[PATCH RESEND] refs: Always pass old object name to reftx hook

[Patrick Steinhardt <ps@pks.im>]

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Inputs of the reference-transaction hook currently depends on the
command which is being run. For example if the command `git update-ref
$REF $A $B` is executed, it will receive "$B $A $REF" as input, but if
the command `git update-ref $REF $A` is executed without providing the
old value, then it will receive "0*40 $A $REF" as input. This is due to
the fact that we directly write queued transaction updates into the
hook's standard input, which will not contain the old object value in
case it wasn't provided.

While this behaviour reflects what is happening as part of the
repository, it doesn't feel like it is useful. The main intent of the
reference-transaction hook is to be able to completely audit all
reference updates, no matter where they come from. As such, it makes a
lot more sense to always provide actual values instead of what the user
wanted. Furthermore, it's impossible for the hook to distinguish whether
this is intended to be a branch creation or a branch update without
doing additional digging with the current format.

Fix the issue by storing the old object value into the queued
transaction update operation if it wasn't provided by the caller.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 Documentation/githooks.txt       |  6 ++++++
 refs/files-backend.c             |  8 ++++++++
 refs/packed-backend.c            |  2 ++
 t/t1416-ref-transaction-hooks.sh | 12 ++++++------
 4 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 1f3b57d04d..e7baf0e2a0 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -492,6 +492,12 @@ receives on standard input a line of the format:
 
   <old-value> SP <new-value> SP <ref-name> LF
 
+where `<old-value>` is the old object name stored in the ref,
+`<new-value>` is the new object name to be stored in the ref and
+`<ref-name>` is the full name of the ref.
+When creating a new ref, `<old-value>` is 40 `0`.
+When deleting an old ref, `<new-value>` is 40 `0`.
+
 The exit status of the hook is ignored for any state except for the
 "prepared" state. In the "prepared" state, a non-zero exit status will
 cause the transaction to be aborted. The hook will not be called with
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 04e85e7002..5b10d3822b 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -2452,6 +2452,9 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 				ret = TRANSACTION_GENERIC_ERROR;
 				goto out;
 			}
+
+			if (!(update->flags & REF_HAVE_OLD))
+				oidcpy(&update->old_oid, &lock->old_oid);
 		} else {
 			/*
 			 * Create a new update for the reference this
@@ -2474,6 +2477,9 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 			goto out;
 		}
 
+		if (!(update->flags & REF_HAVE_OLD))
+			oidcpy(&update->old_oid, &lock->old_oid);
+
 		/*
 		 * If this update is happening indirectly because of a
 		 * symref update, record the old OID in the parent
@@ -2484,6 +2490,8 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 		     parent_update = parent_update->parent_update) {
 			struct ref_lock *parent_lock = parent_update->backend_data;
 			oidcpy(&parent_lock->old_oid, &lock->old_oid);
+			if (!(parent_update->flags & REF_HAVE_OLD))
+				oidcpy(&parent_update->old_oid, &lock->old_oid);
 		}
 	}
 
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index b912f2505f..08f0feee3d 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1178,6 +1178,8 @@ static int write_with_updates(struct packed_ref_store *refs,
 						    oid_to_hex(&update->old_oid));
 					goto error;
 				}
+			} else {
+				oidcpy(&update->old_oid, iter->oid);
 			}
 
 			/* Now figure out what to use for the new value: */
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
index f6e741c6c0..111533682a 100755
--- a/t/t1416-ref-transaction-hooks.sh
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -52,12 +52,12 @@ test_expect_success 'hook gets all queued updates in prepared state' '
 		fi
 	EOF
 	cat >expect <<-EOF &&
-		$ZERO_OID $POST_OID HEAD
-		$ZERO_OID $POST_OID refs/heads/master
+		$PRE_OID $POST_OID HEAD
+		$PRE_OID $POST_OID refs/heads/master
 	EOF
 	git update-ref HEAD POST <<-EOF &&
-		update HEAD $ZERO_OID $POST_OID
-		update refs/heads/master $ZERO_OID $POST_OID
+		update HEAD $PRE_OID $POST_OID
+		update refs/heads/master $PRE_OID $POST_OID
 	EOF
 	test_cmp expect actual
 '
@@ -75,8 +75,8 @@ test_expect_success 'hook gets all queued updates in committed state' '
 		fi
 	EOF
 	cat >expect <<-EOF &&
-		$ZERO_OID $POST_OID HEAD
-		$ZERO_OID $POST_OID refs/heads/master
+		$PRE_OID $POST_OID HEAD
+		$PRE_OID $POST_OID refs/heads/master
 	EOF
 	git update-ref HEAD POST &&
 	test_cmp expect actual
-- 
2.30.0


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