[PATCH v2] cache-tree: do not lazy-fetch merge tree

[Jonathan Tan <jonathantanmy@google.com>]

When cherry-picking (for example), new trees may be constructed. During
this process, Git constructs the new tree in a struct strbuf, computes
the OID of the new tree, and checks if the new OID already exists on
disk. However, in a partial clone, the disk check causes a lazy fetch to
occur, which is both unnecessary (because we have the tree in the struct
strbuf) and likely to fail (because the remote probably doesn't have
this tree).

Do not lazy fetch in this situation.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
As requested in What's Cooking [1], here's a patch with an updated
commit message. Otherwise, the patch is exactly the same.

[1] https://public-inbox.org/git/xmqqd0gcm2zm.fsf@gitster-ct.c.googlers.com/
---
 cache-tree.c             |  2 +-
 t/t0410-partial-clone.sh | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/cache-tree.c b/cache-tree.c
index c22161f987..9e596893bc 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -407,7 +407,7 @@ static int update_one(struct cache_tree *it,
 	if (repair) {
 		struct object_id oid;
 		hash_object_file(buffer.buf, buffer.len, tree_type, &oid);
-		if (has_object_file(&oid))
+		if (has_object_file_with_flags(&oid, OBJECT_INFO_SKIP_FETCH_OBJECT))
 			oidcpy(&it->oid, &oid);
 		else
 			to_invalidate = 1;
diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index 6415063980..3e434b6a81 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh
@@ -492,6 +492,20 @@ test_expect_success 'gc stops traversal when a missing but promised object is re
 	! grep "$TREE_HASH" out
 '
 
+test_expect_success 'do not fetch when checking existence of tree we construct ourselves' '
+	rm -rf repo &&
+	test_create_repo repo &&
+	test_commit -C repo base &&
+	test_commit -C repo side1 &&
+	git -C repo checkout base &&
+	test_commit -C repo side2 &&
+
+	git -C repo config core.repositoryformatversion 1 &&
+	git -C repo config extensions.partialclone "arbitrary string" &&
+
+	git -C repo cherry-pick side1
+'
+
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
-- 
2.23.0.162.g0b9fbb3734-goog