}
 
-	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return ret;
 }
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index f61a73ec25..5e0dd7d08e 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -27,6 +27,12 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 void packed_refs_unlock(struct ref_store *ref_store);
 int packed_refs_is_locked(struct ref_store *ref_store);
 
+int packed_refs_delete_refs(struct ref_store *ref_store,
+			    struct ref_transaction *transaction,
+			    const char *msg,
+			    struct string_list *refnames,
+			    unsigned int flags);
+
 /*
  * Return true if `transaction` really needs to be carried out against
  * the specified packed_ref_store, or false if it can be skipped
-- 
2.34.1


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[PATCH 2/6] refs: allow passing flags when beginning transactions

[Patrick Steinhardt]

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We do not currently have any flags when creating reference transactions,
but we'll add one to disable execution of the reference transaction hook
in some cases.

Allow passing flags to `ref_store_transaction_begin()` to prepare for
this change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c                |  8 +++++---
 refs.h                |  3 ++-
 refs/files-backend.c  | 10 +++++-----
 refs/packed-backend.c |  2 +-
 refs/refs-internal.h  |  1 +
 sequencer.c           |  2 +-
 6 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/refs.c b/refs.c
index 996ac27164..1c8a0c01f1 100644
--- a/refs.c
+++ b/refs.c
@@ -800,7 +800,7 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 
-	transaction = ref_store_transaction_begin(refs, &err);
+	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname, old_oid,
 				   flags, msg, &err) ||
@@ -1005,6 +1005,7 @@ int read_ref_at(struct ref_store *refs, const char *refname,
 }
 
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    unsigned int flags,
 						    struct strbuf *err)
 {
 	struct ref_transaction *tr;
@@ -1012,12 +1013,13 @@ struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
 
 	CALLOC_ARRAY(tr, 1);
 	tr->ref_store = refs;
+	tr->flags = flags;
 	return tr;
 }
 
 struct ref_transaction *ref_transaction_begin(struct strbuf *err)
 {
-	return ref_store_transaction_begin(get_main_ref_store(the_repository), err);
+	return ref_store_transaction_begin(get_main_ref_store(the_repository), 0, err);
 }
 
 void ref_transaction_free(struct ref_transaction *transaction)
@@ -1148,7 +1150,7 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 	struct strbuf err = STRBUF_INIT;
 	int ret = 0;
 
-	t = ref_store_transaction_begin(refs, &err);
+	t = ref_store_transaction_begin(refs, 0, &err);
 	if (!t ||
 	    ref_transaction_update(t, refname, new_oid, old_oid, flags, msg,
 				   &err) ||
diff --git a/refs.h b/refs.h
index 45c34e99e3..e3d3b7aa14 100644
--- a/refs.h
+++ b/refs.h
@@ -231,7 +231,7 @@ char *repo_default_branch_name(struct repository *r, int quiet);
  *         struct strbuf err = STRBUF_INIT;
  *         int ret = 0;
  *
- *         transaction = ref_store_transaction_begin(refs, &err);
+ *         transaction = ref_store_transaction_begin(refs, 0, &err);
  *         if (!transaction ||
  *             ref_transaction_update(...) ||
  *             ref_transaction_create(...) ||
@@ -551,6 +551,7 @@ enum action_on_err {
  * be freed by calling ref_transaction_free().
  */
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    unsigned int flags,
 						    struct strbuf *err);
 struct ref_transaction *ref_transaction_begin(struct strbuf *err);
 
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 453adc38ea..95583dcd00 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1122,7 +1122,7 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base, &err);
+	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1193,7 +1193,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
 	if (!transaction)
 		return -1;
 
@@ -1260,7 +1260,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
 	if (!transaction)
 		goto error;
 
@@ -2767,7 +2767,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store, err);
+						refs->packed_ref_store, 0, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3038,7 +3038,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, err);
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index cb19b5291e..ac4d92460e 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1535,7 +1535,7 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	 * updates into a single transaction.
 	 */
 
-	transaction = ref_store_transaction_begin(ref_store, &err);
+	transaction = ref_store_transaction_begin(ref_store, 0, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index fb2c58ce3b..5bec9014ac 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -213,6 +213,7 @@ struct ref_transaction {
 	size_t nr;
 	enum ref_transaction_state state;
 	void *backend_data;
+	unsigned int flags;
 };
 
 /*
diff --git a/sequencer.c b/sequencer.c
index b4135a78c9..78122e0f0c 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3599,7 +3599,7 @@ static int do_label(struct repository *r, const char *name, int len)
 	strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
 	strbuf_addf(&msg, "rebase (label) '%.*s'", len, name);
 
-	transaction = ref_store_transaction_begin(refs, &err);
+	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction) {
 		error("%s", err.buf);
 		ret = -1;
-- 
2.34.1


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[PATCH 3/6] refs: allow skipping the reference-transaction hook

[Patrick Steinhardt]

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The reference-transaction hook is executing whenever we prepare, commit
or abort a reference transaction. While this is mostly intentional, in
case of the files backend we're leaking the implementation detail that
the store is in fact a composite store with one loose and one packed
backend to the caller. So while we want to execute the hook for all
logical updates, executing it for such implementation details is
unexpected.

Prepare for a fix by adding a new flag which allows to skip execution of
the hook.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c | 3 +++
 refs.h | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/refs.c b/refs.c
index 1c8a0c01f1..462e629d37 100644
--- a/refs.c
+++ b/refs.c
@@ -2076,6 +2076,9 @@ static int run_transaction_hook(struct ref_transaction *transaction,
 	const char *hook;
 	int ret = 0, i;
 
+	if (transaction->flags & REF_TRANSACTION_SKIP_HOOK)
+		return 0;
+
 	hook = find_hook("reference-transaction");
 	if (!hook)
 		return ret;
diff --git a/refs.h b/refs.h
index e3d3b7aa14..840fa02613 100644
--- a/refs.h
+++ b/refs.h
@@ -546,6 +546,11 @@ enum action_on_err {
 	UPDATE_REFS_QUIET_ON_ERR
 };
 
+/*
+ * Skip executing the reference-transaction hook.
+ */
+#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
+
 /*
  * Begin a reference transaction.  The reference transaction must
  * be freed by calling ref_transaction_free().
-- 
2.34.1


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[PATCH 4/6] refs: demonstrate excessive execution of the reference-transaction hook

[Patrick Steinhardt]

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Add tests which demonstate which demonstrates that we're executing the
reference-transaction hook too often in some cases, which thus leaks
implementation details about the reference store's implementation
itself. Behaviour will be fixed in follow-up commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 t/t1416-ref-transaction-hooks.sh | 64 ++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index 6c941027a8..0567fbdf0b 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -136,4 +136,68 @@ test_expect_success 'interleaving hook calls succeed' '
 	test_cmp expect target-repo.git/actual
 '
 
+test_expect_success 'hook does not get called on packing refs' '
+	# Pack references first such that we are in a known state.
+	git pack-refs --all &&
+
+	write_script .git/hooks/reference-transaction <<-\EOF &&
+		echo "$@" >>actual
+		cat >>actual
+	EOF
+	rm -f actual &&
+
+	git update-ref refs/heads/unpacked-ref $POST_OID &&
+	git pack-refs --all &&
+
+	# We only expect a single hook invocation, which is the call to
+	# git-update-ref(1). But currently, packing refs will also trigger the
+	# hook.
+	cat >expect <<-EOF &&
+		prepared
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		committed
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		prepared
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		committed
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		prepared
+		$POST_OID $ZERO_OID refs/heads/unpacked-ref
+		committed
+		$POST_OID $ZERO_OID refs/heads/unpacked-ref
+	EOF
+
+	test_cmp expect actual
+'
+
+test_expect_success 'deleting packed ref calls hook once' '
+	# Create a reference and pack it.
+	git update-ref refs/heads/to-be-deleted $POST_OID &&
+	git pack-refs --all &&
+
+	write_script .git/hooks/reference-transaction <<-\EOF &&
+		echo "$@" >>actual
+		cat >>actual
+	EOF
+	rm -f actual &&
+
+	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
+
+	# We only expect a single hook invocation, which is the logical
+	# deletion. But currently, we see two interleaving transactions, once
+	# for deleting the loose refs and once for deleting the packed ref.
+	cat >expect <<-EOF &&
+		prepared
+		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
+		prepared
+		$POST_OID $ZERO_OID refs/heads/to-be-deleted
+		committed
+		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
+		committed
+		$POST_OID $ZERO_OID refs/heads/to-be-deleted
+	EOF
+
+	test_cmp expect actual
+'
+
 test_done
-- 
2.34.1


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[PATCH 5/6] refs: do not execute reference-transaction hook on packing refs

[Patrick Steinhardt]

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The reference-transaction hook is supposed to track logical changes to
references, but it currently also gets executed when packing refs in a
repository. This is unexpected and ultimately not all that useful:
packing refs is not supposed to result in any user-visible change to the
refs' state, and it ultimately is an implementation detail of how refs
stores work.

Fix this excessive execution of the hook when packing refs.

Reported-by: Waleed Khan <me@waleedkhan.name>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c             |  6 ++++--
 t/t1416-ref-transaction-hooks.sh | 11 +----------
 2 files changed, 5 insertions(+), 12 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 95583dcd00..f2bc72f81b 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1122,7 +1122,8 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
+	transaction = ref_store_transaction_begin(&refs->base,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1193,7 +1194,8 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index 0567fbdf0b..f9d3d5213f 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -150,21 +150,12 @@ test_expect_success 'hook does not get called on packing refs' '
 	git pack-refs --all &&
 
 	# We only expect a single hook invocation, which is the call to
-	# git-update-ref(1). But currently, packing refs will also trigger the
-	# hook.
+	# git-update-ref(1).
 	cat >expect <<-EOF &&
 		prepared
 		$ZERO_OID $POST_OID refs/heads/unpacked-ref
 		committed
 		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		prepared
-		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		committed
-		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		prepared
-		$POST_OID $ZERO_OID refs/heads/unpacked-ref
-		committed
-		$POST_OID $ZERO_OID refs/heads/unpacked-ref
 	EOF
 
 	test_cmp expect actual
-- 
2.34.1


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[PATCH 6/6] refs: skip hooks when deleting uncovered packed refs

[Patrick Steinhardt]

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When deleting refs from the loose-files refs backend, then we need to be
careful to also delete the same ref from the packed refs backend, if it
exists. If we don't, then deleting the loose ref would "uncover" the
packed ref. We thus always have to queue up deletions of refs for both
the loose and the packed refs backend. This is done in two separate
transactions, where the end result is that the reference-transaction
hook is executed twice for the deleted refs.

This behaviour is quite misleading: it's exposing implementation details
of how the files backend works to the user, in contrast to the logical
updates that we'd really want to expose via the hook. Worse yet, whether
the hook gets executed once or twice depends on how well-packed the
repository is: if the ref only exists as a loose ref, then we execute it
once, otherwise if it is also packed then we execute it twice.

Fix this behaviour and don't execute the reference-transaction hook at
all when refs in the packed-refs backend if it's driven by the files
backend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c             | 9 ++++++---
 t/t1416-ref-transaction-hooks.sh | 7 +------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index f2bc72f81b..0a2b5ea5da 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1262,7 +1262,8 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		goto error;
 
@@ -2769,7 +2770,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store, 0, err);
+						refs->packed_ref_store,
+						REF_TRANSACTION_SKIP_HOOK, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3040,7 +3042,8 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
+							 REF_TRANSACTION_SKIP_HOOK, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index f9d3d5213f..4e1e84a91f 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -175,16 +175,11 @@ test_expect_success 'deleting packed ref calls hook once' '
 	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
 
 	# We only expect a single hook invocation, which is the logical
-	# deletion. But currently, we see two interleaving transactions, once
-	# for deleting the loose refs and once for deleting the packed ref.
+	# deletion.
 	cat >expect <<-EOF &&
-		prepared
-		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
 		prepared
 		$POST_OID $ZERO_OID refs/heads/to-be-deleted
 		committed
-		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
-		committed
 		$POST_OID $ZERO_OID refs/heads/to-be-deleted
 	EOF
 
-- 
2.34.1


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[PATCH v2 0/6] refs: excessive hook execution with packed refs

[Patrick Steinhardt]

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Hi,

this is a resend of version 1 of this patch series to hopefully entice
some reviews. The only change is that v2 is rebased onto the current
main branch at commit e83ba647f7 (The seventh batch, 2022-01-05). The
following was from the orignial cover letter:

As reported by Waleed in [1], the reference-transaction hook is being
executed when packing refs. Given that the hook ideally ought to track
logical updates to refs instead of leaking low-level implementation
details of how the files backend works, this is understandably leading
to some confusion.

This patch series aims to fix that by improving how the tandom of loose
and packed refs backends interact such that we skip executing the hook
when the loose backend:

    - repacks references.
    - needs to delete packed refs when deleting a loose ref would
      uncover that packed ref.

Patrick

[1]: <CAKjfCeBcuYC3OXRVtxxDGWRGOxC38Fb7CNuSh_dMmxpGVip_9Q@mail.gmail.com>

Patrick Steinhardt (6):
  refs: open-code deletion of packed refs
  refs: allow passing flags when beginning transactions
  refs: allow skipping the reference-transaction hook
  refs: demonstrate excessive execution of the reference-transaction
    hook
  refs: do not execute reference-transaction hook on packing refs
  refs: skip hooks when deleting uncovered packed refs

 refs.c                           | 11 +++++--
 refs.h                           |  8 ++++-
 refs/files-backend.c             | 25 +++++++++++-----
 refs/packed-backend.c            | 30 ++++++++++++++-----
 refs/packed-backend.h            |  6 ++++
 refs/refs-internal.h             |  1 +
 sequencer.c                      |  2 +-
 t/t1416-ref-transaction-hooks.sh | 50 ++++++++++++++++++++++++++++++++
 8 files changed, 113 insertions(+), 20 deletions(-)

-- 
2.34.1


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[PATCH v2 1/6] refs: open-code deletion of packed refs

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 5049 bytes --]

When deleting loose refs, then we also have to delete the refs in the
packed backend. This is done by calling `refs_delete_refs()`, which
then uses the packed-backend's logic to delete refs. This doesn't allow
us to exercise any control over the reference transaction which is being
created in the packed backend, which is required in a subsequent commit.

Extract a new function `packed_refs_delete_refs()`, which hosts most of
the logic to delete refs except for creating the transaction itself.
Like this, we can easily create the transactionion in the files backend
and thus exert more control over it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c  | 12 +++++++++---
 refs/packed-backend.c | 28 +++++++++++++++++++++-------
 refs/packed-backend.h |  6 ++++++
 3 files changed, 36 insertions(+), 10 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 90b671025a..5795e54020 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1249,6 +1249,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct ref_transaction *transaction = NULL;
 	struct strbuf err = STRBUF_INIT;
 	int i, result = 0;
 
@@ -1258,10 +1259,14 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
-		packed_refs_unlock(refs->packed_ref_store);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	if (!transaction)
+		goto error;
+
+	result = packed_refs_delete_refs(refs->packed_ref_store,
+					 transaction, msg, refnames, flags);
+	if (result)
 		goto error;
-	}
 
 	packed_refs_unlock(refs->packed_ref_store);
 
@@ -1288,6 +1293,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	else
 		error(_("could not delete references: %s"), err.buf);
 
+	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return -1;
 }
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 67152c664e..e97d67f932 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1522,15 +1522,10 @@ static int packed_initial_transaction_commit(struct ref_store *ref_store,
 static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			     struct string_list *refnames, unsigned int flags)
 {
-	struct packed_ref_store *refs =
-		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
-	struct string_list_item *item;
 	int ret;
 
-	(void)refs; /* We need the check above, but don't use the variable */
-
 	if (!refnames->nr)
 		return 0;
 
@@ -1544,6 +1539,27 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (!transaction)
 		return -1;
 
+	ret = packed_refs_delete_refs(ref_store, transaction,
+				      msg, refnames, flags);
+
+	ref_transaction_free(transaction);
+	return ret;
+}
+
+int packed_refs_delete_refs(struct ref_store *ref_store,
+			    struct ref_transaction *transaction,
+			    const char *msg,
+			    struct string_list *refnames,
+			    unsigned int flags)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct strbuf err = STRBUF_INIT;
+	struct string_list_item *item;
+	int ret;
+
+	(void)(refs); /* We need the check above, but don't use the variable */
+
 	for_each_string_list_item(item, refnames) {
 		if (ref_transaction_delete(transaction, item->string, NULL,
 					   flags, msg, &err)) {
@@ -1554,7 +1570,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	}
 
 	ret = ref_transaction_commit(transaction, &err);
-
 	if (ret) {
 		if (refnames->nr == 1)
 			error(_("could not delete reference %s: %s"),
@@ -1563,7 +1578,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			error(_("could not delete references: %s"), err.buf);
 	}
 
-	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return ret;
 }
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index f61a73ec25..5e0dd7d08e 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -27,6 +27,12 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 void packed_refs_unlock(struct ref_store *ref_store);
 int packed_refs_is_locked(struct ref_store *ref_store);
 
+int packed_refs_delete_refs(struct ref_store *ref_store,
+			    struct ref_transaction *transaction,
+			    const char *msg,
+			    struct string_list *refnames,
+			    unsigned int flags);
+
 /*
  * Return true if `transaction` really needs to be carried out against
  * the specified packed_ref_store, or false if it can be skipped
-- 
2.34.1


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[PATCH v2 2/6] refs: allow passing flags when beginning transactions

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 6362 bytes --]

We do not currently have any flags when creating reference transactions,
but we'll add one to disable execution of the reference transaction hook
in some cases.

