Fix possible deadlock in SharedCache inside a forked child proc (#25158)
Summary:
Related: https://github.com/pytorch/pytorch/issues/24927#issuecomment-524608021
`fork` inherits lock state. So if we happen to unfortunately fork when the `SharedCache` lock is held. We could deadlock in the child process when some code tries to acquire it.
Following pytorch multiprocessing library design, this patch resets the lock to a new object after fork. A similar example from python core lib for `multiprocessing.Queue` is :
```py
class Queue(object):
def __init__(self, ...):
...
self._after_fork()
if sys.platform != 'win32':
register_after_fork(self, Queue._after_fork)
def _after_fork(self):
debug('Queue._after_fork()')
self._notempty = threading.Condition(threading.Lock())
self._buffer = collections.deque()
self._thread = None
self._jointhread = None
self._joincancelled = False
self._closed = False
self._close = None
self._send_bytes = self._writer.send_bytes
self._recv_bytes = self._reader.recv_bytes
self._poll = self._reader.poll
```
https://github.com/python/cpython/blob/d4d60134b29290049e28df54f23493de4f1824b6/Lib/multiprocessing/queues.py#L54-L78
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25158
Differential Revision: D17091227
Pulled By: soumith
fbshipit-source-id: ee7130f47d7bbd42fc34a2598f1f6974d8d7cdb7
