[PATCH v4] index-pack: remove fetch_if_missing=0

[Kousik Sanagavarapu <five231003@gmail.com>]

A collision test is triggered in sha1_object(), whenever there is an
object file in our repo. If our repo is a partial clone, then checking
for this file existence does not lazy-fetch the object (if the object
is missing and if there are one or more promisor remotes) when
fetch_if_missing is set to 0.

Though this global lets us control lazy-fetching in regions of code,
it prevents multi-threading [1].

Hence, use has_object() to check for the existence of an object, which
has the default behavior of not lazy-fetching in a partial clone, even
when fetch_if_missing is set to 1. It is worth mentioning that this is
the only place where there is potential for lazy-fetching and all other
cases [2] are properly handled, making it safe to remove this global
here.

[1] See https://lore.kernel.org/git/xmqqv9sdeeif.fsf@gitster-ct.c.googlers.com/
    and the discussion that follows from it.

[2] These cases are:
    - When we check objects, but we return with 0 early if the object
      doesn't exist.
    - We prefetch delta bases in a partial clone, if we don't have them
      (as the comment outlines).
    - There are some cases where we fsck objects, but lazy-fetching is
      already handled in fsck.

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Kousik Sanagavarapu <five231003@gmail.com>
---

Changes since v3:
- Changed the commit message to give a stronger reason as to
  why we should reduce the use of this global and in this
  process remove it from builtin/index-pack.c

- Also made an addition to the test which I overlooked last time
  and without which the does not make sense

 builtin/index-pack.c     | 11 +----------
 t/t5616-partial-clone.sh | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 6648f2daef..8c0f36a49e 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -800,8 +800,7 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 
 	if (startup_info->have_repository) {
 		read_lock();
-		collision_test_needed =
-			has_object_file_with_flags(oid, OBJECT_INFO_QUICK);
+		collision_test_needed = has_object(the_repository, oid, 0);
 		read_unlock();
 	}
 
@@ -1728,14 +1727,6 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	int report_end_of_input = 0;
 	int hash_algo = 0;
 
-	/*
-	 * index-pack never needs to fetch missing objects except when
-	 * REF_DELTA bases are missing (which are explicitly handled). It only
-	 * accesses the repo to do hash collision checks and to check which
-	 * REF_DELTA bases need to be fetched.
-	 */
-	fetch_if_missing = 0;
-
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage(index_pack_usage);
 
diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index f519d2a87a..41fa7130f1 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -644,6 +644,39 @@ test_expect_success 'repack does not loosen promisor objects' '
 	grep "loosen_unused_packed_objects/loosened:0" trace
 '
 
+test_expect_success 'index-pack does not lazy-fetch
+		     when checking for sha1 collsions' '
+	rm -rf server client another-remote &&
+
+	git init server &&
+	echo "line" >server/file &&
+	git -C server add file &&
+	git -C server commit -am "file" &&
+	git -C server config --local uploadpack.allowFilter 1 &&
+	git -C server config --local uploadpack.allowAnySha1InWant 1 &&
+
+	git clone --no-checkout --filter=blob:none \
+				"file://$(pwd)/server" client &&
+	git -C client config extensions.partialClone 1 &&
+	git -C client config remote.origin.promisor 1 &&
+
+	git clone "file://$(pwd)/server" another-remote &&
+
+	echo "new line" >server/new-file &&
+	git -C server add new-file &&
+	git -C server commit -am "new-file" &&
+	HASH=$(git -C server hash-object new-file) &&
+
+	git -C another-remote pull &&
+
+	# Try to fetch so that "client" will have to do a collision-check.
+	# This should, however, not fetch "new-file" because "client" is a
+	# partial clone.
+	GIT_TRACE_PACKET=git -C client fetch \
+				"file://$(pwd)/another-remote" main &&
+	! grep "want $HASH" trace
+'
+
 test_expect_success 'lazy-fetch in submodule succeeds' '
 	# setup
 	test_config_global protocol.file.allow always &&
-- 
2.25.1