close_lock_file(): if close fails, roll back

If closing an open lockfile fails, then we cannot be sure of the
contents of the lockfile, so there is nothing sensible to do but
delete it. This change also insures that the lock_file object is left
in a defined state in this error path (namely, unlocked).

The only caller that is ultimately affected by this change is
try_merge_strategy() -> write_locked_index(), which can call
close_lock_file() via various execution paths. This caller uses a
static lock_file object which previously could have been reused after
a failed close_lock_file() even though it was still in locked state.
This change causes the lock_file object to be unlocked on failure,
thus fixing this error-handling path.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-lockfile.txt

@@ -162,9 +162,10 @@ close_lock_file::
 	Take a pointer to the `struct lock_file` initialized with an
 	earlier call to `hold_lock_file_for_update` or
 	`hold_lock_file_for_append`, and close the file descriptor.
-	Return 0 upon success or a negative value on failure to
-	close(2). Usually `commit_lock_file` or `rollback_lock_file`
-	should be called after `close_lock_file`.
+	Return 0 upon success. On failure to `close(2)`, return a
+	negative value and rollback the lock file. Usually
+	`commit_lock_file` or `rollback_lock_file` should eventually
+	be called if `close_lock_file` succeeds.
 
 reopen_lock_file::
 

lockfile.c

@@ -37,13 +37,14 @@
  *   lockfile, and owner holds the PID of the process that locked the
  *   file.
  *
- * - Locked, lockfile closed (after close_lock_file()).  Same as the
- *   previous state, except that the lockfile is closed and fd is -1.
+ * - Locked, lockfile closed (after successful close_lock_file()).
+ *   Same as the previous state, except that the lockfile is closed
+ *   and fd is -1.
  *
- * - Unlocked (after commit_lock_file(), rollback_lock_file(), or a
- *   failed attempt to lock).  In this state, filename[0] == '\0' and
- *   fd is -1.  The object is left registered in the lock_file_list,
- *   and on_list is set.
+ * - Unlocked (after commit_lock_file(), rollback_lock_file(), a
+ *   failed attempt to lock, or a failed close_lock_file()). In this
+ *   state, filename[0] == '\0' and fd is -1. The object is left
+ *   registered in the lock_file_list, and on_list is set.
  */
 
 static struct lock_file *lock_file_list;
@@ -284,7 +285,13 @@ int close_lock_file(struct lock_file *lk)
 		return 0;
 
 	lk->fd = -1;
-	return close(fd);
+	if (close(fd)) {
+		int save_errno = errno;
+		rollback_lock_file(lk);
+		errno = save_errno;
+		return -1;
+	}
+	return 0;
 }
 
 int reopen_lock_file(struct lock_file *lk)
@@ -330,7 +337,8 @@ void rollback_lock_file(struct lock_file *lk)
 	if (!lk->filename[0])
 		return;
 
-	close_lock_file(lk);
-	unlink_or_warn(lk->filename);
-	lk->filename[0] = 0;
+	if (!close_lock_file(lk)) {
+		unlink_or_warn(lk->filename);
+		lk->filename[0] = 0;
+	}
 }