Fix multiprocessing hang on FSErrors

fs/errors.py

@@ -59,11 +59,15 @@ class MissingInfoNamespace(AttributeError):
 
     def __init__(self, namespace):
         # type: (Text) -> None
+        self.namespace=namespace
         msg = "namespace '{}' is required for this attribute"
         super(MissingInfoNamespace, self).__init__(
             msg.format(namespace)
         )
 
+    def __reduce__(self):
+        return type(self), (self.namespace,)
+
 
 @six.python_2_unicode_compatible
 class FSError(Exception):
@@ -121,6 +125,10 @@ def new_func(*args, **kwargs):
                 raise cls(exc=e)
         return new_func  # type: ignore
 
+    def __reduce__(self):
+        return type(self), (self._msg, self.exc)
+
+
 class PathError(FSError):
     """Base exception for errors to do with a path string.
     """
@@ -132,6 +140,9 @@ def __init__(self, path, msg=None):
         self.path = path
         super(PathError, self).__init__(msg=msg)
 
+    def __reduce__(self):
+        return type(self), (self.path, self._msg)
+
 
 class NoSysPath(PathError):
     """The filesystem does not provide *sys paths* to the resource.
@@ -151,6 +162,9 @@ def __init__(self, path, purpose, msg=None):
         self.purpose = purpose
         super(NoURL, self).__init__(path, msg=msg)
 
+    def __reduce__(self):
+        return type(self), (self.path, self.purpose, self._msg)
+
 
 class InvalidPath(PathError):
     """Path can't be mapped on to the underlaying filesystem.
@@ -184,6 +198,9 @@ def __init__(self,
         self.errno = getattr(exc, "errno", None)
         super(OperationFailed, self).__init__(msg=msg)
 
+    def __reduce__(self):
+        return type(self), (self.path, self.exc, self._msg)
+
 
 class Unsupported(OperationFailed):
     """Operation not supported by the filesystem.
@@ -239,6 +256,9 @@ def __init__(self, path, exc=None, msg=None):
         self.exc = exc
         super(ResourceError, self).__init__(msg=msg)
 
+    def __reduce__(self):
+        return type(self), (self.path, self.exc, self._msg)
+
 
 class ResourceNotFound(ResourceError):
     """Required resource not found.
@@ -325,7 +345,11 @@ class IllegalBackReference(ValueError):
 
     def __init__(self, path):
         # type: (Text) -> None
+        self.path = path
         _msg = \
             "path '{path}' contains back-references outside of filesystem"
         _msg = _msg.format(path=path)
         super(IllegalBackReference, self).__init__(_msg)
+
+    def __reduce__(self):
+        return type(self), (self.path,)

tests/test_errors.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+import multiprocessing
 import unittest
 
 from six import text_type
@@ -26,6 +27,27 @@ def test_unsupported(self):
             "not supported"
         )
 
+    def test_raise_in_multiprocessing(self):
+        # Without the __reduce__ methods in FSError subclasses, this test will hang forever.
+        tests = [
+            [errors.ResourceNotFound, 'some_path'],
+            [errors.FilesystemClosed],
+            [errors.CreateFailed],
+            [errors.NoSysPath, 'some_path'],
+            [errors.NoURL, 'some_path', 'some_purpose'],
+            [errors.Unsupported]
+        ]
+        try:
+            pool = multiprocessing.Pool(1)
+            for args in tests:
+                with self.assertRaises(args[0]):
+                    pool.apply(_multiprocessing_test_task, args)
+        finally:
+            pool.close()
+
+def _multiprocessing_test_task(err, *args):
+    raise err(*args)
+
 
 class TestCreateFailed(unittest.TestCase):