Check that we can serialize the warning detail attr

changelog/379.bugfix.rst

@@ -0,0 +1,2 @@
+Warning attributes are checked to make sure they can be dumped prior to
+serializing the warning for submission to the master node.

testing/acceptance_test.py

@@ -1,5 +1,6 @@
 import os
 import re
+import sys
 import textwrap
 
 import py
@@ -803,6 +804,39 @@ def test_func(tmpdir):
         result = testdir.runpytest(n)
         result.stdout.fnmatch_lines(["*UserWarning*foo.txt*", "*1 passed, 1 warnings*"])
 
+    @pytest.mark.parametrize("n", ["-n0", "-n1"])
+    def test_unserializable_warning_details(self, testdir, n):
+        """Check that warnings with unserializable _WARNING_DETAILS are
+        handled correctly (#379).
+        """
+        if sys.version_info[0] < 3:
+            # The issue is only present in Python 3 warnings
+            return
+        testdir.makepyfile(
+            """
+            import warnings, pytest
+            import socket
+            import gc
+            def abuse_socket():
+                s = socket.socket()
+                del s
+
+            # Deliberately provoke a ResourceWarning for an unclosed socket.
+            # The socket itself will end up attached as a value in
+            # _WARNING_DETAIL. We need to test that it is not serialized
+            # (it can't be, so the test will fail if we try to).
+            @pytest.mark.filterwarnings('always')
+            def test_func(tmpdir):
+                abuse_socket()
+                gc.collect()
+        """
+        )
+        testdir.syspathinsert()
+        result = testdir.runpytest(n)
+        result.stdout.fnmatch_lines(
+            ["*ResourceWarning*unclosed*", "*1 passed, 1 warnings*"]
+        )
+
 
 class TestNodeFailure:
     def test_load_single(self, testdir):

xdist/remote.py

@@ -218,7 +218,15 @@ def serialize_warning_message(warning_message):
     for attr_name in warning_message._WARNING_DETAILS:
         if attr_name in ("message", "category"):
             continue
-        result[attr_name] = getattr(warning_message, attr_name)
+        attr = getattr(warning_message, attr_name)
+        # Check if we can serialize the warning detail, marking `None` otherwise
+        # Note that we need to define the attr (even as `None`) to allow deserializing
+        try:
+            dumps(attr)
+        except DumpError:
+            result[attr_name] = repr(attr)
+        else:
+            result[attr_name] = attr
     return result