Allow passing flags to `ref_store_transaction_begin()` to prepare for
this change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c                |  8 +++++---
 refs.h                |  3 ++-
 refs/files-backend.c  | 10 +++++-----
 refs/packed-backend.c |  2 +-
 refs/refs-internal.h  |  1 +
 sequencer.c           |  2 +-
 6 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/refs.c b/refs.c
index 4da4996c4d..7415864b62 100644
--- a/refs.c
+++ b/refs.c
@@ -800,7 +800,7 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 
-	transaction = ref_store_transaction_begin(refs, &err);
+	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname, old_oid,
 				   flags, msg, &err) ||
@@ -1005,6 +1005,7 @@ int read_ref_at(struct ref_store *refs, const char *refname,
 }
 
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    unsigned int flags,
 						    struct strbuf *err)
 {
 	struct ref_transaction *tr;
@@ -1012,12 +1013,13 @@ struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
 
 	CALLOC_ARRAY(tr, 1);
 	tr->ref_store = refs;
+	tr->flags = flags;
 	return tr;
 }
 
 struct ref_transaction *ref_transaction_begin(struct strbuf *err)
 {
-	return ref_store_transaction_begin(get_main_ref_store(the_repository), err);
+	return ref_store_transaction_begin(get_main_ref_store(the_repository), 0, err);
 }
 
 void ref_transaction_free(struct ref_transaction *transaction)
@@ -1156,7 +1158,7 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 	struct strbuf err = STRBUF_INIT;
 	int ret = 0;
 
-	t = ref_store_transaction_begin(refs, &err);
+	t = ref_store_transaction_begin(refs, 0, &err);
 	if (!t ||
 	    ref_transaction_update(t, refname, new_oid, old_oid, flags, msg,
 				   &err) ||
diff --git a/refs.h b/refs.h
index 92360e55a2..31f7bf9642 100644
--- a/refs.h
+++ b/refs.h
@@ -231,7 +231,7 @@ char *repo_default_branch_name(struct repository *r, int quiet);
  *         struct strbuf err = STRBUF_INIT;
  *         int ret = 0;
  *
- *         transaction = ref_store_transaction_begin(refs, &err);
+ *         transaction = ref_store_transaction_begin(refs, 0, &err);
  *         if (!transaction ||
  *             ref_transaction_update(...) ||
  *             ref_transaction_create(...) ||
@@ -573,6 +573,7 @@ enum action_on_err {
  * be freed by calling ref_transaction_free().
  */
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    unsigned int flags,
 						    struct strbuf *err);
 struct ref_transaction *ref_transaction_begin(struct strbuf *err);
 
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 5795e54020..ecf88cee04 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1121,7 +1121,7 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base, &err);
+	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1192,7 +1192,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
 	if (!transaction)
 		return -1;
 
@@ -1259,7 +1259,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
 	if (!transaction)
 		goto error;
 
@@ -2773,7 +2773,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store, err);
+						refs->packed_ref_store, 0, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3044,7 +3044,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, err);
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index e97d67f932..cfebcd0b46 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1535,7 +1535,7 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	 * updates into a single transaction.
 	 */
 
-	transaction = ref_store_transaction_begin(ref_store, &err);
+	transaction = ref_store_transaction_begin(ref_store, 0, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 46a839539e..a0af63f162 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -213,6 +213,7 @@ struct ref_transaction {
 	size_t nr;
 	enum ref_transaction_state state;
 	void *backend_data;
+	unsigned int flags;
 };
 
 /*
diff --git a/sequencer.c b/sequencer.c
index 6abd72160c..61edd39d7a 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3588,7 +3588,7 @@ static int do_label(struct repository *r, const char *name, int len)
 	strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
 	strbuf_addf(&msg, "rebase (label) '%.*s'", len, name);
 
-	transaction = ref_store_transaction_begin(refs, &err);
+	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction) {
 		error("%s", err.buf);
 		ret = -1;
-- 
2.34.1


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[PATCH v2 3/6] refs: allow skipping the reference-transaction hook

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 1449 bytes --]

The reference-transaction hook is executing whenever we prepare, commit
or abort a reference transaction. While this is mostly intentional, in
case of the files backend we're leaking the implementation detail that
the store is in fact a composite store with one loose and one packed
backend to the caller. So while we want to execute the hook for all
logical updates, executing it for such implementation details is
unexpected.

Prepare for a fix by adding a new flag which allows to skip execution of
the hook.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c | 3 +++
 refs.h | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/refs.c b/refs.c
index 7415864b62..526bf5ed97 100644
--- a/refs.c
+++ b/refs.c
@@ -2084,6 +2084,9 @@ static int run_transaction_hook(struct ref_transaction *transaction,
 	const char *hook;
 	int ret = 0, i;
 
+	if (transaction->flags & REF_TRANSACTION_SKIP_HOOK)
+		return 0;
+
 	hook = find_hook("reference-transaction");
 	if (!hook)
 		return ret;
diff --git a/refs.h b/refs.h
index 31f7bf9642..d4056f9fe2 100644
--- a/refs.h
+++ b/refs.h
@@ -568,6 +568,11 @@ enum action_on_err {
 	UPDATE_REFS_QUIET_ON_ERR
 };
 
+/*
+ * Skip executing the reference-transaction hook.
+ */
+#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
+
 /*
  * Begin a reference transaction.  The reference transaction must
  * be freed by calling ref_transaction_free().
-- 
2.34.1


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[PATCH v2 4/6] refs: demonstrate excessive execution of the reference-transaction hook

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 2645 bytes --]

Add tests which demonstate which demonstrates that we're executing the
reference-transaction hook too often in some cases, which thus leaks
implementation details about the reference store's implementation
itself. Behaviour will be fixed in follow-up commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 t/t1416-ref-transaction-hooks.sh | 64 ++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index 6c941027a8..0567fbdf0b 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -136,4 +136,68 @@ test_expect_success 'interleaving hook calls succeed' '
 	test_cmp expect target-repo.git/actual
 '
 
+test_expect_success 'hook does not get called on packing refs' '
+	# Pack references first such that we are in a known state.
+	git pack-refs --all &&
+
+	write_script .git/hooks/reference-transaction <<-\EOF &&
+		echo "$@" >>actual
+		cat >>actual
+	EOF
+	rm -f actual &&
+
+	git update-ref refs/heads/unpacked-ref $POST_OID &&
+	git pack-refs --all &&
+
+	# We only expect a single hook invocation, which is the call to
+	# git-update-ref(1). But currently, packing refs will also trigger the
+	# hook.
+	cat >expect <<-EOF &&
+		prepared
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		committed
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		prepared
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		committed
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		prepared
+		$POST_OID $ZERO_OID refs/heads/unpacked-ref
+		committed
+		$POST_OID $ZERO_OID refs/heads/unpacked-ref
+	EOF
+
+	test_cmp expect actual
+'
+
+test_expect_success 'deleting packed ref calls hook once' '
+	# Create a reference and pack it.
+	git update-ref refs/heads/to-be-deleted $POST_OID &&
+	git pack-refs --all &&
+
+	write_script .git/hooks/reference-transaction <<-\EOF &&
+		echo "$@" >>actual
+		cat >>actual
+	EOF
+	rm -f actual &&
+
+	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
+
+	# We only expect a single hook invocation, which is the logical
+	# deletion. But currently, we see two interleaving transactions, once
+	# for deleting the loose refs and once for deleting the packed ref.
+	cat >expect <<-EOF &&
+		prepared
+		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
+		prepared
+		$POST_OID $ZERO_OID refs/heads/to-be-deleted
+		committed
+		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
+		committed
+		$POST_OID $ZERO_OID refs/heads/to-be-deleted
+	EOF
+
+	test_cmp expect actual
+'
+
 test_done
-- 
2.34.1


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[PATCH v2 5/6] refs: do not execute reference-transaction hook on packing refs

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 2581 bytes --]

The reference-transaction hook is supposed to track logical changes to
references, but it currently also gets executed when packing refs in a
repository. This is unexpected and ultimately not all that useful:
packing refs is not supposed to result in any user-visible change to the
refs' state, and it ultimately is an implementation detail of how refs
stores work.

Fix this excessive execution of the hook when packing refs.

Reported-by: Waleed Khan <me@waleedkhan.name>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c             |  6 ++++--
 t/t1416-ref-transaction-hooks.sh | 11 +----------
 2 files changed, 5 insertions(+), 12 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index ecf88cee04..3c0f3027fe 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1121,7 +1121,8 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
+	transaction = ref_store_transaction_begin(&refs->base,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1192,7 +1193,8 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index 0567fbdf0b..f9d3d5213f 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -150,21 +150,12 @@ test_expect_success 'hook does not get called on packing refs' '
 	git pack-refs --all &&
 
 	# We only expect a single hook invocation, which is the call to
-	# git-update-ref(1). But currently, packing refs will also trigger the
-	# hook.
+	# git-update-ref(1).
 	cat >expect <<-EOF &&
 		prepared
 		$ZERO_OID $POST_OID refs/heads/unpacked-ref
 		committed
 		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		prepared
-		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		committed
-		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		prepared
-		$POST_OID $ZERO_OID refs/heads/unpacked-ref
-		committed
-		$POST_OID $ZERO_OID refs/heads/unpacked-ref
 	EOF
 
 	test_cmp expect actual
-- 
2.34.1


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[PATCH v2 6/6] refs: skip hooks when deleting uncovered packed refs

[Patrick Steinhardt]

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When deleting refs from the loose-files refs backend, then we need to be
careful to also delete the same ref from the packed refs backend, if it
exists. If we don't, then deleting the loose ref would "uncover" the
packed ref. We thus always have to queue up deletions of refs for both
the loose and the packed refs backend. This is done in two separate
transactions, where the end result is that the reference-transaction
hook is executed twice for the deleted refs.

This behaviour is quite misleading: it's exposing implementation details
of how the files backend works to the user, in contrast to the logical
updates that we'd really want to expose via the hook. Worse yet, whether
the hook gets executed once or twice depends on how well-packed the
repository is: if the ref only exists as a loose ref, then we execute it
once, otherwise if it is also packed then we execute it twice.

Fix this behaviour and don't execute the reference-transaction hook at
all when refs in the packed-refs backend if it's driven by the files
backend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c             | 9 ++++++---
 t/t1416-ref-transaction-hooks.sh | 7 +------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 3c0f3027fe..9a20cb8fa8 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1261,7 +1261,8 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		goto error;
 
@@ -2775,7 +2776,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store, 0, err);
+						refs->packed_ref_store,
+						REF_TRANSACTION_SKIP_HOOK, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3046,7 +3048,8 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
+							 REF_TRANSACTION_SKIP_HOOK, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index f9d3d5213f..4e1e84a91f 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -175,16 +175,11 @@ test_expect_success 'deleting packed ref calls hook once' '
 	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
 
 	# We only expect a single hook invocation, which is the logical
-	# deletion. But currently, we see two interleaving transactions, once
-	# for deleting the loose refs and once for deleting the packed ref.
+	# deletion.
 	cat >expect <<-EOF &&
-		prepared
-		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
 		prepared
 		$POST_OID $ZERO_OID refs/heads/to-be-deleted
 		committed
-		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
-		committed
 		$POST_OID $ZERO_OID refs/heads/to-be-deleted
 	EOF
 
-- 
2.34.1


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Re: [PATCH v2 0/6] refs: excessive hook execution with packed refs

[Junio C Hamano]

Patrick Steinhardt <ps@pks.im> writes:

> Hi,
>
> this is a resend of version 1 of this patch series to hopefully entice
> some reviews. The only change is that v2 is rebased onto the current
> main branch at commit e83ba647f7 (The seventh batch, 2022-01-05). The
> following was from the orignial cover letter:

I'll add Ævar, who has been making a lot of changes to the refs
subsystem, and Han-Wen, whose work to add a new ref backend may need
to interact with this change, as possible stake-holders to the CC list.

> As reported by Waleed in [1], the reference-transaction hook is being
> executed when packing refs. Given that the hook ideally ought to track
> logical updates to refs instead of leaking low-level implementation
> details of how the files backend works, this is understandably leading
> to some confusion.
>
> This patch series aims to fix that by improving how the tandom of loose
> and packed refs backends interact such that we skip executing the hook
> when the loose backend:
>
>     - repacks references.
>     - needs to delete packed refs when deleting a loose ref would
>       uncover that packed ref.
>
> Patrick
>
> [1]: <CAKjfCeBcuYC3OXRVtxxDGWRGOxC38Fb7CNuSh_dMmxpGVip_9Q@mail.gmail.com>
>
> Patrick Steinhardt (6):
>   refs: open-code deletion of packed refs
>   refs: allow passing flags when beginning transactions
>   refs: allow skipping the reference-transaction hook
>   refs: demonstrate excessive execution of the reference-transaction
>     hook
>   refs: do not execute reference-transaction hook on packing refs
>   refs: skip hooks when deleting uncovered packed refs
>
>  refs.c                           | 11 +++++--
>  refs.h                           |  8 ++++-
>  refs/files-backend.c             | 25 +++++++++++-----
>  refs/packed-backend.c            | 30 ++++++++++++++-----
>  refs/packed-backend.h            |  6 ++++
>  refs/refs-internal.h             |  1 +
>  sequencer.c                      |  2 +-
>  t/t1416-ref-transaction-hooks.sh | 50 ++++++++++++++++++++++++++++++++
>  8 files changed, 113 insertions(+), 20 deletions(-)

Re: [PATCH v2 1/6] refs: open-code deletion of packed refs

[Junio C Hamano]

Patrick Steinhardt <ps@pks.im> writes:

This breaks the static analysis job at
https://github.com/git/git/runs/4744820766

>  refs/packed-backend.h |  6 ++++++
> diff --git a/refs/packed-backend.h b/refs/packed-backend.h
> index f61a73ec25..5e0dd7d08e 100644
> --- a/refs/packed-backend.h
> +++ b/refs/packed-backend.h
> @@ -27,6 +27,12 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
>  void packed_refs_unlock(struct ref_store *ref_store);
>  int packed_refs_is_locked(struct ref_store *ref_store);
>  
> +int packed_refs_delete_refs(struct ref_store *ref_store,
> +			    struct ref_transaction *transaction,
> +			    const char *msg,
> +			    struct string_list *refnames,
> +			    unsigned int flags);
> +
>  /*
>   * Return true if `transaction` really needs to be carried out against
>   * the specified packed_ref_store, or false if it can be skipped

    HDR reftable/block.h
In file included from refs/packed-backend.hcc:2:0:
./refs/packed-backend.h:33:15: error: ‘struct string_list’ declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
        struct string_list *refnames,
               ^~~~~~~~~~~
cc1: all warnings being treated as errors
make: *** [refs/packed-backend.hco] Error 1
make: *** Waiting for unfinished jobs....
Makefile:3029: recipe for target 'refs/packed-backend.hco' failed
Error: Process completed with exit code 1.

Re: [PATCH v2 4/6] refs: demonstrate excessive execution of the reference-transaction hook

[Junio C Hamano]

Patrick Steinhardt <ps@pks.im> writes:

> Add tests which demonstate which demonstrates that we're executing the

You demonstrate too often, which may be the point of the test, but
looks wrong.

I actually think this should be done as part of the fix to the code
itself, which presumably is a single-liner to tell the "skip when
running delete in packed-refs backend".  IOW, just fix the code and
test how the externally observable behaviour of the code should be
in new tests, in the same commit.

> diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
> index 6c941027a8..0567fbdf0b 100755
> --- a/t/t1416-ref-transaction-hooks.sh
> +++ b/t/t1416-ref-transaction-hooks.sh
> @@ -136,4 +136,68 @@ test_expect_success 'interleaving hook calls succeed' '
>  	test_cmp expect target-repo.git/actual
>  '
>  
> +test_expect_success 'hook does not get called on packing refs' '
> +	# Pack references first such that we are in a known state.
> +	git pack-refs --all &&
> +
> +	write_script .git/hooks/reference-transaction <<-\EOF &&
> +		echo "$@" >>actual
> +		cat >>actual
> +	EOF
> +	rm -f actual &&
> +
> +	git update-ref refs/heads/unpacked-ref $POST_OID &&
> +	git pack-refs --all &&
> +
> +	# We only expect a single hook invocation, which is the call to
> +	# git-update-ref(1). But currently, packing refs will also trigger the
> +	# hook.
> +	cat >expect <<-EOF &&
> +		prepared
> +		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> +		committed
> +		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> +		prepared
> +		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> +		committed
> +		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> +		prepared
> +		$POST_OID $ZERO_OID refs/heads/unpacked-ref
> +		committed
> +		$POST_OID $ZERO_OID refs/heads/unpacked-ref
> +	EOF
> +
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'deleting packed ref calls hook once' '
> +	# Create a reference and pack it.
> +	git update-ref refs/heads/to-be-deleted $POST_OID &&
> +	git pack-refs --all &&
> +
> +	write_script .git/hooks/reference-transaction <<-\EOF &&
> +		echo "$@" >>actual
> +		cat >>actual
> +	EOF
> +	rm -f actual &&
> +
> +	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
> +
> +	# We only expect a single hook invocation, which is the logical
> +	# deletion. But currently, we see two interleaving transactions, once
> +	# for deleting the loose refs and once for deleting the packed ref.
> +	cat >expect <<-EOF &&
> +		prepared
> +		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> +		prepared
> +		$POST_OID $ZERO_OID refs/heads/to-be-deleted
> +		committed
> +		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> +		committed
> +		$POST_OID $ZERO_OID refs/heads/to-be-deleted
> +	EOF
> +
> +	test_cmp expect actual
> +'
> +
>  test_done

Re: [PATCH v2 1/6] refs: open-code deletion of packed refs

[Junio C Hamano]

Patrick Steinhardt <ps@pks.im> writes:

> When deleting loose refs, then we also have to delete the refs in the
> packed backend. This is done by calling `refs_delete_refs()`, which
> then uses the packed-backend's logic to delete refs. This doesn't allow
> us to exercise any control over the reference transaction which is being
> created in the packed backend, which is required in a subsequent commit.
>
> Extract a new function `packed_refs_delete_refs()`, which hosts most of
> the logic to delete refs except for creating the transaction itself.
> Like this, we can easily create the transactionion in the files backend

ionion?

> and thus exert more control over it.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---

The title might be technically correct, but I think the primary
point, the benefit this step brings to the system, is now we have a
helper function we can call in a transaction we create ourselves.

    refs: extract packed_refs_delete_refs() to be used in our own transaction

or something along that line, perhaps?  I dunno.

>  refs/files-backend.c  | 12 +++++++++---
>  refs/packed-backend.c | 28 +++++++++++++++++++++-------
>  refs/packed-backend.h |  6 ++++++
>  3 files changed, 36 insertions(+), 10 deletions(-)
>
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 90b671025a..5795e54020 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -1249,6 +1249,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
>  {
>  	struct files_ref_store *refs =
>  		files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
> +	struct ref_transaction *transaction = NULL;
>  	struct strbuf err = STRBUF_INIT;
>  	int i, result = 0;
>  
> @@ -1258,10 +1259,14 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
>  	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
>  		goto error;
>  
> -	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
> -		packed_refs_unlock(refs->packed_ref_store);
> +	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
> +	if (!transaction)
> +		goto error;
> +
> +	result = packed_refs_delete_refs(refs->packed_ref_store,
> +					 transaction, msg, refnames, flags);
> +	if (result)
>  		goto error;
> -	}

Without looking at the later patches, my guess is that we'd create a
single transaction for packed-refs backend and then issue multiple
delete requests in that single transaction?

Ah, no.  We are already deleting multiple refs in one go, so that is
not what is happening here.  Makes readers a bit curious.  Hopefully
we will find it out in a later patch.

