[PATCH] clone: use "quick" lookup while following tags

[Jeff King <peff@peff.net>]

On Sat, Mar 28, 2020 at 10:40:23AM -0400, Jeff King wrote:

> So I guess the problem is not with shallow clones specifically, but they
> lead us to not having fetched the commits pointed to by tags, which
> leads to us trying to fault in those commits (and their trees) rather
> than realizing that we weren't meant to have them. And the size of the
> local repo balloons because you're fetching all those commits one by
> one, and not getting the benefit of the deltas you would when you do a
> single --filter=blob:none fetch.
> 
> I guess we need something like this:

The issue is actually with --single-branch, which is implied by --depth.
But the fix is the same either way.

Here it is with a commit message and test.

-- >8 --
Subject: [PATCH] clone: use "quick" lookup while following tags

When cloning with --single-branch, we implement git-fetch's usual
tag-following behavior, grabbing any tag objects that point to objects
we have locally.

When we're a partial clone, though, our has_object_file() check will
actually lazy-fetch each tag. That not only defeats the purpose of
--single-branch, but it does it incredibly slowly, potentially kicking
off a new fetch for each tag. This is even worse for a shallow clone,
which implies --single-branch, because even tags which are supersets of
each other will be fetched individually.

We can fix this by passing OBJECT_INFO_SKIP_FETCH_OBJECT to the call,
which is what git-fetch does in this case.

Likewise, let's include OBJECT_INFO_QUICK, as that's what git-fetch
does. The rationale is discussed in 5827a03545 (fetch: use "quick"
has_sha1_file for tag following, 2016-10-13), but here the tradeoff
would apply even more so because clone is very unlikely to be racing
with another process repacking our newly-created repository.

This may provide a very small speedup even in the non-partial case case,
as we'd avoid calling reprepare_packed_git() for each tag (though in
practice, we'd only have a single packfile, so that reprepare should be
quite cheap).

Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/clone.c          | 4 +++-
 t/t5616-partial-clone.sh | 8 ++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/builtin/clone.c b/builtin/clone.c
index d8b1f413aa..9da6459f1d 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -643,7 +643,9 @@ static void write_followtags(const struct ref *refs, const char *msg)
 			continue;
 		if (ends_with(ref->name, "^{}"))
 			continue;
-		if (!has_object_file(&ref->old_oid))
+		if (!has_object_file_with_flags(&ref->old_oid,
+						OBJECT_INFO_QUICK |
+						OBJECT_INFO_SKIP_FETCH_OBJECT))
 			continue;
 		update_ref(msg, ref->name, &ref->old_oid, NULL, 0,
 			   UPDATE_REFS_DIE_ON_ERR);
diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index 77bb91e976..8f0d81a27e 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -415,6 +415,14 @@ test_expect_success 'verify fetch downloads only one pack when updating refs' '
 	test_line_count = 3 pack-list
 '
 
+test_expect_success 'single-branch tag following respects partial clone' '
+	git clone --single-branch -b B --filter=blob:none \
+		"file://$(pwd)/srv.bare" single &&
+	git -C single rev-parse --verify refs/tags/B &&
+	git -C single rev-parse --verify refs/tags/A &&
+	test_must_fail git -C single rev-parse --verify refs/tags/C
+'
+
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
-- 
2.26.0.408.gebd8a4413c