Handle unserializable warning arguments

Fix #349

changelog/349.bugfix.rst

@@ -0,0 +1 @@
+Correctly handle warnings created with arguments that can't be serialized during the transfer from workers to master node.

testing/acceptance_test.py

@@ -766,6 +766,23 @@ class TestWarnings:
         result = testdir.runpytest(n)
         result.stdout.fnmatch_lines(["*MyWarning*", "*1 passed, 1 warnings*"])
 
+    @pytest.mark.parametrize("n", ["-n0", "-n1"])
+    def test_unserializable_arguments(self, testdir, n):
+        """Check that warnings with unserializable arguments are handled correctly (#349)."""
+        testdir.makepyfile(
+            """
+            import warnings, pytest
+
+            def test_func(tmpdir):
+                fn = (tmpdir / 'foo.txt').ensure(file=1)
+                with fn.open('r') as f:
+                    warnings.warn(UserWarning("foo", f))
+        """
+        )
+        testdir.syspathinsert()
+        result = testdir.runpytest(n)
+        result.stdout.fnmatch_lines(["*UserWarning*foo.txt*", "*1 passed, 1 warnings*"])
+
 
 class TestNodeFailure:
     def test_load_single(self, testdir):

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 = None
-            message = cls(*data["message_args"])
+        if data["message_args"] is not None:
-        except TypeError:
+            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"],