> diff --git a/refs/packed-backend.h b/refs/packed-backend.h
> index f61a73ec25..5e0dd7d08e 100644
> --- a/refs/packed-backend.h
> +++ b/refs/packed-backend.h
> @@ -27,6 +27,12 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
>  void packed_refs_unlock(struct ref_store *ref_store);
>  int packed_refs_is_locked(struct ref_store *ref_store);
>  
> +int packed_refs_delete_refs(struct ref_store *ref_store,
> +			    struct ref_transaction *transaction,
> +			    const char *msg,
> +			    struct string_list *refnames,
> +			    unsigned int flags);
> +
>  /*
>   * Return true if `transaction` really needs to be carried out against
>   * the specified packed_ref_store, or false if it can be skipped

Re: [PATCH v2 2/6] refs: allow passing flags when beginning transactions

[Junio C Hamano]

Patrick Steinhardt <ps@pks.im> writes:

> We do not currently have any flags when creating reference transactions,
> but we'll add one to disable execution of the reference transaction hook
> in some cases.
>
> Allow passing flags to `ref_store_transaction_begin()` to prepare for
> this change.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  refs.c                |  8 +++++---
>  refs.h                |  3 ++-
>  refs/files-backend.c  | 10 +++++-----
>  refs/packed-backend.c |  2 +-
>  refs/refs-internal.h  |  1 +
>  sequencer.c           |  2 +-
>  6 files changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/refs.c b/refs.c
> index 4da4996c4d..7415864b62 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -800,7 +800,7 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
>  	struct ref_transaction *transaction;
>  	struct strbuf err = STRBUF_INIT;
>  
> -	transaction = ref_store_transaction_begin(refs, &err);
> +	transaction = ref_store_transaction_begin(refs, 0, &err);

Ah, OK, the "later" came soon enough.  It wasn't that we wanted to
cram multiple calls for "delete" in a single transaction.  It was
that we wanted to tweak how each transaction works by passing a
flag word.

>  struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
> +						    unsigned int flags,
>  						    struct strbuf *err)

OK.  And in this step, everybody passes 0 (i.e. no bit set) to the
function.

Re: [PATCH v2 6/6] refs: skip hooks when deleting uncovered packed refs

[Junio C Hamano]

Patrick Steinhardt <ps@pks.im> writes:

> When deleting refs from the loose-files refs backend, then we need to be
> careful to also delete the same ref from the packed refs backend, if it
> exists. If we don't, then deleting the loose ref would "uncover" the
> packed ref. We thus always have to queue up deletions of refs for both
> the loose and the packed refs backend. This is done in two separate
> transactions, where the end result is that the reference-transaction
> hook is executed twice for the deleted refs.
>
> This behaviour is quite misleading: it's exposing implementation details
> of how the files backend works to the user, in contrast to the logical
> updates that we'd really want to expose via the hook. Worse yet, whether
> the hook gets executed once or twice depends on how well-packed the
> repository is: if the ref only exists as a loose ref, then we execute it
> once, otherwise if it is also packed then we execute it twice.

If the ref only exists as a packed ref, what happens? ...

> Fix this behaviour and don't execute the reference-transaction hook at
> all when refs in the packed-refs backend if it's driven by the files
> backend.

... We try to remove from the loose backend, which would say "nah,
it did not exist in my store".  I am not sure if it should execute
the delete hook in such a case for the ref.  But if it does not, not
running the hook in the ref transaction for packed backend driven by
the loose backend would mean nobody notifies the deletion of the
ref, no?

To me, it seems that the only case this strategy would work
correctly is when the files backend calls deletion hook for a
request to delete nonexistent ref, which by itself sounds like a
problem.

> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  refs/files-backend.c             | 9 ++++++---
>  t/t1416-ref-transaction-hooks.sh | 7 +------
>  2 files changed, 7 insertions(+), 9 deletions(-)
>
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 3c0f3027fe..9a20cb8fa8 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -1261,7 +1261,8 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
>  	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
>  		goto error;
>  
> -	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
> +	transaction = ref_store_transaction_begin(refs->packed_ref_store,
> +						  REF_TRANSACTION_SKIP_HOOK, &err);
>  	if (!transaction)
>  		goto error;
>  
> @@ -2775,7 +2776,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
>  			 */
>  			if (!packed_transaction) {
>  				packed_transaction = ref_store_transaction_begin(
> -						refs->packed_ref_store, 0, err);
> +						refs->packed_ref_store,
> +						REF_TRANSACTION_SKIP_HOOK, err);
>  				if (!packed_transaction) {
>  					ret = TRANSACTION_GENERIC_ERROR;
>  					goto cleanup;
> @@ -3046,7 +3048,8 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
>  				 &affected_refnames))
>  		BUG("initial ref transaction called with existing refs");
>  
> -	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
> +	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
> +							 REF_TRANSACTION_SKIP_HOOK, err);
>  	if (!packed_transaction) {
>  		ret = TRANSACTION_GENERIC_ERROR;
>  		goto cleanup;
> diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
> index f9d3d5213f..4e1e84a91f 100755
> --- a/t/t1416-ref-transaction-hooks.sh
> +++ b/t/t1416-ref-transaction-hooks.sh
> @@ -175,16 +175,11 @@ test_expect_success 'deleting packed ref calls hook once' '
>  	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
>  
>  	# We only expect a single hook invocation, which is the logical
> -	# deletion. But currently, we see two interleaving transactions, once
> -	# for deleting the loose refs and once for deleting the packed ref.
> +	# deletion.
>  	cat >expect <<-EOF &&
> -		prepared
> -		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
>  		prepared
>  		$POST_OID $ZERO_OID refs/heads/to-be-deleted
>  		committed
> -		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> -		committed
>  		$POST_OID $ZERO_OID refs/heads/to-be-deleted
>  	EOF

Re: [PATCH v2 4/6] refs: demonstrate excessive execution of the reference-transaction hook

[Eric Sunshine]

On Fri, Jan 7, 2022 at 3:09 PM Patrick Steinhardt <ps@pks.im> wrote:
> Add tests which demonstate which demonstrates that we're executing the
> reference-transaction hook too often in some cases, which thus leaks
> implementation details about the reference store's implementation
> itself. Behaviour will be fixed in follow-up commits.

s/which demonstate which demonstrates/which demonstrate/

> Signed-off-by: Patrick Steinhardt <ps@pks.im>

Re: [PATCH v2 4/6] refs: demonstrate excessive execution of the reference-transaction hook

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 3555 bytes --]

On Fri, Jan 07, 2022 at 05:31:04PM -0800, Junio C Hamano wrote:
> Patrick Steinhardt <ps@pks.im> writes:
> 
> > Add tests which demonstate which demonstrates that we're executing the
> 
> You demonstrate too often, which may be the point of the test, but
> looks wrong.
> 
> I actually think this should be done as part of the fix to the code
> itself, which presumably is a single-liner to tell the "skip when
> running delete in packed-refs backend".  IOW, just fix the code and
> test how the externally observable behaviour of the code should be
> in new tests, in the same commit.

The reason why I chose to split this out into a separate commit is that
it makes it easier to see what behaviour exactly is changing. If it was
a single step, then a reader would only see the post-image behaviour but
cannot reason about the pre-image behaviour without puzzling everything
together.

So personally I'd prefer to keep it as a separate step, but I'm not
opposed to merging them if you still disagree with my reasoning above.

Patrick

> > diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
> > index 6c941027a8..0567fbdf0b 100755
> > --- a/t/t1416-ref-transaction-hooks.sh
> > +++ b/t/t1416-ref-transaction-hooks.sh
> > @@ -136,4 +136,68 @@ test_expect_success 'interleaving hook calls succeed' '
> >  	test_cmp expect target-repo.git/actual
> >  '
> >  
> > +test_expect_success 'hook does not get called on packing refs' '
> > +	# Pack references first such that we are in a known state.
> > +	git pack-refs --all &&
> > +
> > +	write_script .git/hooks/reference-transaction <<-\EOF &&
> > +		echo "$@" >>actual
> > +		cat >>actual
> > +	EOF
> > +	rm -f actual &&
> > +
> > +	git update-ref refs/heads/unpacked-ref $POST_OID &&
> > +	git pack-refs --all &&
> > +
> > +	# We only expect a single hook invocation, which is the call to
> > +	# git-update-ref(1). But currently, packing refs will also trigger the
> > +	# hook.
> > +	cat >expect <<-EOF &&
> > +		prepared
> > +		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> > +		committed
> > +		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> > +		prepared
> > +		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> > +		committed
> > +		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> > +		prepared
> > +		$POST_OID $ZERO_OID refs/heads/unpacked-ref
> > +		committed
> > +		$POST_OID $ZERO_OID refs/heads/unpacked-ref
> > +	EOF
> > +
> > +	test_cmp expect actual
> > +'
> > +
> > +test_expect_success 'deleting packed ref calls hook once' '
> > +	# Create a reference and pack it.
> > +	git update-ref refs/heads/to-be-deleted $POST_OID &&
> > +	git pack-refs --all &&
> > +
> > +	write_script .git/hooks/reference-transaction <<-\EOF &&
> > +		echo "$@" >>actual
> > +		cat >>actual
> > +	EOF
> > +	rm -f actual &&
> > +
> > +	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
> > +
> > +	# We only expect a single hook invocation, which is the logical
> > +	# deletion. But currently, we see two interleaving transactions, once
> > +	# for deleting the loose refs and once for deleting the packed ref.
> > +	cat >expect <<-EOF &&
> > +		prepared
> > +		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> > +		prepared
> > +		$POST_OID $ZERO_OID refs/heads/to-be-deleted
> > +		committed
> > +		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> > +		committed
> > +		$POST_OID $ZERO_OID refs/heads/to-be-deleted
> > +	EOF
> > +
> > +	test_cmp expect actual
> > +'
> > +
> >  test_done

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Re: [PATCH v2 6/6] refs: skip hooks when deleting uncovered packed refs

[Patrick Steinhardt]

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On Fri, Jan 07, 2022 at 06:01:04PM -0800, Junio C Hamano wrote:
> Patrick Steinhardt <ps@pks.im> writes:
> 
> > When deleting refs from the loose-files refs backend, then we need to be
> > careful to also delete the same ref from the packed refs backend, if it
> > exists. If we don't, then deleting the loose ref would "uncover" the
> > packed ref. We thus always have to queue up deletions of refs for both
> > the loose and the packed refs backend. This is done in two separate
> > transactions, where the end result is that the reference-transaction
> > hook is executed twice for the deleted refs.
> >
> > This behaviour is quite misleading: it's exposing implementation details
> > of how the files backend works to the user, in contrast to the logical
> > updates that we'd really want to expose via the hook. Worse yet, whether
> > the hook gets executed once or twice depends on how well-packed the
> > repository is: if the ref only exists as a loose ref, then we execute it
> > once, otherwise if it is also packed then we execute it twice.
> 
> If the ref only exists as a packed ref, what happens? ...
> 
> > Fix this behaviour and don't execute the reference-transaction hook at
> > all when refs in the packed-refs backend if it's driven by the files
> > backend.
> 
> ... We try to remove from the loose backend, which would say "nah,
> it did not exist in my store".  I am not sure if it should execute
> the delete hook in such a case for the ref.  But if it does not, not
> running the hook in the ref transaction for packed backend driven by
> the loose backend would mean nobody notifies the deletion of the
> ref, no?

This is shown in the test I've added, "deleting packed ref calls hook
once". We create a new reference and pack it such that it doesn't exist
as loose ref anymore, but only as a packed one. Updating that ref
would've caused us to execute the hook twice before, once via the
packed-backend and once via the loose-backend. With my fix we only
execute it once via the loose-backend, even if it doesn't currently know
it. This works because the loose-backend has to create a lock for the
nonexistent reference such that no concurrent call touches the same ref.

> To me, it seems that the only case this strategy would work
> correctly is when the files backend calls deletion hook for a
> request to delete nonexistent ref, which by itself sounds like a
> problem.

It does so only if the ref exists in either the loose or packed backend
though. If trying to update a ref which exists in neither of those, then
the reference transaction would fail with an "unable to resolve
reference" error in `lock_raw_ref()`.

So this should behave as expected: deleting a packed ref calls the hook
once, deleting a nonexistent ref fails and doesn't call the hook at all.

Patrick

> > Signed-off-by: Patrick Steinhardt <ps@pks.im>
> > ---
> >  refs/files-backend.c             | 9 ++++++---
> >  t/t1416-ref-transaction-hooks.sh | 7 +------
> >  2 files changed, 7 insertions(+), 9 deletions(-)
> >
> > diff --git a/refs/files-backend.c b/refs/files-backend.c
> > index 3c0f3027fe..9a20cb8fa8 100644
> > --- a/refs/files-backend.c
> > +++ b/refs/files-backend.c
> > @@ -1261,7 +1261,8 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
> >  	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
> >  		goto error;
> >  
> > -	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
> > +	transaction = ref_store_transaction_begin(refs->packed_ref_store,
> > +						  REF_TRANSACTION_SKIP_HOOK, &err);
> >  	if (!transaction)
> >  		goto error;
> >  
> > @@ -2775,7 +2776,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
> >  			 */
> >  			if (!packed_transaction) {
> >  				packed_transaction = ref_store_transaction_begin(
> > -						refs->packed_ref_store, 0, err);
> > +						refs->packed_ref_store,
> > +						REF_TRANSACTION_SKIP_HOOK, err);
> >  				if (!packed_transaction) {
> >  					ret = TRANSACTION_GENERIC_ERROR;
> >  					goto cleanup;
> > @@ -3046,7 +3048,8 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
> >  				 &affected_refnames))
> >  		BUG("initial ref transaction called with existing refs");
> >  
> > -	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
> > +	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
> > +							 REF_TRANSACTION_SKIP_HOOK, err);
> >  	if (!packed_transaction) {
> >  		ret = TRANSACTION_GENERIC_ERROR;
> >  		goto cleanup;
> > diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
> > index f9d3d5213f..4e1e84a91f 100755
> > --- a/t/t1416-ref-transaction-hooks.sh
> > +++ b/t/t1416-ref-transaction-hooks.sh
> > @@ -175,16 +175,11 @@ test_expect_success 'deleting packed ref calls hook once' '
> >  	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
> >  
> >  	# We only expect a single hook invocation, which is the logical
> > -	# deletion. But currently, we see two interleaving transactions, once
> > -	# for deleting the loose refs and once for deleting the packed ref.
> > +	# deletion.
> >  	cat >expect <<-EOF &&
> > -		prepared
> > -		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> >  		prepared
> >  		$POST_OID $ZERO_OID refs/heads/to-be-deleted
> >  		committed
> > -		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> > -		committed
> >  		$POST_OID $ZERO_OID refs/heads/to-be-deleted
> >  	EOF

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Re: [PATCH v2 6/6] refs: skip hooks when deleting uncovered packed refs

[Junio C Hamano]

Patrick Steinhardt <ps@pks.im> writes:

> It does so only if the ref exists in either the loose or packed backend
> though. If trying to update a ref which exists in neither of those, then
> the reference transaction would fail with an "unable to resolve
> reference" error in `lock_raw_ref()`.
>
> So this should behave as expected: deleting a packed ref calls the hook
> once, deleting a nonexistent ref fails and doesn't call the hook at all.

OK.  Your explanation deserves to be in the log message for those
who ask questions next time, not lost in the archive in a message
just answering my question and explaining it only for me.  After
all, that is why I ask questions in my reviews.

Thanks.

Re: [PATCH v2 1/6] refs: open-code deletion of packed refs

[Junio C Hamano]

Patrick Steinhardt <ps@pks.im> writes:

> diff --git a/refs/packed-backend.h b/refs/packed-backend.h
> index f61a73ec25..5e0dd7d08e 100644
> --- a/refs/packed-backend.h
> +++ b/refs/packed-backend.h
> @@ -27,6 +27,12 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
>  void packed_refs_unlock(struct ref_store *ref_store);
>  int packed_refs_is_locked(struct ref_store *ref_store);
>  
> +int packed_refs_delete_refs(struct ref_store *ref_store,
> +			    struct ref_transaction *transaction,
> +			    const char *msg,
> +			    struct string_list *refnames,
> +			    unsigned int flags);
> +
>  /*
>   * Return true if `transaction` really needs to be carried out against
>   * the specified packed_ref_store, or false if it can be skipped

This breaks "make hdr-check" (which is part of CI -- static-analysis).

----- >8 --------- >8 --------- >8 --------- >8 -----
From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 12 Jan 2022 16:24:13 -0800
Subject: [PATCH] fixup! refs: open-code deletion of packed refs

---
 refs/packed-backend.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index 5e0dd7d08e..a2cca5d9a3 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -3,6 +3,7 @@
 
 struct repository;
 struct ref_transaction;
+struct string_list;
 
 /*
  * Support for storing references in a `packed-refs` file.
-- 
2.35.0-rc0-185-gedd802a149

[PATCH v3 0/6] refs: excessive hook execution with packed refs

[Patrick Steinhardt]

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Hi,

this is the third version of this patch series, which addresses an issue
where the reference-transaction hook is being invoked twice when
deleting refs both in the packed-refs and loose-refs file.

The following things changed in comparison to v2:

    - Fixed some typos in commit messages.

    - Improved the subject of the first patch to more clearly highlight
      the purpose of it, not only say what the patch does.

    - Fixed a missing declaration for `struct string_list`.

    - Clarified why the last patch does the right thing even in case the
      ref only exists in the packed-refs backend, and in case it doesn't
      exist in either of the backends.

Thanks for your feedback!

Patrick

Patrick Steinhardt (6):
  refs: extract packed_refs_delete_refs() to allow control of
    transaction
  refs: allow passing flags when beginning transactions
  refs: allow skipping the reference-transaction hook
  refs: demonstrate excessive execution of the reference-transaction
    hook
  refs: do not execute reference-transaction hook on packing refs
  refs: skip hooks when deleting uncovered packed refs

 refs.c                           | 11 +++++--
 refs.h                           |  8 ++++-
 refs/files-backend.c             | 25 +++++++++++-----
 refs/packed-backend.c            | 30 ++++++++++++++-----
 refs/packed-backend.h            |  7 +++++
 refs/refs-internal.h             |  1 +
 sequencer.c                      |  2 +-
 t/t1416-ref-transaction-hooks.sh | 50 ++++++++++++++++++++++++++++++++
 8 files changed, 114 insertions(+), 20 deletions(-)

Range-diff against v2:
1:  0739f085b2 ! 1:  abbc28822b refs: open-code deletion of packed refs
    @@ Metadata
     Author: Patrick Steinhardt <ps@pks.im>
     
      ## Commit message ##
    -    refs: open-code deletion of packed refs
    +    refs: extract packed_refs_delete_refs() to allow control of transaction
     
         When deleting loose refs, then we also have to delete the refs in the
         packed backend. This is done by calling `refs_delete_refs()`, which
    @@ Commit message
     
         Extract a new function `packed_refs_delete_refs()`, which hosts most of
         the logic to delete refs except for creating the transaction itself.
    -    Like this, we can easily create the transactionion in the files backend
    +    Like this, we can easily create the transaction in the files backend
         and thus exert more control over it.
     
