fsck: check index files in all worktrees

We check the index file for the main worktree, but completely ignore the
index files in other worktrees. These should be checked, too, as they
are part of the repository state (and in particular, errors in those
index files may cause repo-wide operations like "git gc" to complain).

Reported-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fsck.c

@@ -984,10 +984,24 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 	}
 
 	if (keep_cache_objects) {
+		struct worktree **worktrees, **p;
+
 		verify_index_checksum = 1;
 		verify_ce_order = 1;
-		repo_read_index(the_repository);
-		fsck_index(the_repository->index);
+
+		worktrees = get_worktrees();
+		for (p = worktrees; *p; p++) {
+			struct worktree *wt = *p;
+			struct index_state istate =
+				INDEX_STATE_INIT(the_repository);
+
+			if (read_index_from(&istate,
+					    worktree_git_path(wt, "index"),
+					    get_worktree_git_dir(wt)) > 0)
+				fsck_index(&istate);
+			discard_index(&istate);
+		}
+		free_worktrees(worktrees);
 	}
 
 	check_connectivity();

t/t1450-fsck.sh

@@ -1023,4 +1023,16 @@ test_expect_success 'fsck error on gitattributes with excessive size' '
 	test_cmp expected actual
 '
 
+test_expect_success 'fsck detects problems in worktree index' '
+	test_when_finished "git worktree remove -f wt" &&
+	git worktree add wt &&
+
+	echo "this will be removed to break the worktree index" >wt/file &&
+	git -C wt add file &&
+	blob=$(git -C wt rev-parse :file) &&
+	remove_object $blob &&
+
+	test_must_fail git fsck
+'
+
 test_done