Fix Pdb

Summary: Pull Request resolved: https://github.com/pytorch/fairseq/pull/551

Differential Revision: D14295227

Pulled By: myleott

fbshipit-source-id: 404f2a2697a62ce0dbf22e5ab2e1cf932acc83ac

fairseq/pdb.py

@@ -14,26 +14,27 @@ import sys
 __all__ = ['set_trace']
 
 
+_stdin_fd = sys.stdin.fileno()
+_stdin = [None]
+_stdin_lock = multiprocessing.Lock()
+
+
 class MultiprocessingPdb(pdb.Pdb):
     """A Pdb wrapper that works in a multiprocessing environment.
 
     Usage: `from fairseq import pdb; pdb.set_trace()`
     """
 
-    _stdin_fd = sys.stdin.fileno()
-    _stdin = None
-    _stdin_lock = multiprocessing.Lock()
-
     def __init__(self):
         pdb.Pdb.__init__(self, nosigint=True)
 
     def _cmdloop(self):
         stdin_bak = sys.stdin
-        with self._stdin_lock:
+        with _stdin_lock:
             try:
-                if not self._stdin:
-                    self._stdin = os.fdopen(self._stdin_fd)
-                sys.stdin = self._stdin
+                if not _stdin[0]:
+                    _stdin[0] = os.fdopen(_stdin_fd)
+                sys.stdin = _stdin[0]
                 self.cmdloop()
             finally:
                 sys.stdin = stdin_bak