git-apply: add --allow-empty flag

Some users or scripts will pipe "git diff"
output to "git apply" when replaying diffs
or commits. In these cases, they will rely
on the return value of "git apply" to know
whether the diff was applied successfully.

However, for empty commits, "git apply" will
fail. This complicates scripts since they
have to either buffer the diff and check
its length, or run diff again with "exit-code",
essentially doing the diff twice.

Add the "--allow-empty" flag to "git apply"
which allows it to handle both empty diffs
and empty commits created by "git format-patch
--always" by doing nothing and returning 0.

Add tests for both with and without --allow-empty.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-apply.txt

@@ -16,7 +16,7 @@ SYNOPSIS
 	  [--ignore-space-change | --ignore-whitespace]
 	  [--whitespace=(nowarn|warn|fix|error|error-all)]
 	  [--exclude=<path>] [--include=<path>] [--directory=<root>]
-	  [--verbose | --quiet] [--unsafe-paths] [<patch>...]
+	  [--verbose | --quiet] [--unsafe-paths] [--allow-empty] [<patch>...]
 
 DESCRIPTION
 -----------
@@ -256,6 +256,10 @@ When `git apply` is used as a "better GNU patch", the user can pass
 the `--unsafe-paths` option to override this safety check.  This option
 has no effect when `--index` or `--cached` is in use.
 
+--allow-empty::
+	Don't return error for patches containing no diff. This includes
+	empty patches and patches with commit text only.
+
 CONFIGURATION
 -------------
 

apply.c

@@ -4752,8 +4752,10 @@ static int apply_patch(struct apply_state *state,
 	}
 
 	if (!list && !skipped_patch) {
-		error(_("unrecognized input"));
-		res = -128;
+		if (!state->allow_empty) {
+			error(_("No valid patches in input (allow with \"--allow-empty\")"));
+			res = -128;
+		}
 		goto end;
 	}
 
@@ -5081,6 +5083,8 @@ int apply_parse_options(int argc, const char **argv,
 		OPT_CALLBACK(0, "directory", state, N_("root"),
 			N_("prepend <root> to all filenames"),
 			apply_option_parse_directory),
+		OPT_BOOL(0, "allow-empty", &state->allow_empty,
+			N_("don't return error for empty patches")),
 		OPT_END()
 	};
 

apply.h

@@ -66,6 +66,7 @@ struct apply_state {
 	int threeway;
 	int unidiff_zero;
 	int unsafe_paths;
+	int allow_empty;
 
 	/* Other non boolean parameters */
 	struct repository *repo;

t/t4126-apply-empty.sh

@@ -9,6 +9,8 @@ test_expect_success setup '
 	git add empty &&
 	test_tick &&
 	git commit -m initial &&
+	git commit --allow-empty -m "empty commit" &&
+	git format-patch --always HEAD~ >empty.patch &&
 	for i in a b c d e
 	do
 		echo $i
@@ -25,30 +27,42 @@ test_expect_success setup '
 '
 
 test_expect_success 'apply empty' '
-	git reset --hard &&
 	rm -f missing &&
+	test_when_finished "git reset --hard" &&
 	git apply patch0 &&
 	test_cmp expect empty
 '
 
+test_expect_success 'apply empty patch fails' '
+	test_when_finished "git reset --hard" &&
+	test_must_fail git apply empty.patch &&
+	test_must_fail git apply - </dev/null
+'
+
+test_expect_success 'apply with --allow-empty succeeds' '
+	test_when_finished "git reset --hard" &&
+	git apply --allow-empty empty.patch &&
+	git apply --allow-empty - </dev/null
+'
+
 test_expect_success 'apply --index empty' '
-	git reset --hard &&
 	rm -f missing &&
+	test_when_finished "git reset --hard" &&
 	git apply --index patch0 &&
 	test_cmp expect empty &&
 	git diff --exit-code
 '
 
 test_expect_success 'apply create' '
-	git reset --hard &&
 	rm -f missing &&
+	test_when_finished "git reset --hard" &&
 	git apply patch1 &&
 	test_cmp expect missing
 '
 
 test_expect_success 'apply --index create' '
-	git reset --hard &&
 	rm -f missing &&
+	test_when_finished "git reset --hard" &&
 	git apply --index patch1 &&
 	test_cmp expect missing &&
 	git diff --exit-code