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
    @@ refs/packed-backend.c: static int packed_delete_refs(struct ref_store *ref_store
      }
     
      ## refs/packed-backend.h ##
    +@@
    + 
    + struct repository;
    + struct ref_transaction;
    ++struct string_list;
    + 
    + /*
    +  * Support for storing references in a `packed-refs` file.
     @@ refs/packed-backend.h: int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
      void packed_refs_unlock(struct ref_store *ref_store);
      int packed_refs_is_locked(struct ref_store *ref_store);
2:  629be01d50 = 2:  9dd172a757 refs: allow passing flags when beginning transactions
3:  550d89a323 = 3:  be826bae3b refs: allow skipping the reference-transaction hook
4:  b52e59cdac ! 4:  662a6e6244 refs: demonstrate excessive execution of the reference-transaction hook
    @@ Metadata
      ## Commit message ##
         refs: demonstrate excessive execution of the reference-transaction hook
     
    -    Add tests which demonstate which demonstrates that we're executing the
    +    Add tests which demonstate that we're executing the
         reference-transaction hook too often in some cases, which thus leaks
         implementation details about the reference store's implementation
         itself. Behaviour will be fixed in follow-up commits.
5:  1539e9711f = 5:  d83f309b9c refs: do not execute reference-transaction hook on packing refs
6:  0fbf68dbf4 ! 6:  279eadc41c refs: skip hooks when deleting uncovered packed refs
    @@ Commit message
     
         Fix this behaviour and don't execute the reference-transaction hook at
         all when refs in the packed-refs backend if it's driven by the files
    -    backend.
    +    backend. This works as expected even in case the refs to be deleted only
    +    exist in the packed-refs backend because the loose-backend always queues
    +    refs in its own transaction even if they don't exist such that they can
    +    be locked for concurrent creation. And it also does the right thing in
    +    case neither of the backends has the ref because that would cause the
    +    transaction to fail completely.
     
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
     
-- 
2.34.1


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[PATCH v3 1/6] refs: extract packed_refs_delete_refs() to allow control of transaction

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 5205 bytes --]

When deleting loose refs, then we also have to delete the refs in the
packed backend. This is done by calling `refs_delete_refs()`, which
then uses the packed-backend's logic to delete refs. This doesn't allow
us to exercise any control over the reference transaction which is being
created in the packed backend, which is required in a subsequent commit.

Extract a new function `packed_refs_delete_refs()`, which hosts most of
the logic to delete refs except for creating the transaction itself.
Like this, we can easily create the transaction in the files backend
and thus exert more control over it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c  | 12 +++++++++---
 refs/packed-backend.c | 28 +++++++++++++++++++++-------
 refs/packed-backend.h |  7 +++++++
 3 files changed, 37 insertions(+), 10 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 90b671025a..5795e54020 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1249,6 +1249,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct ref_transaction *transaction = NULL;
 	struct strbuf err = STRBUF_INIT;
 	int i, result = 0;
 
@@ -1258,10 +1259,14 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
-		packed_refs_unlock(refs->packed_ref_store);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	if (!transaction)
+		goto error;
+
+	result = packed_refs_delete_refs(refs->packed_ref_store,
+					 transaction, msg, refnames, flags);
+	if (result)
 		goto error;
-	}
 
 	packed_refs_unlock(refs->packed_ref_store);
 
@@ -1288,6 +1293,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	else
 		error(_("could not delete references: %s"), err.buf);
 
+	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return -1;
 }
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 67152c664e..e97d67f932 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1522,15 +1522,10 @@ static int packed_initial_transaction_commit(struct ref_store *ref_store,
 static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			     struct string_list *refnames, unsigned int flags)
 {
-	struct packed_ref_store *refs =
-		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
-	struct string_list_item *item;
 	int ret;
 
-	(void)refs; /* We need the check above, but don't use the variable */
-
 	if (!refnames->nr)
 		return 0;
 
@@ -1544,6 +1539,27 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (!transaction)
 		return -1;
 
+	ret = packed_refs_delete_refs(ref_store, transaction,
+				      msg, refnames, flags);
+
+	ref_transaction_free(transaction);
+	return ret;
+}
+
+int packed_refs_delete_refs(struct ref_store *ref_store,
+			    struct ref_transaction *transaction,
+			    const char *msg,
+			    struct string_list *refnames,
+			    unsigned int flags)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct strbuf err = STRBUF_INIT;
+	struct string_list_item *item;
+	int ret;
+
+	(void)(refs); /* We need the check above, but don't use the variable */
+
 	for_each_string_list_item(item, refnames) {
 		if (ref_transaction_delete(transaction, item->string, NULL,
 					   flags, msg, &err)) {
@@ -1554,7 +1570,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	}
 
 	ret = ref_transaction_commit(transaction, &err);
-
 	if (ret) {
 		if (refnames->nr == 1)
 			error(_("could not delete reference %s: %s"),
@@ -1563,7 +1578,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			error(_("could not delete references: %s"), err.buf);
 	}
 
-	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return ret;
 }
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index f61a73ec25..a2cca5d9a3 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -3,6 +3,7 @@
 
 struct repository;
 struct ref_transaction;
+struct string_list;
 
 /*
  * Support for storing references in a `packed-refs` file.
@@ -27,6 +28,12 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 void packed_refs_unlock(struct ref_store *ref_store);
 int packed_refs_is_locked(struct ref_store *ref_store);
 
+int packed_refs_delete_refs(struct ref_store *ref_store,
+			    struct ref_transaction *transaction,
+			    const char *msg,
+			    struct string_list *refnames,
+			    unsigned int flags);
+
 /*
  * Return true if `transaction` really needs to be carried out against
  * the specified packed_ref_store, or false if it can be skipped
-- 
2.34.1


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[PATCH v3 2/6] refs: allow passing flags when beginning transactions

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 6362 bytes --]

We do not currently have any flags when creating reference transactions,
but we'll add one to disable execution of the reference transaction hook
in some cases.

Allow passing flags to `ref_store_transaction_begin()` to prepare for
this change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c                |  8 +++++---
 refs.h                |  3 ++-
 refs/files-backend.c  | 10 +++++-----
 refs/packed-backend.c |  2 +-
 refs/refs-internal.h  |  1 +
 sequencer.c           |  2 +-
 6 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/refs.c b/refs.c
index 4da4996c4d..7415864b62 100644
--- a/refs.c
+++ b/refs.c
@@ -800,7 +800,7 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 
-	transaction = ref_store_transaction_begin(refs, &err);
+	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname, old_oid,
 				   flags, msg, &err) ||
@@ -1005,6 +1005,7 @@ int read_ref_at(struct ref_store *refs, const char *refname,
 }
 
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    unsigned int flags,
 						    struct strbuf *err)
 {
 	struct ref_transaction *tr;
@@ -1012,12 +1013,13 @@ struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
 
 	CALLOC_ARRAY(tr, 1);
 	tr->ref_store = refs;
+	tr->flags = flags;
 	return tr;
 }
 
 struct ref_transaction *ref_transaction_begin(struct strbuf *err)
 {
-	return ref_store_transaction_begin(get_main_ref_store(the_repository), err);
+	return ref_store_transaction_begin(get_main_ref_store(the_repository), 0, err);
 }
 
 void ref_transaction_free(struct ref_transaction *transaction)
@@ -1156,7 +1158,7 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 	struct strbuf err = STRBUF_INIT;
 	int ret = 0;
 
-	t = ref_store_transaction_begin(refs, &err);
+	t = ref_store_transaction_begin(refs, 0, &err);
 	if (!t ||
 	    ref_transaction_update(t, refname, new_oid, old_oid, flags, msg,
 				   &err) ||
diff --git a/refs.h b/refs.h
index 92360e55a2..31f7bf9642 100644
--- a/refs.h
+++ b/refs.h
@@ -231,7 +231,7 @@ char *repo_default_branch_name(struct repository *r, int quiet);
  *         struct strbuf err = STRBUF_INIT;
  *         int ret = 0;
  *
- *         transaction = ref_store_transaction_begin(refs, &err);
+ *         transaction = ref_store_transaction_begin(refs, 0, &err);
  *         if (!transaction ||
  *             ref_transaction_update(...) ||
  *             ref_transaction_create(...) ||
@@ -573,6 +573,7 @@ enum action_on_err {
  * be freed by calling ref_transaction_free().
  */
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    unsigned int flags,
 						    struct strbuf *err);
 struct ref_transaction *ref_transaction_begin(struct strbuf *err);
 
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 5795e54020..ecf88cee04 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1121,7 +1121,7 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base, &err);
+	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1192,7 +1192,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
 	if (!transaction)
 		return -1;
 
@@ -1259,7 +1259,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
 	if (!transaction)
 		goto error;
 
@@ -2773,7 +2773,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store, err);
+						refs->packed_ref_store, 0, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3044,7 +3044,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, err);
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index e97d67f932..cfebcd0b46 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1535,7 +1535,7 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	 * updates into a single transaction.
 	 */
 
-	transaction = ref_store_transaction_begin(ref_store, &err);
+	transaction = ref_store_transaction_begin(ref_store, 0, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 46a839539e..a0af63f162 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -213,6 +213,7 @@ struct ref_transaction {
 	size_t nr;
 	enum ref_transaction_state state;
 	void *backend_data;
+	unsigned int flags;
 };
 
 /*
diff --git a/sequencer.c b/sequencer.c
index 6abd72160c..61edd39d7a 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3588,7 +3588,7 @@ static int do_label(struct repository *r, const char *name, int len)
 	strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
 	strbuf_addf(&msg, "rebase (label) '%.*s'", len, name);
 
-	transaction = ref_store_transaction_begin(refs, &err);
+	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction) {
 		error("%s", err.buf);
 		ret = -1;
-- 
2.34.1


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[PATCH v3 3/6] refs: allow skipping the reference-transaction hook

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 1449 bytes --]

The reference-transaction hook is executing whenever we prepare, commit
or abort a reference transaction. While this is mostly intentional, in
case of the files backend we're leaking the implementation detail that
the store is in fact a composite store with one loose and one packed
backend to the caller. So while we want to execute the hook for all
logical updates, executing it for such implementation details is
unexpected.

Prepare for a fix by adding a new flag which allows to skip execution of
the hook.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c | 3 +++
 refs.h | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/refs.c b/refs.c
index 7415864b62..526bf5ed97 100644
--- a/refs.c
+++ b/refs.c
@@ -2084,6 +2084,9 @@ static int run_transaction_hook(struct ref_transaction *transaction,
 	const char *hook;
 	int ret = 0, i;
 
+	if (transaction->flags & REF_TRANSACTION_SKIP_HOOK)
+		return 0;
+
 	hook = find_hook("reference-transaction");
 	if (!hook)
 		return ret;
diff --git a/refs.h b/refs.h
index 31f7bf9642..d4056f9fe2 100644
--- a/refs.h
+++ b/refs.h
@@ -568,6 +568,11 @@ enum action_on_err {
 	UPDATE_REFS_QUIET_ON_ERR
 };
 
+/*
+ * Skip executing the reference-transaction hook.
+ */
+#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
+
 /*
  * Begin a reference transaction.  The reference transaction must
  * be freed by calling ref_transaction_free().
-- 
2.34.1


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[PATCH v3 4/6] refs: demonstrate excessive execution of the reference-transaction hook

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 2626 bytes --]

Add tests which demonstate that we're executing the
reference-transaction hook too often in some cases, which thus leaks
implementation details about the reference store's implementation
itself. Behaviour will be fixed in follow-up commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 t/t1416-ref-transaction-hooks.sh | 64 ++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index 6c941027a8..0567fbdf0b 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -136,4 +136,68 @@ test_expect_success 'interleaving hook calls succeed' '
 	test_cmp expect target-repo.git/actual
 '
 
+test_expect_success 'hook does not get called on packing refs' '
+	# Pack references first such that we are in a known state.
+	git pack-refs --all &&
+
+	write_script .git/hooks/reference-transaction <<-\EOF &&
+		echo "$@" >>actual
+		cat >>actual
+	EOF
+	rm -f actual &&
+
+	git update-ref refs/heads/unpacked-ref $POST_OID &&
+	git pack-refs --all &&
+
+	# We only expect a single hook invocation, which is the call to
+	# git-update-ref(1). But currently, packing refs will also trigger the
+	# hook.
+	cat >expect <<-EOF &&
+		prepared
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		committed
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		prepared
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		committed
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		prepared
+		$POST_OID $ZERO_OID refs/heads/unpacked-ref
+		committed
+		$POST_OID $ZERO_OID refs/heads/unpacked-ref
+	EOF
+
+	test_cmp expect actual
+'
+
+test_expect_success 'deleting packed ref calls hook once' '
+	# Create a reference and pack it.
+	git update-ref refs/heads/to-be-deleted $POST_OID &&
+	git pack-refs --all &&
+
+	write_script .git/hooks/reference-transaction <<-\EOF &&
+		echo "$@" >>actual
+		cat >>actual
+	EOF
+	rm -f actual &&
+
+	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
+
+	# We only expect a single hook invocation, which is the logical
+	# deletion. But currently, we see two interleaving transactions, once
+	# for deleting the loose refs and once for deleting the packed ref.
+	cat >expect <<-EOF &&
+		prepared
+		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
+		prepared
+		$POST_OID $ZERO_OID refs/heads/to-be-deleted
+		committed
+		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
+		committed
+		$POST_OID $ZERO_OID refs/heads/to-be-deleted
+	EOF
+
+	test_cmp expect actual
+'
+
 test_done
-- 
2.34.1


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[PATCH v3 5/6] refs: do not execute reference-transaction hook on packing refs

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 2581 bytes --]

The reference-transaction hook is supposed to track logical changes to
references, but it currently also gets executed when packing refs in a
repository. This is unexpected and ultimately not all that useful:
packing refs is not supposed to result in any user-visible change to the
refs' state, and it ultimately is an implementation detail of how refs
stores work.

Fix this excessive execution of the hook when packing refs.

Reported-by: Waleed Khan <me@waleedkhan.name>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c             |  6 ++++--
 t/t1416-ref-transaction-hooks.sh | 11 +----------
 2 files changed, 5 insertions(+), 12 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index ecf88cee04..3c0f3027fe 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1121,7 +1121,8 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
+	transaction = ref_store_transaction_begin(&refs->base,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1192,7 +1193,8 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index 0567fbdf0b..f9d3d5213f 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -150,21 +150,12 @@ test_expect_success 'hook does not get called on packing refs' '
 	git pack-refs --all &&
 
 	# We only expect a single hook invocation, which is the call to
-	# git-update-ref(1). But currently, packing refs will also trigger the
-	# hook.
+	# git-update-ref(1).
 	cat >expect <<-EOF &&
 		prepared
 		$ZERO_OID $POST_OID refs/heads/unpacked-ref
 		committed
 		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		prepared
-		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		committed
-		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		prepared
-		$POST_OID $ZERO_OID refs/heads/unpacked-ref
-		committed
-		$POST_OID $ZERO_OID refs/heads/unpacked-ref
 	EOF
 
 	test_cmp expect actual
-- 
2.34.1


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[PATCH v3 6/6] refs: skip hooks when deleting uncovered packed refs

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 3947 bytes --]

When deleting refs from the loose-files refs backend, then we need to be
careful to also delete the same ref from the packed refs backend, if it
exists. If we don't, then deleting the loose ref would "uncover" the
packed ref. We thus always have to queue up deletions of refs for both
the loose and the packed refs backend. This is done in two separate
transactions, where the end result is that the reference-transaction
hook is executed twice for the deleted refs.

This behaviour is quite misleading: it's exposing implementation details
of how the files backend works to the user, in contrast to the logical
updates that we'd really want to expose via the hook. Worse yet, whether
the hook gets executed once or twice depends on how well-packed the
repository is: if the ref only exists as a loose ref, then we execute it
once, otherwise if it is also packed then we execute it twice.

Fix this behaviour and don't execute the reference-transaction hook at
all when refs in the packed-refs backend if it's driven by the files
backend. This works as expected even in case the refs to be deleted only
exist in the packed-refs backend because the loose-backend always queues
refs in its own transaction even if they don't exist such that they can
be locked for concurrent creation. And it also does the right thing in
case neither of the backends has the ref because that would cause the
transaction to fail completely.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c             | 9 ++++++---
 t/t1416-ref-transaction-hooks.sh | 7 +------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 3c0f3027fe..9a20cb8fa8 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1261,7 +1261,8 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		goto error;
 
@@ -2775,7 +2776,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store, 0, err);
+						refs->packed_ref_store,
+						REF_TRANSACTION_SKIP_HOOK, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3046,7 +3048,8 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
+							 REF_TRANSACTION_SKIP_HOOK, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index f9d3d5213f..4e1e84a91f 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -175,16 +175,11 @@ test_expect_success 'deleting packed ref calls hook once' '
 	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
 
 	# We only expect a single hook invocation, which is the logical
-	# deletion. But currently, we see two interleaving transactions, once
-	# for deleting the loose refs and once for deleting the packed ref.
+	# deletion.
 	cat >expect <<-EOF &&
-		prepared
-		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
 		prepared
 		$POST_OID $ZERO_OID refs/heads/to-be-deleted
 		committed
-		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
-		committed
 		$POST_OID $ZERO_OID refs/heads/to-be-deleted
 	EOF
 
-- 
2.34.1


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Re: [PATCH v3 1/6] refs: extract packed_refs_delete_refs() to allow control of transaction

[Ævar Arnfjörð Bjarmason]

On Thu, Jan 13 2022, Patrick Steinhardt wrote:

> [[PGP Signed Part:Undecided]]
> When deleting loose refs, then we also have to delete the refs in the
> packed backend. This is done by calling `refs_delete_refs()`, which
> then uses the packed-backend's logic to delete refs. This doesn't allow
> us to exercise any control over the reference transaction which is being
> created in the packed backend, which is required in a subsequent commit.
>
> Extract a new function `packed_refs_delete_refs()`, which hosts most of
> the logic to delete refs except for creating the transaction itself.
> Like this, we can easily create the transaction in the files backend
> and thus exert more control over it.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  refs/files-backend.c  | 12 +++++++++---
>  refs/packed-backend.c | 28 +++++++++++++++++++++-------
>  refs/packed-backend.h |  7 +++++++
>  3 files changed, 37 insertions(+), 10 deletions(-)
>
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 90b671025a..5795e54020 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -1249,6 +1249,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
>  {
>  	struct files_ref_store *refs =
>  		files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
> +	struct ref_transaction *transaction = NULL;

nit: "NULL initialization never needed?
(re: https://lore.kernel.org/git/220113.86bl0gvtq3.gmgdl@evledraar.gmail.com/)

>  	struct strbuf err = STRBUF_INIT;
>  	int i, result = 0;
>  
> @@ -1258,10 +1259,14 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
>  	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
>  		goto error;
>  
> -	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
> -		packed_refs_unlock(refs->packed_ref_store);
> +	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
> +	if (!transaction)
> +		goto error;
> +
> +	result = packed_refs_delete_refs(refs->packed_ref_store,
> +					 transaction, msg, refnames, flags);
> +	if (result)
>  		goto error;
> -	}
>  
>  	packed_refs_unlock(refs->packed_ref_store);
>  
> @@ -1288,6 +1293,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
>  	else
>  		error(_("could not delete references: %s"), err.buf);
>  
> +	ref_transaction_free(transaction);
>  	strbuf_release(&err);
>  	return -1;
>  }
> diff --git a/refs/packed-backend.c b/refs/packed-backend.c
> index 67152c664e..e97d67f932 100644
> --- a/refs/packed-backend.c
> +++ b/refs/packed-backend.c
> @@ -1522,15 +1522,10 @@ static int packed_initial_transaction_commit(struct ref_store *ref_store,
>  static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
>  			     struct string_list *refnames, unsigned int flags)
>  {
> -	struct packed_ref_store *refs =
> -		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
>  	struct strbuf err = STRBUF_INIT;
>  	struct ref_transaction *transaction;
> -	struct string_list_item *item;
>  	int ret;
>  
> -	(void)refs; /* We need the check above, but don't use the variable */
> -
>  	if (!refnames->nr)
>  		return 0;
>  
> @@ -1544,6 +1539,27 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
>  	if (!transaction)
>  		return -1;
>  
> +	ret = packed_refs_delete_refs(ref_store, transaction,
> +				      msg, refnames, flags);
> +
> +	ref_transaction_free(transaction);
> +	return ret;
> +}
> +
> +int packed_refs_delete_refs(struct ref_store *ref_store,
> +			    struct ref_transaction *transaction,
> +			    const char *msg,
> +			    struct string_list *refnames,
> +			    unsigned int flags)
> +{
> +	struct packed_ref_store *refs =
> +		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
> +	struct strbuf err = STRBUF_INIT;
> +	struct string_list_item *item;
> +	int ret;
> +
> +	(void)(refs); /* We need the check above, but don't use the variable */
> +
>  	for_each_string_list_item(item, refnames) {
>  		if (ref_transaction_delete(transaction, item->string, NULL,
>  					   flags, msg, &err)) {

I see you're just moving this code around, but FWIW we can just do this
(also in the pre-image):

	int packed_refs_delete_refs(...)
	{
		[declare variables]
                
                /* Assert ref store sanity */
                packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs")

                [...]
	}

Not sure it's good to change it around just for this mostly-move, just a
note...

