[PATCH] fetch: commit references fetched before backfilling tags

In 0e358de64a (fetch: use batched reference updates, 2025-05-19), the
fetch code was modified to use batched updates to provide a good
performance improvement. Wherein batched updates were used to fetch both
references and backfill tags.

When using batched updates, the references aren't yet committed to disk
when we start backfilling tags. This means in situations such as shallow
fetching the negotiation during backfilling tags, the client doesn't
have any references to report in the 'have' section. Since backfilling
doesn't use a depth limit, this can cause the server to send all the
objects present in the repository.

Fix this by committing the previous batched update and initiating a new
one for backfilling tags. Also add a test which captures this regression.

While this does make it a little slower than master, due to creation of
two transactions, It is still faster than not using batched updates:

Benchmark 1: fetch: many refs (refformat = reftable, refcount = 10000, revision = 0e358de64a9e014575d11ef884bfc9beb931e37f~1)
  Time (mean ± σ):      1.468 s ±  0.041 s    [User: 0.839 s, System: 0.587 s]
  Range (min … max):    1.427 s …  1.558 s    10 runs

Benchmark 2: fetch: many refs (refformat = reftable, refcount = 10000, revision = HEAD)
  Time (mean ± σ):      84.4 ms ±   1.7 ms    [User: 60.9 ms, System: 25.8 ms]
  Range (min … max):    81.4 ms …  88.6 ms    29 runs

Summary
  fetch: many refs (refformat = reftable, refcount = 10000, revision = HEAD) ran
   17.38 ± 0.61 times faster than fetch: many refs (refformat = reftable, refcount = 10000, revision = 0e358de64a9e014575d11ef884bfc9beb931e37f~1)

Reported-by: Mitja Bezenšek <redacted>
Signed-off-by: Karthik Nayak <redacted>
---
The issue was reported by Mitja Bezenšek on GitLab's Git repo [1].

[1]: https://gitlab.com/gitlab-org/git/-/work_items/799
---
 builtin/fetch.c  | 21 +++++++++++++++++++++
 t/t5510-fetch.sh | 17 +++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/builtin/fetch.c b/builtin/fetch.c
index b2decc6cfd..68b04d0f8a 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c

@@ -2076,6 +2076,27 @@ static int do_fetch(struct transport *transport,
 		struct ref *tags_ref_map = NULL, **tail = &tags_ref_map;
 
 		find_non_local_tags(remote_refs, transaction, &tags_ref_map, &tail);
+
+		/*
+		 * Backfilling tags has no depth limit. If we don't commit
+		 * the fetched references, the backfill will report no refs
+		 * in the 'have' section of the negotiation. This can cause
+		 * the server to send all objects.
+		 */
+		if (tags_ref_map && !atomic_fetch) {
+			retcode |= commit_ref_transaction(&transaction, false,
+							  transport->remote->name,
+							  &rejected_refs, &err);
+
+			transaction = ref_store_transaction_begin(get_main_ref_store(the_repository),
+								  REF_TRANSACTION_ALLOW_FAILURE, &err);
+			if (!transaction) {
+				free_refs(tags_ref_map);
+				retcode = -1;
+				goto cleanup;
+			}
+		}
+
 		if (tags_ref_map) {
 			/*
 			 * If backfilling of tags fails then we want to tell

diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 300bd5396d..81865c1ecc 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh

@@ -1942,6 +1942,23 @@ test_expect_success "backfill tags when providing a refspec" '
 	test_cmp expect actual
 '
 
+test_expect_success 'shallow fetch does not fetch objects again for tags' '
+	test_when_finished rm -rf source target trace &&
+
+	git init source &&
+	test_commit_bulk -C source 10 &&
+	git -C source tag -a tag -m tag HEAD~2 &&
+	HEAD_OID=$(git -C source rev-parse HEAD) &&
+
+	git init target &&
+	git -C target remote add origin ../source &&
+	GIT_TEST_PROTOCOL_VERSION=2 GIT_TRACE_PACKET=$(pwd)/trace \
+		git -C target fetch --depth 5 origin &&
+
+	test $(grep -c "fetch> command=fetch" trace) -gt 2 &&
+	test $(grep -c "fetch> have $HEAD_OID" trace) -eq 2
+'
+
 test_expect_success REFFILES "FETCH_HEAD is updated even if ref updates fail" '
 	test_when_finished rm -rf base repo &&
 

---
base-commit: 6de20f6092dcf9bdb1c8efe03db4b70c82b423dd
change-id: 20261007-799-shallow-fetch-with-tags-d62b3efb29f0


Thanks
- Karthik