rwlock: flush and fsync

It is important that we ensure that rwlocked contents are flushed to
disk, because otherwise other rwlock readers might get outdated state.

Kudos @shcheklein for catching this one.

dvc/rwlock.py

@@ -50,6 +50,9 @@ def _edit_rwlock(lock_dir):
     yield lock
     with open(path, "w+") as fobj:
         json.dump(lock, fobj)
+        # NOTE: flush and fsync to ensure that rwlock contents are saved
+        fobj.flush()
+        os.fsync(fobj.fileno())
 
 
 def _infos_to_str(infos):