> @@ -1554,7 +1570,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
>  	}
>  
>  	ret = ref_transaction_commit(transaction, &err);
> -

Stray whitespace change.

Re: [PATCH v3 5/6] refs: do not execute reference-transaction hook on packing refs

[Ævar Arnfjörð Bjarmason]

On Thu, Jan 13 2022, Patrick Steinhardt wrote:

> [[PGP Signed Part:Undecided]]
> The reference-transaction hook is supposed to track logical changes to
> references, but it currently also gets executed when packing refs in a
> repository. This is unexpected and ultimately not all that useful:
> packing refs is not supposed to result in any user-visible change to the
> refs' state, and it ultimately is an implementation detail of how refs
> stores work.
>
> Fix this excessive execution of the hook when packing refs.
>
> Reported-by: Waleed Khan <me@waleedkhan.name>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  refs/files-backend.c             |  6 ++++--
>  t/t1416-ref-transaction-hooks.sh | 11 +----------
>  2 files changed, 5 insertions(+), 12 deletions(-)
>
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index ecf88cee04..3c0f3027fe 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -1121,7 +1121,8 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
>  	if (check_refname_format(r->name, 0))
>  		return;
>  
> -	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
> +	transaction = ref_store_transaction_begin(&refs->base,
> +						  REF_TRANSACTION_SKIP_HOOK, &err);
>  	if (!transaction)
>  		goto cleanup;
>  	ref_transaction_add_update(
> @@ -1192,7 +1193,8 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
>  	struct strbuf err = STRBUF_INIT;
>  	struct ref_transaction *transaction;
>  
> -	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
> +	transaction = ref_store_transaction_begin(refs->packed_ref_store,
> +						  REF_TRANSACTION_SKIP_HOOK, &err);
>  	if (!transaction)
>  		return -1;
>  
> diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
> index 0567fbdf0b..f9d3d5213f 100755
> --- a/t/t1416-ref-transaction-hooks.sh
> +++ b/t/t1416-ref-transaction-hooks.sh
> @@ -150,21 +150,12 @@ test_expect_success 'hook does not get called on packing refs' '
>  	git pack-refs --all &&
>  
>  	# We only expect a single hook invocation, which is the call to
> -	# git-update-ref(1). But currently, packing refs will also trigger the
> -	# hook.
> +	# git-update-ref(1).
>  	cat >expect <<-EOF &&
>  		prepared
>  		$ZERO_OID $POST_OID refs/heads/unpacked-ref
>  		committed
>  		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> -		prepared
> -		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> -		committed
> -		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> -		prepared
> -		$POST_OID $ZERO_OID refs/heads/unpacked-ref
> -		committed
> -		$POST_OID $ZERO_OID refs/heads/unpacked-ref
>  	EOF
>  
>  	test_cmp expect actual

I wondered how we'd deal with cases where the loose ref != the
corresponding packed ref, but I can't think of ones where it won't be
invisible externally, i.e. we'll correctly update the packed-refs and
delete that loose ref as part of this transaction.

I do wonder if the docs also need updating, currently they say:

    [The hook] executes whenever a reference transaction is prepared,
    committed or aborted[...]

But now we'll explicitly exclude certain classes of
transactions. Perhaps we should expand:

    "The hook does not cover symbolic references (but that may change in
    the future)."

Into some list of types of changes we intentionally exclude, might
include in the future etc.

Re: [PATCH v3 6/6] refs: skip hooks when deleting uncovered packed refs

[Ævar Arnfjörð Bjarmason]

On Thu, Jan 13 2022, Patrick Steinhardt wrote:

> [[PGP Signed Part:Undecided]]
> When deleting refs from the loose-files refs backend, then we need to be
> careful to also delete the same ref from the packed refs backend, if it
> exists. If we don't, then deleting the loose ref would "uncover" the
> packed ref. We thus always have to queue up deletions of refs for both
> the loose and the packed refs backend. This is done in two separate
> transactions, where the end result is that the reference-transaction
> hook is executed twice for the deleted refs.

But do we (which would be an issue before this series) delete the loose
and and then the packed one, thus racily exposing the stale ref to any
concurrent repository reader, or do we first update the packed ref to
the valu of the now-locked loose ref to avoid such a race?

> [...]
> Fix this behaviour and don't execute the reference-transaction hook at
> all when refs in the packed-refs backend if it's driven by the files
> backend. This works as expected even in case the refs to be deleted only
> exist in the packed-refs backend because the loose-backend always queues
> refs in its own transaction even if they don't exist such that they can
> be locked for concurrent creation. And it also does the right thing in
> case neither of the backends has the ref because that would cause the
> transaction to fail completely.

I do wonder if the fundimental approach here is the right
one. I.e. changing the hook to only expose "real" updates, as opposed to
leaving it as a lower-level facility to listed in on any sort of ref
updates.

In such a scenario we could imagine adding a third parameter or
otherwise flag the update as "real" to the hook, so a dumber hook
consumer could ignore the more verbose inter-transactional chatter.

I say that because this change does the right thing for the use-case you
have in mind, but if you e.g. imagine a more gentle background-friendly
"gc" such a thing could be implemented by backing off as soon as it sees
an ongoing transaction being started.

With my ae35e16cd43 (reflog expire: don't lock reflogs using previously
seen OID, 2021-08-23) not getting that more chatty data should be be OK
for such a hypothetical hook.

But we might have more avoidable tripping over locks as the gc and ref
transaction race one another to lock various things in the repository.

Or maybe nobody cares in practice, just food for thought.

Re: [PATCH v3 3/6] refs: allow skipping the reference-transaction hook

[Ævar Arnfjörð Bjarmason]

On Thu, Jan 13 2022, Patrick Steinhardt wrote:

> [[PGP Signed Part:Undecided]]
> The reference-transaction hook is executing whenever we prepare, commit
> or abort a reference transaction. While this is mostly intentional, in
> case of the files backend we're leaking the implementation detail that
> the store is in fact a composite store with one loose and one packed
> backend to the caller. So while we want to execute the hook for all
> logical updates, executing it for such implementation details is
> unexpected.
>
> Prepare for a fix by adding a new flag which allows to skip execution of
> the hook.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  refs.c | 3 +++
>  refs.h | 5 +++++
>  2 files changed, 8 insertions(+)
>
> diff --git a/refs.c b/refs.c
> index 7415864b62..526bf5ed97 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -2084,6 +2084,9 @@ static int run_transaction_hook(struct ref_transaction *transaction,
>  	const char *hook;
>  	int ret = 0, i;
>  
> +	if (transaction->flags & REF_TRANSACTION_SKIP_HOOK)
> +		return 0;
> +
>  	hook = find_hook("reference-transaction");
>  	if (!hook)
>  		return ret;
> diff --git a/refs.h b/refs.h
> index 31f7bf9642..d4056f9fe2 100644
> --- a/refs.h
> +++ b/refs.h
> @@ -568,6 +568,11 @@ enum action_on_err {
>  	UPDATE_REFS_QUIET_ON_ERR
>  };
>  
> +/*
> + * Skip executing the reference-transaction hook.
> + */
> +#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
> +
>  /*
>   * Begin a reference transaction.  The reference transaction must
>   * be freed by calling ref_transaction_free().

This isn't needed in refs.h, so let's put it in refs-internal.h where
e.g. "enum ref_transaction_state" now lives:
    
    diff --git a/refs.h b/refs.h
    index d4056f9fe26..31f7bf96424 100644
    --- a/refs.h
    +++ b/refs.h
    @@ -568,11 +568,6 @@ enum action_on_err {
            UPDATE_REFS_QUIET_ON_ERR
     };
     
    -/*
    - * Skip executing the reference-transaction hook.
    - */
    -#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
    -
     /*
      * Begin a reference transaction.  The reference transaction must
      * be freed by calling ref_transaction_free().
    diff --git a/refs/refs-internal.h b/refs/refs-internal.h
    index a0af63f162f..87da39243f7 100644
    --- a/refs/refs-internal.h
    +++ b/refs/refs-internal.h
    @@ -201,6 +201,11 @@ enum ref_transaction_state {
            REF_TRANSACTION_CLOSED   = 2
     };
     
    +/*
    + * Skip executing the reference-transaction hook.
    + */
    +#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
    +
     /*
      * Data structure for holding a reference transaction, which can
      * consist of checks and updates to multiple references, carried out

A bit more odd is that this series ends up with a
ref_transaction_begin() that doesn't correspond to its ref_store_*()
parent, i.e. the others pass the ref store for you, but now we omit the
flags too.

I see why you did that, to avoid tweaking every existing
ref_transaction_begin() caller.

But isn't something like the below a better approach? We can introduce a
refs-internal.h-only flag enum, and then just have a new
ref_store_transaction_begin_no_hook() called from these new
files-backend.c users.

The diff on top is a bit verbose, but the exposed API is cleaner
(presumably no "public" user should be allowed to skip the hook), and
the overall diff if this is squashed in is smaller.

diff --git a/refs.c b/refs.c
index 526bf5ed97a..ffaf34e9710 100644
--- a/refs.c
+++ b/refs.c
@@ -800,7 +800,7 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 
-	transaction = ref_store_transaction_begin(refs, 0, &err);
+	transaction = ref_store_transaction_begin(refs, &err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname, old_oid,
 				   flags, msg, &err) ||
@@ -1004,22 +1004,36 @@ int read_ref_at(struct ref_store *refs, const char *refname,
 	return 1;
 }
 
-struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
-						    unsigned int flags,
-						    struct strbuf *err)
+static struct ref_transaction *ref_store_transaction_begin_1(struct ref_store *refs,
+							     unsigned int no_hook,
+							     struct strbuf *err)
 {
 	struct ref_transaction *tr;
 	assert(err);
 
 	CALLOC_ARRAY(tr, 1);
 	tr->ref_store = refs;
-	tr->flags = flags;
+	if (no_hook)
+		tr->flags &= REF_TRANSACTION_SKIP_HOOK;
 	return tr;
 }
 
 struct ref_transaction *ref_transaction_begin(struct strbuf *err)
 {
-	return ref_store_transaction_begin(get_main_ref_store(the_repository), 0, err);
+	return ref_store_transaction_begin_1(get_main_ref_store(the_repository),
+					     0, err);
+}
+
+struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    struct strbuf *err)
+{
+	return ref_store_transaction_begin_1(refs, 0, err);
+}
+
+struct ref_transaction *ref_store_transaction_begin_no_hook(struct ref_store *refs,
+							    struct strbuf *err)
+{
+	return ref_store_transaction_begin_1(refs, 1, err);
 }
 
 void ref_transaction_free(struct ref_transaction *transaction)
@@ -1158,7 +1172,7 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 	struct strbuf err = STRBUF_INIT;
 	int ret = 0;
 
-	t = ref_store_transaction_begin(refs, 0, &err);
+	t = ref_store_transaction_begin(refs, &err);
 	if (!t ||
 	    ref_transaction_update(t, refname, new_oid, old_oid, flags, msg,
 				   &err) ||
diff --git a/refs.h b/refs.h
index 31f7bf96424..3663e0347ff 100644
--- a/refs.h
+++ b/refs.h
@@ -573,8 +573,9 @@ enum action_on_err {
  * be freed by calling ref_transaction_free().
  */
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
-						    unsigned int flags,
 						    struct strbuf *err);
+struct ref_transaction *ref_store_transaction_begin_no_hook(struct ref_store *refs,
+							    struct strbuf *err);
 struct ref_transaction *ref_transaction_begin(struct strbuf *err);
 
 /*
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 9a20cb8fa89..f85c8f3a692 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1121,8 +1121,7 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base,
-						  REF_TRANSACTION_SKIP_HOOK, &err);
+	transaction = ref_store_transaction_begin_no_hook(&refs->base, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1193,8 +1192,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store,
-						  REF_TRANSACTION_SKIP_HOOK, &err);
+	transaction = ref_store_transaction_begin_no_hook(refs->packed_ref_store, &err);
 	if (!transaction)
 		return -1;
 
@@ -1261,8 +1259,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store,
-						  REF_TRANSACTION_SKIP_HOOK, &err);
+	transaction = ref_store_transaction_begin_no_hook(refs->packed_ref_store, &err);
 	if (!transaction)
 		goto error;
 
@@ -2775,9 +2772,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 * packed-refs if it exists there.
 			 */
 			if (!packed_transaction) {
-				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store,
-						REF_TRANSACTION_SKIP_HOOK, err);
+				packed_transaction = ref_store_transaction_begin_no_hook(
+						refs->packed_ref_store, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3048,8 +3044,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
-							 REF_TRANSACTION_SKIP_HOOK, err);
+	packed_transaction = ref_store_transaction_begin_no_hook(refs->packed_ref_store, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index cfebcd0b46e..e97d67f9321 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1535,7 +1535,7 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	 * updates into a single transaction.
 	 */
 
-	transaction = ref_store_transaction_begin(ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(ref_store, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 87da39243f7..8ff9e0e6e3a 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -204,7 +204,9 @@ enum ref_transaction_state {
 /*
  * Skip executing the reference-transaction hook.
  */
-#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
+enum ref_transaction_flags {
+	REF_TRANSACTION_SKIP_HOOK = 1 << 0,
+};
 
 /*
  * Data structure for holding a reference transaction, which can
@@ -218,7 +220,7 @@ struct ref_transaction {
 	size_t nr;
 	enum ref_transaction_state state;
 	void *backend_data;
-	unsigned int flags;
+	enum ref_transaction_flags flags;
 };
 
 /*
diff --git a/sequencer.c b/sequencer.c
index 61edd39d7a4..6abd72160cc 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3588,7 +3588,7 @@ static int do_label(struct repository *r, const char *name, int len)
 	strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
 	strbuf_addf(&msg, "rebase (label) '%.*s'", len, name);
 
-	transaction = ref_store_transaction_begin(refs, 0, &err);
+	transaction = ref_store_transaction_begin(refs, &err);
 	if (!transaction) {
 		error("%s", err.buf);
 		ret = -1;

Re: [PATCH v2 0/6] refs: excessive hook execution with packed refs

[Han-Wen Nienhuys]

On Fri, Jan 7, 2022 at 11:17 PM Junio C Hamano <gitster@pobox.com> wrote:
> > this is a resend of version 1 of this patch series to hopefully entice
> > some reviews. The only change is that v2 is rebased onto the current
> > main branch at commit e83ba647f7 (The seventh batch, 2022-01-05). The
> > following was from the orignial cover letter:
>
> I'll add Ævar, who has been making a lot of changes to the refs
> subsystem, and Han-Wen, whose work to add a new ref backend may need
> to interact with this change, as possible stake-holders to the CC list.

Thanks for the consideration, Jun. As the hook is called from refs.c,
it should not make a difference for reftable.

I looked over the patches. I didn't look at the bottom change to
packed/loose refs as I'm not an expert.

The individual transaction updates already support their own flags, so
this change generates some confusion. Consider:

int refs_delete_ref(struct ref_store *refs, const char *msg,
    const char *refname,
    const struct object_id *old_oid,
    unsigned int flags)

how would one delete a ref skipping the transaction hook? It will be
easy for someone to pass the SKIP_TRANSACTION_HOOK to
refs_delete_ref(), with surprising results.

It might make sense to not introduce a new flag namespace, but use
update->flags instead. You'd have to add your new flag after
REF_SKIP_REFNAME_VERIFICATION.
Bonus is that you can unittest the new flag using the existing
ref-store helper without extra work. (check that a transaction with &
without the flag works as expected.)

other than that, looks OK to me.

-- 
Han-Wen Nienhuys - Google Munich
I work 80%. Don't expect answers from me on Fridays.
--
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Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
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Re: [PATCH v2 0/6] refs: excessive hook execution with packed refs

[Patrick Steinhardt]

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On Thu, Jan 13, 2022 at 07:24:19PM +0100, Han-Wen Nienhuys wrote:
> On Fri, Jan 7, 2022 at 11:17 PM Junio C Hamano <gitster@pobox.com> wrote:
> > > this is a resend of version 1 of this patch series to hopefully entice
> > > some reviews. The only change is that v2 is rebased onto the current
> > > main branch at commit e83ba647f7 (The seventh batch, 2022-01-05). The
> > > following was from the orignial cover letter:
> >
> > I'll add Ævar, who has been making a lot of changes to the refs
> > subsystem, and Han-Wen, whose work to add a new ref backend may need
> > to interact with this change, as possible stake-holders to the CC list.
> 
> Thanks for the consideration, Jun. As the hook is called from refs.c,
> it should not make a difference for reftable.
> 
> I looked over the patches. I didn't look at the bottom change to
> packed/loose refs as I'm not an expert.
> 
> The individual transaction updates already support their own flags, so
> this change generates some confusion. Consider:
> 
> int refs_delete_ref(struct ref_store *refs, const char *msg,
>     const char *refname,
>     const struct object_id *old_oid,
>     unsigned int flags)
> 
> how would one delete a ref skipping the transaction hook? It will be
> easy for someone to pass the SKIP_TRANSACTION_HOOK to
> refs_delete_ref(), with surprising results.
> 
> It might make sense to not introduce a new flag namespace, but use
> update->flags instead. You'd have to add your new flag after
> REF_SKIP_REFNAME_VERIFICATION.
> Bonus is that you can unittest the new flag using the existing
> ref-store helper without extra work. (check that a transaction with &
> without the flag works as expected.)
> 
> other than that, looks OK to me.

Thanks for your feedback!

In fact the first version I had locally did exactly that. But I found
the end version result harder to reason about, most importantly because
it's not a 100% clear to the reader whether all callsites which delete
refs in the packed-backend via the files-backend are adapted or whether
any of the callsites was missing. By having the flag in a central place
it's immediately clear that the hook won't be run at all, which is
exactly what we want here.

