submodule--helper: add push-check subcommand

Add the 'push-check' subcommand to submodule--helper which is used to
check if the provided remote and refspec can be used as part of a push
operation in the submodule.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1105,6 +1105,50 @@ static int resolve_remote_submodule_branch(int argc, const char **argv,
 	return 0;
 }
 
+static int push_check(int argc, const char **argv, const char *prefix)
+{
+	struct remote *remote;
+
+	if (argc < 2)
+		die("submodule--helper push-check requires at least 1 argument");
+
+	/*
+	 * The remote must be configured.
+	 * This is to avoid pushing to the exact same URL as the parent.
+	 */
+	remote = pushremote_get(argv[1]);
+	if (!remote || remote->origin == REMOTE_UNCONFIGURED)
+		die("remote '%s' not configured", argv[1]);
+
+	/* Check the refspec */
+	if (argc > 2) {
+		int i, refspec_nr = argc - 2;
+		struct ref *local_refs = get_local_heads();
+		struct refspec *refspec = parse_push_refspec(refspec_nr,
+							     argv + 2);
+
+		for (i = 0; i < refspec_nr; i++) {
+			struct refspec *rs = refspec + i;
+
+			if (rs->pattern || rs->matching)
+				continue;
+
+			/*
+			 * LHS must match a single ref
+			 * NEEDSWORK: add logic to special case 'HEAD' once
+			 * working with submodules in a detached head state
+			 * ceases to be the norm.
+			 */
+			if (count_refspec_match(rs->src, local_refs, NULL) != 1)
+				die("src refspec '%s' must name a ref",
+				    rs->src);
+		}
+		free_refspec(refspec_nr, refspec);
+	}
+
+	return 0;
+}
+
 static int absorb_git_dirs(int argc, const char **argv, const char *prefix)
 {
 	int i;
@@ -1170,6 +1214,7 @@ static struct cmd_struct commands[] = {
 	{"resolve-relative-url-test", resolve_relative_url_test, 0},
 	{"init", module_init, SUPPORT_SUPER_PREFIX},
 	{"remote-branch", resolve_remote_submodule_branch, 0},
+	{"push-check", push_check, 0},
 	{"absorb-git-dirs", absorb_git_dirs, SUPPORT_SUPER_PREFIX},
 	{"is-active", is_active, 0},
 };