Fix multiprocessing hang on FSErrors (PyFilesystem#184)

* Fix multiprocessing hang on FSErrors

* Fixed whitespace

Author: Spacerat

fs/errors.py

@@ -60,9 +60,13 @@ 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):
@@ -133,7 +137,10 @@ def new_func(*args, **kwargs):
 
         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.
     """
@@ -145,6 +152,9 @@ 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.
@@ -164,6 +174,9 @@ 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.
@@ -198,6 +211,9 @@ def __init__(
         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.
@@ -253,6 +269,9 @@ 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.
@@ -339,6 +358,11 @@ class IllegalBackReference(ValueError):
 
     def __init__(self, path):
         # type: (Text) -> None
-        _msg = "path '{path}' contains back-references outside of filesystem"
-        _msg = _msg.format(path=path)
-        super(IllegalBackReference, self).__init__(_msg)
+        self.path = path
+        msg = (
+            "path '{path}' contains back-references outside of filesystem"
+        ).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):