Patrick

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Re: [PATCH v3 1/6] refs: extract packed_refs_delete_refs() to allow control of transaction

[Patrick Steinhardt]

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On Thu, Jan 13, 2022 at 01:43:18PM +0100, Ævar Arnfjörð Bjarmason wrote:
> 
> On Thu, Jan 13 2022, Patrick Steinhardt wrote:
> 
> > [[PGP Signed Part:Undecided]]
> > When deleting loose refs, then we also have to delete the refs in the
> > packed backend. This is done by calling `refs_delete_refs()`, which
> > then uses the packed-backend's logic to delete refs. This doesn't allow
> > us to exercise any control over the reference transaction which is being
> > created in the packed backend, which is required in a subsequent commit.
> >
> > Extract a new function `packed_refs_delete_refs()`, which hosts most of
> > the logic to delete refs except for creating the transaction itself.
> > Like this, we can easily create the transaction in the files backend
> > and thus exert more control over it.
> >
> > Signed-off-by: Patrick Steinhardt <ps@pks.im>
> > ---
> >  refs/files-backend.c  | 12 +++++++++---
> >  refs/packed-backend.c | 28 +++++++++++++++++++++-------
> >  refs/packed-backend.h |  7 +++++++
> >  3 files changed, 37 insertions(+), 10 deletions(-)
> >
> > diff --git a/refs/files-backend.c b/refs/files-backend.c
> > index 90b671025a..5795e54020 100644
> > --- a/refs/files-backend.c
> > +++ b/refs/files-backend.c
> > @@ -1249,6 +1249,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
> >  {
> >  	struct files_ref_store *refs =
> >  		files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
> > +	struct ref_transaction *transaction = NULL;
> 
> nit: "NULL initialization never needed?
> (re: https://lore.kernel.org/git/220113.86bl0gvtq3.gmgdl@evledraar.gmail.com/)

It is needed: if locking the packed-refs file fails we `goto error;` and
try to free the transaction without it having been initialized yet.

> >  	struct strbuf err = STRBUF_INIT;
> >  	int i, result = 0;
> >  
> > @@ -1258,10 +1259,14 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
> >  	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
> >  		goto error;
> >  
> > -	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
> > -		packed_refs_unlock(refs->packed_ref_store);
> > +	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
> > +	if (!transaction)
> > +		goto error;
> > +
> > +	result = packed_refs_delete_refs(refs->packed_ref_store,
> > +					 transaction, msg, refnames, flags);
> > +	if (result)
> >  		goto error;
> > -	}
> >  
> >  	packed_refs_unlock(refs->packed_ref_store);
> >  
> > @@ -1288,6 +1293,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
> >  	else
> >  		error(_("could not delete references: %s"), err.buf);
> >  
> > +	ref_transaction_free(transaction);
> >  	strbuf_release(&err);
> >  	return -1;
> >  }
> > diff --git a/refs/packed-backend.c b/refs/packed-backend.c
> > index 67152c664e..e97d67f932 100644
> > --- a/refs/packed-backend.c
> > +++ b/refs/packed-backend.c
> > @@ -1522,15 +1522,10 @@ static int packed_initial_transaction_commit(struct ref_store *ref_store,
> >  static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
> >  			     struct string_list *refnames, unsigned int flags)
> >  {
> > -	struct packed_ref_store *refs =
> > -		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
> >  	struct strbuf err = STRBUF_INIT;
> >  	struct ref_transaction *transaction;
> > -	struct string_list_item *item;
> >  	int ret;
> >  
> > -	(void)refs; /* We need the check above, but don't use the variable */
> > -
> >  	if (!refnames->nr)
> >  		return 0;
> >  
> > @@ -1544,6 +1539,27 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
> >  	if (!transaction)
> >  		return -1;
> >  
> > +	ret = packed_refs_delete_refs(ref_store, transaction,
> > +				      msg, refnames, flags);
> > +
> > +	ref_transaction_free(transaction);
> > +	return ret;
> > +}
> > +
> > +int packed_refs_delete_refs(struct ref_store *ref_store,
> > +			    struct ref_transaction *transaction,
> > +			    const char *msg,
> > +			    struct string_list *refnames,
> > +			    unsigned int flags)
> > +{
> > +	struct packed_ref_store *refs =
> > +		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
> > +	struct strbuf err = STRBUF_INIT;
> > +	struct string_list_item *item;
> > +	int ret;
> > +
> > +	(void)(refs); /* We need the check above, but don't use the variable */
> > +
> >  	for_each_string_list_item(item, refnames) {
> >  		if (ref_transaction_delete(transaction, item->string, NULL,
> >  					   flags, msg, &err)) {
> 
> I see you're just moving this code around, but FWIW we can just do this
> (also in the pre-image):
> 
> 	int packed_refs_delete_refs(...)
> 	{
> 		[declare variables]
>                 
>                 /* Assert ref store sanity */
>                 packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs")
> 
>                 [...]
> 	}
> 
> Not sure it's good to change it around just for this mostly-move, just a
> note...

I think this change is trivial enough to make while at it.

Patrick

> > @@ -1554,7 +1570,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
> >  	}
> >  
> >  	ret = ref_transaction_commit(transaction, &err);
> > -
> 
> Stray whitespace change.

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Re: [PATCH v3 5/6] refs: do not execute reference-transaction hook on packing refs

[Patrick Steinhardt]

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On Thu, Jan 13, 2022 at 02:00:10PM +0100, Ævar Arnfjörð Bjarmason wrote:
> 
> On Thu, Jan 13 2022, Patrick Steinhardt wrote:
> 
> > [[PGP Signed Part:Undecided]]
> > The reference-transaction hook is supposed to track logical changes to
> > references, but it currently also gets executed when packing refs in a
> > repository. This is unexpected and ultimately not all that useful:
> > packing refs is not supposed to result in any user-visible change to the
> > refs' state, and it ultimately is an implementation detail of how refs
> > stores work.
> >
> > Fix this excessive execution of the hook when packing refs.
> >
> > Reported-by: Waleed Khan <me@waleedkhan.name>
> > Signed-off-by: Patrick Steinhardt <ps@pks.im>
> > ---
> >  refs/files-backend.c             |  6 ++++--
> >  t/t1416-ref-transaction-hooks.sh | 11 +----------
> >  2 files changed, 5 insertions(+), 12 deletions(-)
> >
> > diff --git a/refs/files-backend.c b/refs/files-backend.c
> > index ecf88cee04..3c0f3027fe 100644
> > --- a/refs/files-backend.c
> > +++ b/refs/files-backend.c
> > @@ -1121,7 +1121,8 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
> >  	if (check_refname_format(r->name, 0))
> >  		return;
> >  
> > -	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
> > +	transaction = ref_store_transaction_begin(&refs->base,
> > +						  REF_TRANSACTION_SKIP_HOOK, &err);
> >  	if (!transaction)
> >  		goto cleanup;
> >  	ref_transaction_add_update(
> > @@ -1192,7 +1193,8 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
> >  	struct strbuf err = STRBUF_INIT;
> >  	struct ref_transaction *transaction;
> >  
> > -	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
> > +	transaction = ref_store_transaction_begin(refs->packed_ref_store,
> > +						  REF_TRANSACTION_SKIP_HOOK, &err);
> >  	if (!transaction)
> >  		return -1;
> >  
> > diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
> > index 0567fbdf0b..f9d3d5213f 100755
> > --- a/t/t1416-ref-transaction-hooks.sh
> > +++ b/t/t1416-ref-transaction-hooks.sh
> > @@ -150,21 +150,12 @@ test_expect_success 'hook does not get called on packing refs' '
> >  	git pack-refs --all &&
> >  
> >  	# We only expect a single hook invocation, which is the call to
> > -	# git-update-ref(1). But currently, packing refs will also trigger the
> > -	# hook.
> > +	# git-update-ref(1).
> >  	cat >expect <<-EOF &&
> >  		prepared
> >  		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> >  		committed
> >  		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> > -		prepared
> > -		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> > -		committed
> > -		$ZERO_OID $POST_OID refs/heads/unpacked-ref
> > -		prepared
> > -		$POST_OID $ZERO_OID refs/heads/unpacked-ref
> > -		committed
> > -		$POST_OID $ZERO_OID refs/heads/unpacked-ref
> >  	EOF
> >  
> >  	test_cmp expect actual
> 
> I wondered how we'd deal with cases where the loose ref != the
> corresponding packed ref, but I can't think of ones where it won't be
> invisible externally, i.e. we'll correctly update the packed-refs and
> delete that loose ref as part of this transaction.

With the previous code we'd see two hook executions with different old
OIDs. Given that we only care about logical updates though the user'd
only want to see the one which deletes the user-visible OID, which is
what's stored in the loose ref. And with the fixes in this series that's
the hook invocation we retain.

> I do wonder if the docs also need updating, currently they say:
> 
>     [The hook] executes whenever a reference transaction is prepared,
>     committed or aborted[...]
> 
> But now we'll explicitly exclude certain classes of
> transactions. Perhaps we should expand:
> 
>     "The hook does not cover symbolic references (but that may change in
>     the future)."
> 
> Into some list of types of changes we intentionally exclude, might
> include in the future etc.

Well, from the user's perspective we do execute the hook whenever we
drive a reference transaction: all modifications to the files backend
are still visible to the hook after the changes in this series. The
issue is that with the files backend being a combination of two
backends, we essentially saw a subset of refs executing the hook twice,
which really is an implementation detail.

Patrick

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Re: [PATCH v3 6/6] refs: skip hooks when deleting uncovered packed refs

[Patrick Steinhardt]

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On Thu, Jan 13, 2022 at 02:04:38PM +0100, Ævar Arnfjörð Bjarmason wrote:
> 
> On Thu, Jan 13 2022, Patrick Steinhardt wrote:
> 
> > [[PGP Signed Part:Undecided]]
> > When deleting refs from the loose-files refs backend, then we need to be
> > careful to also delete the same ref from the packed refs backend, if it
> > exists. If we don't, then deleting the loose ref would "uncover" the
> > packed ref. We thus always have to queue up deletions of refs for both
> > the loose and the packed refs backend. This is done in two separate
> > transactions, where the end result is that the reference-transaction
> > hook is executed twice for the deleted refs.
> 
> But do we (which would be an issue before this series) delete the loose
> and and then the packed one, thus racily exposing the stale ref to any
> concurrent repository reader, or do we first update the packed ref to
> the valu of the now-locked loose ref to avoid such a race?

We first commit the packed-refs file so that the stale ref is not
exposed.

> > [...]
> > Fix this behaviour and don't execute the reference-transaction hook at
> > all when refs in the packed-refs backend if it's driven by the files
> > backend. This works as expected even in case the refs to be deleted only
> > exist in the packed-refs backend because the loose-backend always queues
> > refs in its own transaction even if they don't exist such that they can
> > be locked for concurrent creation. And it also does the right thing in
> > case neither of the backends has the ref because that would cause the
> > transaction to fail completely.
> 
> I do wonder if the fundimental approach here is the right
> one. I.e. changing the hook to only expose "real" updates, as opposed to
> leaving it as a lower-level facility to listed in on any sort of ref
> updates.
> 
> In such a scenario we could imagine adding a third parameter or
> otherwise flag the update as "real" to the hook, so a dumber hook
> consumer could ignore the more verbose inter-transactional chatter.
> 
> I say that because this change does the right thing for the use-case you
> have in mind, but if you e.g. imagine a more gentle background-friendly
> "gc" such a thing could be implemented by backing off as soon as it sees
> an ongoing transaction being started.

I've mostly been acting on the original report by Waleed. And I tend to
agree with his report given that we also got a workaround at GitLab
which filters out reference transactions which only consist of force
deletions because they're likely to be pruning refs in the packed
backend which are about to be uncovered. The result is that execution of
the reftx hook is dependent on how well-packed a repository's refs are:
when refs are packed we execute the hook twice, whereas we execute it
once when it's not well-packed. This is surprising behaviour, even
though one can definitely argue that it's just working as intended.

I think ultimately the question boils down to whether we want to treat
the files backend as a single compound backend and whether the reftx
hook should treat it like that. If we treat it as a single backend, then
we shouldn't report a change in refs when pruning about-to-be-uncovered
refs given that it wouldn't have been visible, but it's only internal
cleanup. And neither should we report ref changes when repacking refs
into a single file given that from the backend's perspective nothing is
about to change.

Patrick

> With my ae35e16cd43 (reflog expire: don't lock reflogs using previously
> seen OID, 2021-08-23) not getting that more chatty data should be be OK
> for such a hypothetical hook.
> 
> But we might have more avoidable tripping over locks as the gc and ref
> transaction race one another to lock various things in the repository.
> 
> Or maybe nobody cares in practice, just food for thought.

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Re: [PATCH v3 3/6] refs: allow skipping the reference-transaction hook

[Patrick Steinhardt]

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On Thu, Jan 13, 2022 at 02:34:41PM +0100, Ævar Arnfjörð Bjarmason wrote:
> 
> On Thu, Jan 13 2022, Patrick Steinhardt wrote:
> 
> > [[PGP Signed Part:Undecided]]
> > The reference-transaction hook is executing whenever we prepare, commit
> > or abort a reference transaction. While this is mostly intentional, in
> > case of the files backend we're leaking the implementation detail that
> > the store is in fact a composite store with one loose and one packed
> > backend to the caller. So while we want to execute the hook for all
> > logical updates, executing it for such implementation details is
> > unexpected.
> >
> > Prepare for a fix by adding a new flag which allows to skip execution of
> > the hook.
> >
> > Signed-off-by: Patrick Steinhardt <ps@pks.im>
> > ---
> >  refs.c | 3 +++
> >  refs.h | 5 +++++
> >  2 files changed, 8 insertions(+)
> >
> > diff --git a/refs.c b/refs.c
> > index 7415864b62..526bf5ed97 100644
> > --- a/refs.c
> > +++ b/refs.c
> > @@ -2084,6 +2084,9 @@ static int run_transaction_hook(struct ref_transaction *transaction,
> >  	const char *hook;
> >  	int ret = 0, i;
> >  
> > +	if (transaction->flags & REF_TRANSACTION_SKIP_HOOK)
> > +		return 0;
> > +
> >  	hook = find_hook("reference-transaction");
> >  	if (!hook)
> >  		return ret;
> > diff --git a/refs.h b/refs.h
> > index 31f7bf9642..d4056f9fe2 100644
> > --- a/refs.h
> > +++ b/refs.h
> > @@ -568,6 +568,11 @@ enum action_on_err {
> >  	UPDATE_REFS_QUIET_ON_ERR
> >  };
> >  
> > +/*
> > + * Skip executing the reference-transaction hook.
> > + */
> > +#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
> > +
> >  /*
> >   * Begin a reference transaction.  The reference transaction must
> >   * be freed by calling ref_transaction_free().
> 
> This isn't needed in refs.h, so let's put it in refs-internal.h where
> e.g. "enum ref_transaction_state" now lives:

On the other hand we got the flags parameter in the function just two
lines later. A reader would thus wonder "which flags?!" if we didn't
carry the definition of flags nearby and would have to go search the
codebase for the set of supported flags.

>     diff --git a/refs.h b/refs.h
>     index d4056f9fe26..31f7bf96424 100644
>     --- a/refs.h
>     +++ b/refs.h
>     @@ -568,11 +568,6 @@ enum action_on_err {
>             UPDATE_REFS_QUIET_ON_ERR
>      };
>      
>     -/*
>     - * Skip executing the reference-transaction hook.
>     - */
>     -#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
>     -
>      /*
>       * Begin a reference transaction.  The reference transaction must
>       * be freed by calling ref_transaction_free().
>     diff --git a/refs/refs-internal.h b/refs/refs-internal.h
>     index a0af63f162f..87da39243f7 100644
>     --- a/refs/refs-internal.h
>     +++ b/refs/refs-internal.h
>     @@ -201,6 +201,11 @@ enum ref_transaction_state {
>             REF_TRANSACTION_CLOSED   = 2
>      };
>      
>     +/*
>     + * Skip executing the reference-transaction hook.
>     + */
>     +#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
>     +
>      /*
>       * Data structure for holding a reference transaction, which can
>       * consist of checks and updates to multiple references, carried out
> 
> A bit more odd is that this series ends up with a
> ref_transaction_begin() that doesn't correspond to its ref_store_*()
> parent, i.e. the others pass the ref store for you, but now we omit the
> flags too.
> 
> I see why you did that, to avoid tweaking every existing
> ref_transaction_begin() caller.
> 
> But isn't something like the below a better approach? We can introduce a
> refs-internal.h-only flag enum, and then just have a new
> ref_store_transaction_begin_no_hook() called from these new
> files-backend.c users.
> 
> The diff on top is a bit verbose, but the exposed API is cleaner
> (presumably no "public" user should be allowed to skip the hook), and
> the overall diff if this is squashed in is smaller.

While the diff does look sensible it makes me wonder how maintainable it
is in the future. If we ever were to accept another flag then we might
have to duplicate your `ref_store_transaction_begin_no_hook()` to also
carry flags, or add `ref_store_transaciton_begin_another_option()` to
stay consistent.

So for now I tend to prefer the current version given that it feels more
extensible going forward. But I'm happy to change it if others agree
with you.

