Remove empty (& invalid) lock files

* src/filelock.c (current_lock_owner):
Remove empty lock files, as they are necessarily invalid
and can be caused by buggy file systems.
Problem reported by Michal Nazarewicz (bug#72641).

(cherry picked from commit 8b36bfc553b97cf435bdfe1b84abe21c3a605b9f)

diff --git a/src/filelock.c b/src/filelock.c
index c68aacc46fbc3b1c276a8e4d4ec866e9c18844d3..1ae57dc734445894131e3c9cdb8ad3f00def7b67 100644 (file)
--- a/src/filelock.c
+++ b/src/filelock.c
@@ -397,8 +397,8 @@ current_lock_owner (lock_info_type *owner, Lisp_Object lfname)
   if (lfinfolen < 0)
     return errno == ENOENT || errno == ENOTDIR ? 0 : errno;
 
-  /* Examine lock file contents.  */
-  if (true)
+  /* If the lock file seems valid, return a value based on its contents.  */
+  if (lfinfolen)
     {
       if (MAX_LFINFO < lfinfolen)
        return ENAMETOOLONG;
@@ -496,8 +496,11 @@ current_lock_owner (lock_info_type *owner, Lisp_Object lfname)
        return ANOTHER_OWNS_IT;
     }
 
-  /* The owner process is dead or has a strange pid.
-     Try to zap the lockfile.  */
+  /* The owner process is dead or has a strange pid, or the lock file is empty.
+     Try to zap the lockfile.  If the lock file is empty, this assumes
+     the file system is buggy, e.g., <https://bugs.gnu.org/72641>.
+     Emacs never creates empty lock files even temporarily, so removing
+     an empty lock file should be harmless.  */
   return emacs_unlink (SSDATA (lfname)) < 0 ? errno : 0;
 }