branch: skip commit checks when deleting symref branches

Before a branch is deleted, we check that it points to a valid
commit.  With -d we also check that the commit is a merged; this
check is not done with -D.

The reason for that is that commits pointed to by branches should
never go missing; if they do then something broke and it's better
to stop instead of adding to the mess.  And a non-merged commit
may contain changes that are worth preserving, so we require the
stronger option -D instead of -d to get rid of them.

If a branch consists of a symref, these concerns don't apply.
Deleting such a branch can't make a commit become unreferenced,
so we don't need to check if it is merged, or even if it is
actually a valid commit.  Skip them in that case.  This allows
us to delete dangling symref branches.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -214,6 +214,9 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 			die(_("Couldn't look up commit object for HEAD"));
 	}
 	for (i = 0; i < argc; i++, strbuf_release(&bname)) {
+		const char *target;
+		int flags = 0;
+
 		strbuf_branchname(&bname, argv[i]);
 		if (kinds == REF_LOCAL_BRANCH && !strcmp(head, bname.buf)) {
 			error(_("Cannot delete the branch '%s' "
@@ -225,7 +228,9 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 		free(name);
 
 		name = mkpathdup(fmt, bname.buf);
-		if (read_ref(name, sha1)) {
+		target = resolve_ref_unsafe(name, sha1, 0, &flags);
+		if (!target ||
+		    (!(flags & REF_ISSYMREF) && is_null_sha1(sha1))) {
 			error(remote_branch
 			      ? _("remote branch '%s' not found.")
 			      : _("branch '%s' not found."), bname.buf);
@@ -233,7 +238,8 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 			continue;
 		}
 
-		if (check_branch_commit(bname.buf, name, sha1, head_rev, kinds,
+		if (!(flags & REF_ISSYMREF) &&
+		    check_branch_commit(bname.buf, name, sha1, head_rev, kinds,
 					force)) {
 			ret = 1;
 			continue;

t/t3200-branch.sh

@@ -273,6 +273,15 @@ test_expect_success 'deleting a symref' '
 	test_i18ncmp expect actual
 '
 
+test_expect_success 'deleting a dangling symref' '
+	git symbolic-ref refs/heads/dangling-symref nowhere &&
+	test_path_is_file .git/refs/heads/dangling-symref &&
+	echo "Deleted branch dangling-symref (was 0000000)." >expect &&
+	git branch -d dangling-symref >actual &&
+	test_path_is_missing .git/refs/heads/dangling-symref &&
+	test_i18ncmp expect actual
+'
+
 test_expect_success 'renaming a symref is not allowed' \
 '
 	git symbolic-ref refs/heads/master2 refs/heads/master &&