Patrick

> diff --git a/refs.c b/refs.c
> index 526bf5ed97a..ffaf34e9710 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -800,7 +800,7 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
>  	struct ref_transaction *transaction;
>  	struct strbuf err = STRBUF_INIT;
>  
> -	transaction = ref_store_transaction_begin(refs, 0, &err);
> +	transaction = ref_store_transaction_begin(refs, &err);
>  	if (!transaction ||
>  	    ref_transaction_delete(transaction, refname, old_oid,
>  				   flags, msg, &err) ||
> @@ -1004,22 +1004,36 @@ int read_ref_at(struct ref_store *refs, const char *refname,
>  	return 1;
>  }
>  
> -struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
> -						    unsigned int flags,
> -						    struct strbuf *err)
> +static struct ref_transaction *ref_store_transaction_begin_1(struct ref_store *refs,
> +							     unsigned int no_hook,
> +							     struct strbuf *err)
>  {
>  	struct ref_transaction *tr;
>  	assert(err);
>  
>  	CALLOC_ARRAY(tr, 1);
>  	tr->ref_store = refs;
> -	tr->flags = flags;
> +	if (no_hook)
> +		tr->flags &= REF_TRANSACTION_SKIP_HOOK;
>  	return tr;
>  }
>  
>  struct ref_transaction *ref_transaction_begin(struct strbuf *err)
>  {
> -	return ref_store_transaction_begin(get_main_ref_store(the_repository), 0, err);
> +	return ref_store_transaction_begin_1(get_main_ref_store(the_repository),
> +					     0, err);
> +}
> +
> +struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
> +						    struct strbuf *err)
> +{
> +	return ref_store_transaction_begin_1(refs, 0, err);
> +}
> +
> +struct ref_transaction *ref_store_transaction_begin_no_hook(struct ref_store *refs,
> +							    struct strbuf *err)
> +{
> +	return ref_store_transaction_begin_1(refs, 1, err);
>  }
>  
>  void ref_transaction_free(struct ref_transaction *transaction)
> @@ -1158,7 +1172,7 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
>  	struct strbuf err = STRBUF_INIT;
>  	int ret = 0;
>  
> -	t = ref_store_transaction_begin(refs, 0, &err);
> +	t = ref_store_transaction_begin(refs, &err);
>  	if (!t ||
>  	    ref_transaction_update(t, refname, new_oid, old_oid, flags, msg,
>  				   &err) ||
> diff --git a/refs.h b/refs.h
> index 31f7bf96424..3663e0347ff 100644
> --- a/refs.h
> +++ b/refs.h
> @@ -573,8 +573,9 @@ enum action_on_err {
>   * be freed by calling ref_transaction_free().
>   */
>  struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
> -						    unsigned int flags,
>  						    struct strbuf *err);
> +struct ref_transaction *ref_store_transaction_begin_no_hook(struct ref_store *refs,
> +							    struct strbuf *err);
>  struct ref_transaction *ref_transaction_begin(struct strbuf *err);
>  
>  /*
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 9a20cb8fa89..f85c8f3a692 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -1121,8 +1121,7 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
>  	if (check_refname_format(r->name, 0))
>  		return;
>  
> -	transaction = ref_store_transaction_begin(&refs->base,
> -						  REF_TRANSACTION_SKIP_HOOK, &err);
> +	transaction = ref_store_transaction_begin_no_hook(&refs->base, &err);
>  	if (!transaction)
>  		goto cleanup;
>  	ref_transaction_add_update(
> @@ -1193,8 +1192,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
>  	struct strbuf err = STRBUF_INIT;
>  	struct ref_transaction *transaction;
>  
> -	transaction = ref_store_transaction_begin(refs->packed_ref_store,
> -						  REF_TRANSACTION_SKIP_HOOK, &err);
> +	transaction = ref_store_transaction_begin_no_hook(refs->packed_ref_store, &err);
>  	if (!transaction)
>  		return -1;
>  
> @@ -1261,8 +1259,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
>  	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
>  		goto error;
>  
> -	transaction = ref_store_transaction_begin(refs->packed_ref_store,
> -						  REF_TRANSACTION_SKIP_HOOK, &err);
> +	transaction = ref_store_transaction_begin_no_hook(refs->packed_ref_store, &err);
>  	if (!transaction)
>  		goto error;
>  
> @@ -2775,9 +2772,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
>  			 * packed-refs if it exists there.
>  			 */
>  			if (!packed_transaction) {
> -				packed_transaction = ref_store_transaction_begin(
> -						refs->packed_ref_store,
> -						REF_TRANSACTION_SKIP_HOOK, err);
> +				packed_transaction = ref_store_transaction_begin_no_hook(
> +						refs->packed_ref_store, err);
>  				if (!packed_transaction) {
>  					ret = TRANSACTION_GENERIC_ERROR;
>  					goto cleanup;
> @@ -3048,8 +3044,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
>  				 &affected_refnames))
>  		BUG("initial ref transaction called with existing refs");
>  
> -	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
> -							 REF_TRANSACTION_SKIP_HOOK, err);
> +	packed_transaction = ref_store_transaction_begin_no_hook(refs->packed_ref_store, err);
>  	if (!packed_transaction) {
>  		ret = TRANSACTION_GENERIC_ERROR;
>  		goto cleanup;
> diff --git a/refs/packed-backend.c b/refs/packed-backend.c
> index cfebcd0b46e..e97d67f9321 100644
> --- a/refs/packed-backend.c
> +++ b/refs/packed-backend.c
> @@ -1535,7 +1535,7 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
>  	 * updates into a single transaction.
>  	 */
>  
> -	transaction = ref_store_transaction_begin(ref_store, 0, &err);
> +	transaction = ref_store_transaction_begin(ref_store, &err);
>  	if (!transaction)
>  		return -1;
>  
> diff --git a/refs/refs-internal.h b/refs/refs-internal.h
> index 87da39243f7..8ff9e0e6e3a 100644
> --- a/refs/refs-internal.h
> +++ b/refs/refs-internal.h
> @@ -204,7 +204,9 @@ enum ref_transaction_state {
>  /*
>   * Skip executing the reference-transaction hook.
>   */
> -#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
> +enum ref_transaction_flags {
> +	REF_TRANSACTION_SKIP_HOOK = 1 << 0,
> +};
>  
>  /*
>   * Data structure for holding a reference transaction, which can
> @@ -218,7 +220,7 @@ struct ref_transaction {
>  	size_t nr;
>  	enum ref_transaction_state state;
>  	void *backend_data;
> -	unsigned int flags;
> +	enum ref_transaction_flags flags;
>  };
>  
>  /*
> diff --git a/sequencer.c b/sequencer.c
> index 61edd39d7a4..6abd72160cc 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -3588,7 +3588,7 @@ static int do_label(struct repository *r, const char *name, int len)
>  	strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
>  	strbuf_addf(&msg, "rebase (label) '%.*s'", len, name);
>  
> -	transaction = ref_store_transaction_begin(refs, 0, &err);
> +	transaction = ref_store_transaction_begin(refs, &err);
>  	if (!transaction) {
>  		error("%s", err.buf);
>  		ret = -1;

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[PATCH v4 0/6] refs: excessive hook execution with packed refs

[Patrick Steinhardt]

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Hi,

this is the fourth version of this patch series, which addresses an
issue where the reference-transaction hook is being invoked twice when
deleting refs both in the packed-refs and loose-refs file.

The following things changed in comparison to v3:

    - Fixed a memory leak in `files_delete_refs()`.

    - Refactored the `packed_downcast()` invocation such that we don't
      have to mark its unused returned variable as used.

    - Removed a spurious whitespace change.

Patrick

Patrick Steinhardt (6):
  refs: extract packed_refs_delete_refs() to allow control of
    transaction
  refs: allow passing flags when beginning transactions
  refs: allow skipping the reference-transaction hook
  refs: demonstrate excessive execution of the reference-transaction
    hook
  refs: do not execute reference-transaction hook on packing refs
  refs: skip hooks when deleting uncovered packed refs

 refs.c                           | 11 +++++--
 refs.h                           |  8 ++++-
 refs/files-backend.c             | 26 ++++++++++++-----
 refs/packed-backend.c            | 28 +++++++++++++-----
 refs/packed-backend.h            |  7 +++++
 refs/refs-internal.h             |  1 +
 sequencer.c                      |  2 +-
 t/t1416-ref-transaction-hooks.sh | 50 ++++++++++++++++++++++++++++++++
 8 files changed, 114 insertions(+), 19 deletions(-)

Range-diff against v3:
1:  abbc28822b ! 1:  14775046e1 refs: extract packed_refs_delete_refs() to allow control of transaction
    @@ refs/files-backend.c: static int files_delete_refs(struct ref_store *ref_store,
      
      	packed_refs_unlock(refs->packed_ref_store);
      
    +@@ refs/files-backend.c: static int files_delete_refs(struct ref_store *ref_store, const char *msg,
    + 			result |= error(_("could not remove reference %s"), refname);
    + 	}
    + 
    ++	ref_transaction_free(transaction);
    + 	strbuf_release(&err);
    + 	return result;
    + 
     @@ refs/files-backend.c: static int files_delete_refs(struct ref_store *ref_store, const char *msg,
      	else
      		error(_("could not delete references: %s"), err.buf);
    @@ refs/packed-backend.c: static int packed_delete_refs(struct ref_store *ref_store
     +			    struct string_list *refnames,
     +			    unsigned int flags)
     +{
    -+	struct packed_ref_store *refs =
    -+		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
     +	struct strbuf err = STRBUF_INIT;
     +	struct string_list_item *item;
     +	int ret;
     +
    -+	(void)(refs); /* We need the check above, but don't use the variable */
    ++	/* Assert that the ref store refers to a packed backend. */
    ++	packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
     +
      	for_each_string_list_item(item, refnames) {
      		if (ref_transaction_delete(transaction, item->string, NULL,
      					   flags, msg, &err)) {
    -@@ refs/packed-backend.c: static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
    - 	}
    - 
    - 	ret = ref_transaction_commit(transaction, &err);
    --
    - 	if (ret) {
    - 		if (refnames->nr == 1)
    - 			error(_("could not delete reference %s: %s"),
     @@ refs/packed-backend.c: static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
      			error(_("could not delete references: %s"), err.buf);
      	}
2:  9dd172a757 = 2:  d4ac24c8b8 refs: allow passing flags when beginning transactions
3:  be826bae3b = 3:  f4a07fe9a8 refs: allow skipping the reference-transaction hook
4:  662a6e6244 = 4:  a8981baef7 refs: demonstrate excessive execution of the reference-transaction hook
5:  d83f309b9c = 5:  23c344854e refs: do not execute reference-transaction hook on packing refs
6:  279eadc41c = 6:  d6c7d765af refs: skip hooks when deleting uncovered packed refs
-- 
2.34.1


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[PATCH v4 1/6] refs: extract packed_refs_delete_refs() to allow control of transaction

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 5158 bytes --]

When deleting loose refs, then we also have to delete the refs in the
packed backend. This is done by calling `refs_delete_refs()`, which
then uses the packed-backend's logic to delete refs. This doesn't allow
us to exercise any control over the reference transaction which is being
created in the packed backend, which is required in a subsequent commit.

Extract a new function `packed_refs_delete_refs()`, which hosts most of
the logic to delete refs except for creating the transaction itself.
Like this, we can easily create the transaction in the files backend
and thus exert more control over it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c  | 13 ++++++++++---
 refs/packed-backend.c | 26 ++++++++++++++++++++------
 refs/packed-backend.h |  7 +++++++
 3 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 90b671025a..459f18dbc1 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1249,6 +1249,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct ref_transaction *transaction = NULL;
 	struct strbuf err = STRBUF_INIT;
 	int i, result = 0;
 
@@ -1258,10 +1259,14 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
-		packed_refs_unlock(refs->packed_ref_store);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	if (!transaction)
+		goto error;
+
+	result = packed_refs_delete_refs(refs->packed_ref_store,
+					 transaction, msg, refnames, flags);
+	if (result)
 		goto error;
-	}
 
 	packed_refs_unlock(refs->packed_ref_store);
 
@@ -1272,6 +1277,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 			result |= error(_("could not remove reference %s"), refname);
 	}
 
+	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return result;
 
@@ -1288,6 +1294,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	else
 		error(_("could not delete references: %s"), err.buf);
 
+	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return -1;
 }
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 67152c664e..c964dd1617 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1522,15 +1522,10 @@ static int packed_initial_transaction_commit(struct ref_store *ref_store,
 static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			     struct string_list *refnames, unsigned int flags)
 {
-	struct packed_ref_store *refs =
-		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
-	struct string_list_item *item;
 	int ret;
 
-	(void)refs; /* We need the check above, but don't use the variable */
-
 	if (!refnames->nr)
 		return 0;
 
@@ -1544,6 +1539,26 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (!transaction)
 		return -1;
 
+	ret = packed_refs_delete_refs(ref_store, transaction,
+				      msg, refnames, flags);
+
+	ref_transaction_free(transaction);
+	return ret;
+}
+
+int packed_refs_delete_refs(struct ref_store *ref_store,
+			    struct ref_transaction *transaction,
+			    const char *msg,
+			    struct string_list *refnames,
+			    unsigned int flags)
+{
+	struct strbuf err = STRBUF_INIT;
+	struct string_list_item *item;
+	int ret;
+
+	/* Assert that the ref store refers to a packed backend. */
+	packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+
 	for_each_string_list_item(item, refnames) {
 		if (ref_transaction_delete(transaction, item->string, NULL,
 					   flags, msg, &err)) {
@@ -1563,7 +1578,6 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			error(_("could not delete references: %s"), err.buf);
 	}
 
-	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	return ret;
 }
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index f61a73ec25..a2cca5d9a3 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -3,6 +3,7 @@
 
 struct repository;
 struct ref_transaction;
+struct string_list;
 
 /*
  * Support for storing references in a `packed-refs` file.
@@ -27,6 +28,12 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 void packed_refs_unlock(struct ref_store *ref_store);
 int packed_refs_is_locked(struct ref_store *ref_store);
 
+int packed_refs_delete_refs(struct ref_store *ref_store,
+			    struct ref_transaction *transaction,
+			    const char *msg,
+			    struct string_list *refnames,
+			    unsigned int flags);
+
 /*
  * Return true if `transaction` really needs to be carried out against
  * the specified packed_ref_store, or false if it can be skipped
-- 
2.34.1


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[PATCH v4 2/6] refs: allow passing flags when beginning transactions

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 6362 bytes --]

We do not currently have any flags when creating reference transactions,
but we'll add one to disable execution of the reference transaction hook
in some cases.

Allow passing flags to `ref_store_transaction_begin()` to prepare for
this change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c                |  8 +++++---
 refs.h                |  3 ++-
 refs/files-backend.c  | 10 +++++-----
 refs/packed-backend.c |  2 +-
 refs/refs-internal.h  |  1 +
 sequencer.c           |  2 +-
 6 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/refs.c b/refs.c
index 4da4996c4d..7415864b62 100644
--- a/refs.c
+++ b/refs.c
@@ -800,7 +800,7 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 
-	transaction = ref_store_transaction_begin(refs, &err);
+	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname, old_oid,
 				   flags, msg, &err) ||
@@ -1005,6 +1005,7 @@ int read_ref_at(struct ref_store *refs, const char *refname,
 }
 
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    unsigned int flags,
 						    struct strbuf *err)
 {
 	struct ref_transaction *tr;
@@ -1012,12 +1013,13 @@ struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
 
 	CALLOC_ARRAY(tr, 1);
 	tr->ref_store = refs;
+	tr->flags = flags;
 	return tr;
 }
 
 struct ref_transaction *ref_transaction_begin(struct strbuf *err)
 {
-	return ref_store_transaction_begin(get_main_ref_store(the_repository), err);
+	return ref_store_transaction_begin(get_main_ref_store(the_repository), 0, err);
 }
 
 void ref_transaction_free(struct ref_transaction *transaction)
@@ -1156,7 +1158,7 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 	struct strbuf err = STRBUF_INIT;
 	int ret = 0;
 
-	t = ref_store_transaction_begin(refs, &err);
+	t = ref_store_transaction_begin(refs, 0, &err);
 	if (!t ||
 	    ref_transaction_update(t, refname, new_oid, old_oid, flags, msg,
 				   &err) ||
diff --git a/refs.h b/refs.h
index 92360e55a2..31f7bf9642 100644
--- a/refs.h
+++ b/refs.h
@@ -231,7 +231,7 @@ char *repo_default_branch_name(struct repository *r, int quiet);
  *         struct strbuf err = STRBUF_INIT;
  *         int ret = 0;
  *
- *         transaction = ref_store_transaction_begin(refs, &err);
+ *         transaction = ref_store_transaction_begin(refs, 0, &err);
  *         if (!transaction ||
  *             ref_transaction_update(...) ||
  *             ref_transaction_create(...) ||
@@ -573,6 +573,7 @@ enum action_on_err {
  * be freed by calling ref_transaction_free().
  */
 struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
+						    unsigned int flags,
 						    struct strbuf *err);
 struct ref_transaction *ref_transaction_begin(struct strbuf *err);
 
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 459f18dbc1..4d4f0c2099 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1121,7 +1121,7 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base, &err);
+	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1192,7 +1192,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
 	if (!transaction)
 		return -1;
 
@@ -1259,7 +1259,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
 	if (!transaction)
 		goto error;
 
@@ -2774,7 +2774,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store, err);
+						refs->packed_ref_store, 0, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3045,7 +3045,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, err);
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index c964dd1617..0b45598e18 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1535,7 +1535,7 @@ static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 	 * updates into a single transaction.
 	 */
 
-	transaction = ref_store_transaction_begin(ref_store, &err);
+	transaction = ref_store_transaction_begin(ref_store, 0, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 46a839539e..a0af63f162 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -213,6 +213,7 @@ struct ref_transaction {
 	size_t nr;
 	enum ref_transaction_state state;
 	void *backend_data;
+	unsigned int flags;
 };
 
 /*
diff --git a/sequencer.c b/sequencer.c
index 6abd72160c..61edd39d7a 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3588,7 +3588,7 @@ static int do_label(struct repository *r, const char *name, int len)
 	strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
 	strbuf_addf(&msg, "rebase (label) '%.*s'", len, name);
 
-	transaction = ref_store_transaction_begin(refs, &err);
+	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction) {
 		error("%s", err.buf);
 		ret = -1;
-- 
2.34.1


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[PATCH v4 3/6] refs: allow skipping the reference-transaction hook

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 1449 bytes --]

The reference-transaction hook is executing whenever we prepare, commit
or abort a reference transaction. While this is mostly intentional, in
case of the files backend we're leaking the implementation detail that
the store is in fact a composite store with one loose and one packed
backend to the caller. So while we want to execute the hook for all
logical updates, executing it for such implementation details is
unexpected.

Prepare for a fix by adding a new flag which allows to skip execution of
the hook.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c | 3 +++
 refs.h | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/refs.c b/refs.c
index 7415864b62..526bf5ed97 100644
--- a/refs.c
+++ b/refs.c
@@ -2084,6 +2084,9 @@ static int run_transaction_hook(struct ref_transaction *transaction,
 	const char *hook;
 	int ret = 0, i;
 
+	if (transaction->flags & REF_TRANSACTION_SKIP_HOOK)
+		return 0;
+
 	hook = find_hook("reference-transaction");
 	if (!hook)
 		return ret;
diff --git a/refs.h b/refs.h
index 31f7bf9642..d4056f9fe2 100644
--- a/refs.h
+++ b/refs.h
@@ -568,6 +568,11 @@ enum action_on_err {
 	UPDATE_REFS_QUIET_ON_ERR
 };
 
+/*
+ * Skip executing the reference-transaction hook.
+ */
+#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
+
 /*
  * Begin a reference transaction.  The reference transaction must
  * be freed by calling ref_transaction_free().
-- 
2.34.1


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[PATCH v4 4/6] refs: demonstrate excessive execution of the reference-transaction hook

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 2626 bytes --]

Add tests which demonstate that we're executing the
reference-transaction hook too often in some cases, which thus leaks
implementation details about the reference store's implementation
itself. Behaviour will be fixed in follow-up commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 t/t1416-ref-transaction-hooks.sh | 64 ++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index 6c941027a8..0567fbdf0b 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -136,4 +136,68 @@ test_expect_success 'interleaving hook calls succeed' '
 	test_cmp expect target-repo.git/actual
 '
 
+test_expect_success 'hook does not get called on packing refs' '
+	# Pack references first such that we are in a known state.
+	git pack-refs --all &&
+
+	write_script .git/hooks/reference-transaction <<-\EOF &&
+		echo "$@" >>actual
+		cat >>actual
+	EOF
+	rm -f actual &&
+
+	git update-ref refs/heads/unpacked-ref $POST_OID &&
+	git pack-refs --all &&
+
+	# We only expect a single hook invocation, which is the call to
+	# git-update-ref(1). But currently, packing refs will also trigger the
+	# hook.
+	cat >expect <<-EOF &&
+		prepared
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		committed
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		prepared
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		committed
+		$ZERO_OID $POST_OID refs/heads/unpacked-ref
+		prepared
+		$POST_OID $ZERO_OID refs/heads/unpacked-ref
+		committed
+		$POST_OID $ZERO_OID refs/heads/unpacked-ref
+	EOF
+
+	test_cmp expect actual
+'
+
+test_expect_success 'deleting packed ref calls hook once' '
+	# Create a reference and pack it.
+	git update-ref refs/heads/to-be-deleted $POST_OID &&
+	git pack-refs --all &&
+
+	write_script .git/hooks/reference-transaction <<-\EOF &&
+		echo "$@" >>actual
+		cat >>actual
+	EOF
+	rm -f actual &&
+
+	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
+
+	# We only expect a single hook invocation, which is the logical
+	# deletion. But currently, we see two interleaving transactions, once
+	# for deleting the loose refs and once for deleting the packed ref.
+	cat >expect <<-EOF &&
+		prepared
+		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
+		prepared
+		$POST_OID $ZERO_OID refs/heads/to-be-deleted
+		committed
+		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
+		committed
+		$POST_OID $ZERO_OID refs/heads/to-be-deleted
+	EOF
+
+	test_cmp expect actual
+'
+
 test_done
-- 
2.34.1


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[PATCH v4 5/6] refs: do not execute reference-transaction hook on packing refs

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 2581 bytes --]

The reference-transaction hook is supposed to track logical changes to
references, but it currently also gets executed when packing refs in a
repository. This is unexpected and ultimately not all that useful:
packing refs is not supposed to result in any user-visible change to the
refs' state, and it ultimately is an implementation detail of how refs
stores work.

