make ProcessException pickleable (pytorch#70118)

Summary:
Fixes pytorch#70116

Happy to add tests if you let me know the best place to put them.

cc VitalyFedyunin

Pull Request resolved: pytorch#70118

Reviewed By: malfet

Differential Revision: D33255899

Pulled By: ejguan

fbshipit-source-id: 41d495374182eb28bb8bb421e890eca3bddc077b

Author: epwalsh

test/test_multiprocessing_spawn.py

@@ -1,6 +1,7 @@
 # Owner(s): ["module: multiprocessing"]
 
 import os
+import pickle
 import random
 import signal
 import sys
@@ -218,5 +219,15 @@ def test_process_exited(self):
 class ForkTest(TestCase, _TestMultiProcessing):
     start_method = 'fork'
 
+
+class ErrorTest(TestCase):
+    def test_errors_pickleable(self):
+        for error in (
+            mp.ProcessRaisedException("Oh no!", 1, 1),
+            mp.ProcessExitedException("Oh no!", 1, 1, 1),
+        ):
+            pickle.loads(pickle.dumps(error))
+
+
 if __name__ == '__main__':
     run_tests()

torch/multiprocessing/spawn.py

@@ -14,9 +14,13 @@ class ProcessException(Exception):
 
     def __init__(self, msg: str, error_index: int, pid: int):
         super().__init__(msg)
+        self.msg = msg
         self.error_index = error_index
         self.pid = pid
 
+    def __reduce__(self):
+        return type(self), (self.msg, self.error_index, self.pid)
+
 
 class ProcessRaisedException(ProcessException):
     """
@@ -47,6 +51,12 @@ def __init__(
         self.exit_code = exit_code
         self.signal_name = signal_name
 
+    def __reduce__(self):
+        return (
+            type(self),
+            (self.msg, self.error_index, self.pid, self.exit_code, self.signal_name),
+        )
+
 
 def _wrap(fn, i, args, error_queue):
     # prctl(2) is a Linux specific system call.