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 5827a03 (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

builtin/clone.c

@@ -639,7 +639,9 @@ 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);

t/t5616-partial-clone.sh

@@ -405,6 +405,14 @@ 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