branch -d: refuse deleting a branch which is currently checked out

When a branch is checked out by current working tree, deleting the
branch is forbidden. However when the branch is checked out only by
other working trees, deleting incorrectly succeeds.
Use find_shared_symref() to check if the branch is in use, not just
comparing with the current working tree's HEAD.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Kazuki Yamaguchi <k@rhe.jp>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -20,6 +20,7 @@
 #include "utf8.h"
 #include "wt-status.h"
 #include "ref-filter.h"
+#include "worktree.h"
 
 static const char * const builtin_branch_usage[] = {
 	N_("git branch [<options>] [-r | -a] [--merged | --no-merged]"),
@@ -215,16 +216,21 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 		int flags = 0;
 
 		strbuf_branchname(&bname, argv[i]);
-		if (kinds == FILTER_REFS_BRANCHES && !strcmp(head, bname.buf)) {
-			error(_("Cannot delete the branch '%s' "
-			      "which you are currently on."), bname.buf);
-			ret = 1;
-			continue;
+		free(name);
+		name = mkpathdup(fmt, bname.buf);
+
+		if (kinds == FILTER_REFS_BRANCHES) {
+			char *worktree = find_shared_symref("HEAD", name);
+			if (worktree) {
+				error(_("Cannot delete branch '%s' "
+					"checked out at '%s'"),
+				      bname.buf, worktree);
+				free(worktree);
+				ret = 1;
+				continue;
+			}
 		}
 
-		free(name);
-
-		name = mkpathdup(fmt, bname.buf);
 		target = resolve_ref_unsafe(name,
 					    RESOLVE_REF_READING
 					    | RESOLVE_REF_NO_RECURSE

t/t3200-branch.sh

@@ -403,6 +403,12 @@ test_expect_success 'test deleting branch without config' '
 	test_i18ncmp expect actual
 '
 
+test_expect_success 'deleting currently checked out branch fails' '
+	git worktree add -b my7 my7 &&
+	test_must_fail git -C my7 branch -d my7 &&
+	test_must_fail git branch -d my7
+'
+
 test_expect_success 'test --track without .fetch entries' '
 	git branch --track my8 &&
 	test "$(git config branch.my8.remote)" &&