Handle unserializable warning arguments

Fix #349

xdist/remote.py

@@ -12,6 +12,7 @@ import time
 
 import _pytest.hookspec
 import pytest
+from execnet.gateway_base import dumps, DumpError
 
 
 class WorkerInteractor(object):
@@ -181,8 +182,15 @@ def serialize_warning_message(warning_message):
     if isinstance(warning_message.message, Warning):
         message_module = type(warning_message.message).__module__
         message_class_name = type(warning_message.message).__name__
-        message_args = warning_message.message.args
         message_str = str(warning_message.message)
+        # check now if we can serialize the warning arguments (#349)
+        # if not, we will just use the exception message on the master node
+        try:
+            dumps(warning_message.message.args)
+        except DumpError:
+            message_args = None
+        else:
+            message_args = warning_message.message.args
     else:
         message_str = warning_message.message
         message_module = None

xdist/workermanage.py

@@ -426,9 +426,16 @@ def unserialize_warning_message(data):
     if data["message_module"]:
         mod = importlib.import_module(data["message_module"])
         cls = getattr(mod, data["message_class_name"])
-        try:
-            message = cls(*data["message_args"])
-        except TypeError:
+        message = None
+        if data["message_args"] is not None:
+            try:
+                message = cls(*data["message_args"])
+            except TypeError:
+                pass
+        if message is None:
+            # could not recreate the original warning instance;
+            # create a generic Warning instance with the original
+            # message at least
             message_text = "{mod}.{cls}: {msg}".format(
                 mod=data["message_module"],
                 cls=data["message_class_name"],