apply: accept -3/--3way command line option

Begin teaching the three-way merge fallback logic "git am -3" uses
to the underlying "git apply".  It only implements the command line
parsing part, and does not do anything interesting yet, other than
making sure that "--reject" and "--3way" are not given together, and
making "--3way" imply "--index".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -46,6 +46,7 @@ static int apply_with_reject;
 static int apply_verbosely;
 static int allow_overlap;
 static int no_add;
+static int threeway;
 static const char *fake_ancestor;
 static int line_termination = '\n';
 static unsigned int p_context = UINT_MAX;
@@ -3139,6 +3140,12 @@ static int load_preimage(struct image *image,
 	return 0;
 }
 
+static int try_threeway(struct image *image, struct patch *patch,
+			struct stat *st, struct cache_entry *ce)
+{
+	return -1; /* for now */
+}
+
 static int apply_data(struct patch *patch, struct stat *st, struct cache_entry *ce)
 {
 	struct image image;
@@ -3146,8 +3153,11 @@ static int apply_data(struct patch *patch, struct stat *st, struct cache_entry *
 	if (load_preimage(&image, patch, st, ce) < 0)
 		return -1;
 
-	if (apply_fragments(&image, patch) < 0)
-		return -1; /* note with --reject this succeeds. */
+	if (apply_fragments(&image, patch) < 0) {
+		/* Note: with --reject, apply_fragments() returns 0 */
+		if (!threeway || try_threeway(&image, patch, st, ce) < 0)
+			return -1;
+	}
 	patch->result = image.buf;
 	patch->resultsize = image.len;
 	add_to_fn_table(patch);
@@ -4079,6 +4089,8 @@ int cmd_apply(int argc, const char **argv, const char *prefix_)
 			"apply a patch without touching the working tree"),
 		OPT_BOOLEAN(0, "apply", &force_apply,
 			"also apply the patch (use with --stat/--summary/--check)"),
+		OPT_BOOL('3', "3way", &threeway,
+			 "attempt three-way merge if a patch does not apply"),
 		OPT_FILENAME(0, "build-fake-ancestor", &fake_ancestor,
 			"build a temporary index based on embedded index information"),
 		{ OPTION_CALLBACK, 'z', NULL, NULL, NULL,
@@ -4127,6 +4139,15 @@ int cmd_apply(int argc, const char **argv, const char *prefix_)
 	argc = parse_options(argc, argv, prefix, builtin_apply_options,
 			apply_usage, 0);
 
+	if (apply_with_reject && threeway)
+		die("--reject and --3way cannot be used together.");
+	if (cached && threeway)
+		die("--cached and --3way cannot be used together.");
+	if (threeway) {
+		if (is_not_gitdir)
+			die(_("--3way outside a repository"));
+		check_index = 1;
+	}
 	if (apply_with_reject)
 		apply = apply_verbosely = 1;
 	if (!force_apply && (diffstat || numstat || summary || check || fake_ancestor))

t/t4117-apply-reject.sh

@@ -46,6 +46,14 @@ test_expect_success setup '
 	cat file1 >saved.file1
 '
 
+test_expect_success 'apply --reject is incompatible with --3way' '
+	test_when_finished "cat saved.file1 >file1" &&
+	git diff >patch.0 &&
+	git checkout file1 &&
+	test_must_fail git apply --reject --3way patch.0 &&
+	git diff --exit-code
+'
+
 test_expect_success 'apply without --reject should fail' '
 
 	if git apply patch.1