Fix this excessive execution of the hook when packing refs.

Reported-by: Waleed Khan <me@waleedkhan.name>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c             |  6 ++++--
 t/t1416-ref-transaction-hooks.sh | 11 +----------
 2 files changed, 5 insertions(+), 12 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 4d4f0c2099..565929210a 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1121,7 +1121,8 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	if (check_refname_format(r->name, 0))
 		return;
 
-	transaction = ref_store_transaction_begin(&refs->base, 0, &err);
+	transaction = ref_store_transaction_begin(&refs->base,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		goto cleanup;
 	ref_transaction_add_update(
@@ -1192,7 +1193,8 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		return -1;
 
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index 0567fbdf0b..f9d3d5213f 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -150,21 +150,12 @@ test_expect_success 'hook does not get called on packing refs' '
 	git pack-refs --all &&
 
 	# We only expect a single hook invocation, which is the call to
-	# git-update-ref(1). But currently, packing refs will also trigger the
-	# hook.
+	# git-update-ref(1).
 	cat >expect <<-EOF &&
 		prepared
 		$ZERO_OID $POST_OID refs/heads/unpacked-ref
 		committed
 		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		prepared
-		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		committed
-		$ZERO_OID $POST_OID refs/heads/unpacked-ref
-		prepared
-		$POST_OID $ZERO_OID refs/heads/unpacked-ref
-		committed
-		$POST_OID $ZERO_OID refs/heads/unpacked-ref
 	EOF
 
 	test_cmp expect actual
-- 
2.34.1


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[PATCH v4 6/6] refs: skip hooks when deleting uncovered packed refs

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 3947 bytes --]

When deleting refs from the loose-files refs backend, then we need to be
careful to also delete the same ref from the packed refs backend, if it
exists. If we don't, then deleting the loose ref would "uncover" the
packed ref. We thus always have to queue up deletions of refs for both
the loose and the packed refs backend. This is done in two separate
transactions, where the end result is that the reference-transaction
hook is executed twice for the deleted refs.

This behaviour is quite misleading: it's exposing implementation details
of how the files backend works to the user, in contrast to the logical
updates that we'd really want to expose via the hook. Worse yet, whether
the hook gets executed once or twice depends on how well-packed the
repository is: if the ref only exists as a loose ref, then we execute it
once, otherwise if it is also packed then we execute it twice.

Fix this behaviour and don't execute the reference-transaction hook at
all when refs in the packed-refs backend if it's driven by the files
backend. This works as expected even in case the refs to be deleted only
exist in the packed-refs backend because the loose-backend always queues
refs in its own transaction even if they don't exist such that they can
be locked for concurrent creation. And it also does the right thing in
case neither of the backends has the ref because that would cause the
transaction to fail completely.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c             | 9 ++++++---
 t/t1416-ref-transaction-hooks.sh | 7 +------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 565929210a..844918cbd8 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1261,7 +1261,8 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
+	transaction = ref_store_transaction_begin(refs->packed_ref_store,
+						  REF_TRANSACTION_SKIP_HOOK, &err);
 	if (!transaction)
 		goto error;
 
@@ -2776,7 +2777,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
-						refs->packed_ref_store, 0, err);
+						refs->packed_ref_store,
+						REF_TRANSACTION_SKIP_HOOK, err);
 				if (!packed_transaction) {
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto cleanup;
@@ -3047,7 +3049,8 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 				 &affected_refnames))
 		BUG("initial ref transaction called with existing refs");
 
-	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
+							 REF_TRANSACTION_SKIP_HOOK, err);
 	if (!packed_transaction) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index f9d3d5213f..4e1e84a91f 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -175,16 +175,11 @@ test_expect_success 'deleting packed ref calls hook once' '
 	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
 
 	# We only expect a single hook invocation, which is the logical
-	# deletion. But currently, we see two interleaving transactions, once
-	# for deleting the loose refs and once for deleting the packed ref.
+	# deletion.
 	cat >expect <<-EOF &&
-		prepared
-		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
 		prepared
 		$POST_OID $ZERO_OID refs/heads/to-be-deleted
 		committed
-		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
-		committed
 		$POST_OID $ZERO_OID refs/heads/to-be-deleted
 	EOF
 
-- 
2.34.1


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Re: [PATCH v2 0/6] refs: excessive hook execution with packed refs

[Han-Wen Nienhuys]

On Mon, Jan 17, 2022 at 8:18 AM Patrick Steinhardt <ps@pks.im> wrote:
> > It might make sense to not introduce a new flag namespace, but use
> > update->flags instead. You'd have to add your new flag after
> > REF_SKIP_REFNAME_VERIFICATION.
> > Bonus is that you can unittest the new flag using the existing
> > ref-store helper without extra work. (check that a transaction with &
> > without the flag works as expected.)
> >
> > other than that, looks OK to me.
>
> Thanks for your feedback!
>
> In fact the first version I had locally did exactly that. But I found
> the end version result harder to reason about, most importantly because
> it's not a 100% clear to the reader whether all callsites which delete
> refs in the packed-backend via the files-backend are adapted or whether
> any of the callsites was missing. By having the flag in a central place
> it's immediately clear that the hook won't be run at all, which is
> exactly what we want here.

If you do it like this, can you find the callsites that take update
flags (but not transaction flags), and update the signature to say
"update_flags" rather than "flags", and document appropriately?

-- 
Han-Wen Nienhuys - Google Munich
I work 80%. Don't expect answers from me on Fridays.
--

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Re: [PATCH v2 0/6] refs: excessive hook execution with packed refs

[Patrick Steinhardt]

[-- Attachment #1: Type: text/plain, Size: 12498 bytes --]

On Mon, Jan 17, 2022 at 12:37:39PM +0100, Han-Wen Nienhuys wrote:
> On Mon, Jan 17, 2022 at 8:18 AM Patrick Steinhardt <ps@pks.im> wrote:
> > > It might make sense to not introduce a new flag namespace, but use
> > > update->flags instead. You'd have to add your new flag after
> > > REF_SKIP_REFNAME_VERIFICATION.
> > > Bonus is that you can unittest the new flag using the existing
> > > ref-store helper without extra work. (check that a transaction with &
> > > without the flag works as expected.)
> > >
> > > other than that, looks OK to me.
> >
> > Thanks for your feedback!
> >
> > In fact the first version I had locally did exactly that. But I found
> > the end version result harder to reason about, most importantly because
> > it's not a 100% clear to the reader whether all callsites which delete
> > refs in the packed-backend via the files-backend are adapted or whether
> > any of the callsites was missing. By having the flag in a central place
> > it's immediately clear that the hook won't be run at all, which is
> > exactly what we want here.
> 
> If you do it like this, can you find the callsites that take update
> flags (but not transaction flags), and update the signature to say
> "update_flags" rather than "flags", and document appropriately?

All functions which accept "flags" document that they pass the parameter
directly to `ref_transaction_{update,create,delete,verify}` already. And
in the context of those latter functions it cannot possibly be related
to the flags passed to `refs_store_transaction_begin()` because they
already get a transaction as parameter and thus don't have to create
a new one.

So we could apply below patch, and I'm happy to polish and add it to the
series. But it feels to me like it adds churn while only stating things
that are documented already. It could only be me though given that I'm
obviously biased, so please feel free to disagree.

Patrick

-- >8 --

From 91c77a69b6326b1b6cc743ebba6fb0970f0d01c2 Mon Sep 17 00:00:00 2001
Message-Id: <91c77a69b6326b1b6cc743ebba6fb0970f0d01c2.1643008212.git.ps@pks.im>
From: Patrick Steinhardt <ps@pks.im>
Date: Mon, 24 Jan 2022 08:09:45 +0100
Subject: [PATCH] refs: rename generic `flags` field to `update_flags` for
 clarity

---
 refs.c | 50 +++++++++++++++++++++++++-------------------------
 refs.h | 20 ++++++++++----------
 2 files changed, 35 insertions(+), 35 deletions(-)

diff --git a/refs.c b/refs.c
index 526bf5ed97..c15e8bd58d 100644
--- a/refs.c
+++ b/refs.c
@@ -795,7 +795,7 @@ long get_files_ref_lock_timeout_ms(void)
 int refs_delete_ref(struct ref_store *refs, const char *msg,
 		    const char *refname,
 		    const struct object_id *old_oid,
-		    unsigned int flags)
+		    unsigned int update_flags)
 {
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
@@ -803,7 +803,7 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 	transaction = ref_store_transaction_begin(refs, 0, &err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname, old_oid,
-				   flags, msg, &err) ||
+				   update_flags, msg, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		error("%s", err.buf);
 		ref_transaction_free(transaction);
@@ -816,10 +816,10 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 }
 
 int delete_ref(const char *msg, const char *refname,
-	       const struct object_id *old_oid, unsigned int flags)
+	       const struct object_id *old_oid, unsigned int update_flags)
 {
 	return refs_delete_ref(get_main_ref_store(the_repository), msg, refname,
-			       old_oid, flags);
+			       old_oid, update_flags);
 }
 
 static void copy_reflog_msg(struct strbuf *sb, const char *msg)
@@ -1080,12 +1080,12 @@ int ref_transaction_update(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *new_oid,
 			   const struct object_id *old_oid,
-			   unsigned int flags, const char *msg,
+			   unsigned int update_flags, const char *msg,
 			   struct strbuf *err)
 {
 	assert(err);
 
-	if (!(flags & REF_SKIP_REFNAME_VERIFICATION) &&
+	if (!(update_flags & REF_SKIP_REFNAME_VERIFICATION) &&
 	    ((new_oid && !is_null_oid(new_oid)) ?
 		     check_refname_format(refname, REFNAME_ALLOW_ONELEVEL) :
 			   !refname_is_safe(refname))) {
@@ -1094,19 +1094,19 @@ int ref_transaction_update(struct ref_transaction *transaction,
 		return -1;
 	}
 
-	if (flags & ~REF_TRANSACTION_UPDATE_ALLOWED_FLAGS)
-		BUG("illegal flags 0x%x passed to ref_transaction_update()", flags);
+	if (update_flags & ~REF_TRANSACTION_UPDATE_ALLOWED_FLAGS)
+		BUG("illegal flags 0x%x passed to ref_transaction_update()", update_flags);
 
 	/*
 	 * Clear flags outside the allowed set; this should be a noop because
 	 * of the BUG() check above, but it works around a -Wnonnull warning
 	 * with some versions of "gcc -O3".
 	 */
-	flags &= REF_TRANSACTION_UPDATE_ALLOWED_FLAGS;
+	update_flags &= REF_TRANSACTION_UPDATE_ALLOWED_FLAGS;
 
-	flags |= (new_oid ? REF_HAVE_NEW : 0) | (old_oid ? REF_HAVE_OLD : 0);
+	update_flags |= (new_oid ? REF_HAVE_NEW : 0) | (old_oid ? REF_HAVE_OLD : 0);
 
-	ref_transaction_add_update(transaction, refname, flags,
+	ref_transaction_add_update(transaction, refname, update_flags,
 				   new_oid, old_oid, msg);
 	return 0;
 }
@@ -1114,44 +1114,44 @@ int ref_transaction_update(struct ref_transaction *transaction,
 int ref_transaction_create(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *new_oid,
-			   unsigned int flags, const char *msg,
+			   unsigned int update_flags, const char *msg,
 			   struct strbuf *err)
 {
 	if (!new_oid || is_null_oid(new_oid))
 		BUG("create called without valid new_oid");
 	return ref_transaction_update(transaction, refname, new_oid,
-				      null_oid(), flags, msg, err);
+				      null_oid(), update_flags, msg, err);
 }
 
 int ref_transaction_delete(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *old_oid,
-			   unsigned int flags, const char *msg,
+			   unsigned int update_flags, const char *msg,
 			   struct strbuf *err)
 {
 	if (old_oid && is_null_oid(old_oid))
 		BUG("delete called with old_oid set to zeros");
 	return ref_transaction_update(transaction, refname,
 				      null_oid(), old_oid,
-				      flags, msg, err);
+				      update_flags, msg, err);
 }
 
 int ref_transaction_verify(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *old_oid,
-			   unsigned int flags,
+			   unsigned int update_flags,
 			   struct strbuf *err)
 {
 	if (!old_oid)
 		BUG("verify called with old_oid set to NULL");
 	return ref_transaction_update(transaction, refname,
 				      NULL, old_oid,
-				      flags, NULL, err);
+				      update_flags, NULL, err);
 }
 
 int refs_update_ref(struct ref_store *refs, const char *msg,
 		    const char *refname, const struct object_id *new_oid,
-		    const struct object_id *old_oid, unsigned int flags,
+		    const struct object_id *old_oid, unsigned int update_flags,
 		    enum action_on_err onerr)
 {
 	struct ref_transaction *t = NULL;
@@ -1160,7 +1160,7 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 
 	t = ref_store_transaction_begin(refs, 0, &err);
 	if (!t ||
-	    ref_transaction_update(t, refname, new_oid, old_oid, flags, msg,
+	    ref_transaction_update(t, refname, new_oid, old_oid, update_flags, msg,
 				   &err) ||
 	    ref_transaction_commit(t, &err)) {
 		ret = 1;
@@ -1191,10 +1191,10 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 int update_ref(const char *msg, const char *refname,
 	       const struct object_id *new_oid,
 	       const struct object_id *old_oid,
-	       unsigned int flags, enum action_on_err onerr)
+	       unsigned int update_flags, enum action_on_err onerr)
 {
 	return refs_update_ref(get_main_ref_store(the_repository), msg, refname, new_oid,
-			       old_oid, flags, onerr);
+			       old_oid, update_flags, onerr);
 }
 
 char *refs_shorten_unambiguous_ref(struct ref_store *refs,
@@ -2435,21 +2435,21 @@ int initial_ref_transaction_commit(struct ref_transaction *transaction,
 }
 
 int refs_delete_refs(struct ref_store *refs, const char *logmsg,
-		     struct string_list *refnames, unsigned int flags)
+		     struct string_list *refnames, unsigned int update_flags)
 {
 	char *msg;
 	int retval;
 
 	msg = normalize_reflog_message(logmsg);
-	retval = refs->be->delete_refs(refs, msg, refnames, flags);
+	retval = refs->be->delete_refs(refs, msg, refnames, update_flags);
 	free(msg);
 	return retval;
 }
 
 int delete_refs(const char *msg, struct string_list *refnames,
-		unsigned int flags)
+		unsigned int update_flags)
 {
-	return refs_delete_refs(get_main_ref_store(the_repository), msg, refnames, flags);
+	return refs_delete_refs(get_main_ref_store(the_repository), msg, refnames, update_flags);
 }
 
 int refs_rename_ref(struct ref_store *refs, const char *oldref,
diff --git a/refs.h b/refs.h
index d4056f9fe2..5131db7e48 100644
--- a/refs.h
+++ b/refs.h
@@ -442,9 +442,9 @@ int reflog_exists(const char *refname);
 int refs_delete_ref(struct ref_store *refs, const char *msg,
 		    const char *refname,
 		    const struct object_id *old_oid,
-		    unsigned int flags);
+		    unsigned int update_flags);
 int delete_ref(const char *msg, const char *refname,
-	       const struct object_id *old_oid, unsigned int flags);
+	       const struct object_id *old_oid, unsigned int update_flags);
 
 /*
  * Delete the specified references. If there are any problems, emit
@@ -453,9 +453,9 @@ int delete_ref(const char *msg, const char *refname,
  * ref_transaction_delete().
  */
 int refs_delete_refs(struct ref_store *refs, const char *msg,
-		     struct string_list *refnames, unsigned int flags);
+		     struct string_list *refnames, unsigned int update_flags);
 int delete_refs(const char *msg, struct string_list *refnames,
-		unsigned int flags);
+		unsigned int update_flags);
 
 /** Delete a reflog */
 int refs_delete_reflog(struct ref_store *refs, const char *refname);
@@ -680,7 +680,7 @@ int ref_transaction_update(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *new_oid,
 			   const struct object_id *old_oid,
-			   unsigned int flags, const char *msg,
+			   unsigned int update_flags, const char *msg,
 			   struct strbuf *err);
 
 /*
@@ -695,7 +695,7 @@ int ref_transaction_update(struct ref_transaction *transaction,
 int ref_transaction_create(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *new_oid,
-			   unsigned int flags, const char *msg,
+			   unsigned int update_flags, const char *msg,
 			   struct strbuf *err);
 
 /*
@@ -709,7 +709,7 @@ int ref_transaction_create(struct ref_transaction *transaction,
 int ref_transaction_delete(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *old_oid,
-			   unsigned int flags, const char *msg,
+			   unsigned int update_flags, const char *msg,
 			   struct strbuf *err);
 
 /*
@@ -723,7 +723,7 @@ int ref_transaction_delete(struct ref_transaction *transaction,
 int ref_transaction_verify(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *old_oid,
-			   unsigned int flags,
+			   unsigned int update_flags,
 			   struct strbuf *err);
 
 /* Naming conflict (for example, the ref names A and A/B conflict). */
@@ -796,10 +796,10 @@ void ref_transaction_free(struct ref_transaction *transaction);
  */
 int refs_update_ref(struct ref_store *refs, const char *msg, const char *refname,
 		    const struct object_id *new_oid, const struct object_id *old_oid,
-		    unsigned int flags, enum action_on_err onerr);
+		    unsigned int update_flags, enum action_on_err onerr);
 int update_ref(const char *msg, const char *refname,
 	       const struct object_id *new_oid, const struct object_id *old_oid,
-	       unsigned int flags, enum action_on_err onerr);
+	       unsigned int update_flags, enum action_on_err onerr);
 
 int parse_hide_refs_config(const char *var, const char *value, const char *);
 
-- 
2.34.1


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