[PATCH V2] git-apply: Allow simultaneous --cached and --3way options

[Jerry Zhang <jerry@skydio.com>]

Previously, --cached and --3way were not
allowed to be used together, since --3way
wrote conflict markers into the working tree.

These changes change semantics so that if
these flags are given together and there is
a conflict, the conflicting objects are left
at a higher order in the cache, and the command
will return non-zero. If there is no conflict,
the patch is applied directly to cache as
expected and the command will return 0.

The user can use `git diff` to view the contents
of the conflict, or `git checkout -m -- .` to
regenerate the conflict markers in the working
directory.

With the combined --3way and --cached flags,
The conflict markers won't be written to the
working directory, so there is no point in
attempting rerere.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
Signed-off-by: Jerry Zhang <jerryxzha@googlemail.com>
---
 Documentation/git-apply.txt | 3 ++-
 apply.c                     | 5 ++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/Documentation/git-apply.txt b/Documentation/git-apply.txt
index 91d9a8601c..392882d9a5 100644
--- a/Documentation/git-apply.txt
+++ b/Documentation/git-apply.txt
@@ -89,7 +89,8 @@ OPTIONS
 	and we have those blobs available locally, possibly leaving the
 	conflict markers in the files in the working tree for the user to
 	resolve.  This option implies the `--index` option, and is incompatible
-	with the `--reject` and the `--cached` options.
+	with the `--reject` option. When used with the --cached option, any conflicts
+    are left at higher stages in the cache.
 
 --build-fake-ancestor=<file>::
 	Newer 'git diff' output has embedded 'index information'
diff --git a/apply.c b/apply.c
index 6695a931e9..e59c77a1b7 100644
--- a/apply.c
+++ b/apply.c
@@ -133,8 +133,6 @@ int check_apply_state(struct apply_state *state, int force_apply)
 
 	if (state->apply_with_reject && state->threeway)
 		return error(_("--reject and --3way cannot be used together."));
-	if (state->cached && state->threeway)
-		return error(_("--cached and --3way cannot be used together."));
 	if (state->threeway) {
 		if (is_not_gitdir)
 			return error(_("--3way outside a repository"));
@@ -4646,7 +4644,8 @@ static int write_out_results(struct apply_state *state, struct patch *list)
 		}
 		string_list_clear(&cpath, 0);
 
-		repo_rerere(state->repo, 0);
+		if (!state->cached)
+			repo_rerere(state->repo, 0);
 	}
 
 	return errs;
-- 
2.29.0