[PATCH V3] git-apply: skip threeway in add / rename cases

[Jerry Zhang <jerry@skydio.com>]

Certain invocations of "git apply --3way"
will attempt threeway and fail due to
missing objects, even though git is able
to fall back on apply_fragments and
apply the patch successfully with a return
value of 0. To fix, return early from
try_threeway() in the following cases:

When the patch is a rename and no lines have
changed. In this case, "git diff" doesn't
record the blob info, so 3way is neither
possible nor necessary.

When the patch is an addition and there is
no add/add conflict, i.e. direct_to_threeway
is false. In this case, threeway will fail
since the preimage is not in cache, but isn't
necessary anyway since there is no conflict.

This fixes a few unecessary error prints
when applying these kinds of patches with
--3way.

It also fixes a reported issue where applying
a concatenation of several git produced patches
will fail when those patches involve a deletion
followed by creation of the same file. Added a
test for this case too.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
(test provided by <i@zenithal.me>)
---
V2->V3:
- Updated commit title and message to be more
general, and indicate that it also fixes the
delete-then-new bug. Added test.

 apply.c                   |  4 +++-
 t/t4108-apply-threeway.sh | 14 ++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/apply.c b/apply.c
index fed195250b..afc1c6510e 100644
--- a/apply.c
+++ b/apply.c
@@ -3580,11 +3580,13 @@ static int try_threeway(struct apply_state *state,
 	char *img;
 	struct image tmp_image;
 
 	/* No point falling back to 3-way merge in these cases */
 	if (patch->is_delete ||
-	    S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode))
+	    S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode) ||
+	    (patch->is_new && !patch->direct_to_threeway) ||
+	    (patch->is_rename && !patch->lines_added && !patch->lines_deleted))
 		return -1;
 
 	/* Preimage the patch was prepared for */
 	if (patch->is_new)
 		write_object_file("", 0, blob_type, &pre_oid);
diff --git a/t/t4108-apply-threeway.sh b/t/t4108-apply-threeway.sh
index cc3aa3314a..daad50d2d2 100755
--- a/t/t4108-apply-threeway.sh
+++ b/t/t4108-apply-threeway.sh
@@ -273,6 +273,20 @@ test_expect_success 'apply full-index patch with 3way' '
 
 	# Apply must succeed.
 	git apply --3way --index bin.diff
 '
 
+test_expect_success 'apply delete then new patch with 3way' '
+	git reset --hard main &&
+    test_write_lines 1 > delnew &&
+	git add delnew &&
+    git commit -m "delnew" &&
+    rm delnew &&
+    git diff >> delete-then-new.patch &&
+    git diff HEAD~ HEAD >> delete-then-new.patch &&
+
+    git checkout -- . &&
+	# Apply must succeed.
+	git apply --3way delete-then-new.patch
+'
+
 test_done
-- 
2.32.0.1314.g6ed4fcc4cc