Fix isolated pytest assertion rendering in worker process

subprojects/robotpy-wpilib/tests/test_pytest_plugins.py

@@ -166,6 +166,26 @@ def test_robot_failure_output(robot):
     assert robot_pid_one != robot_pid_two
 
 
+def test_isolated_plugin_assertion_rendering(pytester):
+    _make_robot_module(pytester)
+    _configure_isolated_plugin(pytester)
+    pytester.makepyfile(test_isolated="""
+def test_robot_assertion_rendering(robot):
+    assert "x" == "y"
+""")
+
+    result = pytester.runpytest_subprocess("-vv")
+
+    result.assert_outcomes(failed=1)
+    result.stdout.fnmatch_lines(
+        [
+            "*test_isolated.py::test_robot_assertion_rendering FAILED*",
+            "*assert 'x' == 'y'*",
+        ]
+    )
+    assert not any("_pytest/config/__init__.py" in line for line in result.outlines)
+
+
 def test_isolated_plugin_no_duplicate_verbose_output(pytester):
     _make_robot_module(pytester)
     _configure_isolated_plugin(pytester)

subprojects/robotpy-wpilib/wpilib/testing/pytest_isolated_tests_plugin.py

@@ -18,6 +18,24 @@
 from .pytest_plugin import RobotTestingPlugin
 
 
+class _NullTerminalWriter:
+    def _highlight(self, source, lexer="python"):
+        return source
+
+
+class _NullTerminalReporter:
+    """Minimal terminal reporter used in worker processes."""
+
+    def __init__(self):
+        self._tw = _NullTerminalWriter()
+
+    def write(self, *args, **kwargs):
+        pass
+
+    def line(self, *args, **kwargs):
+        pass
+
+
 def _enable_faulthandler():
     #
     # In the event of a segfault, faulthandler will dump the currently
@@ -66,6 +84,15 @@ def sendevent(self, name: str, **kwargs: object):
     @pytest.hookimpl(wrapper=True)
     def pytest_sessionstart(self, session: pytest.Session):
         self.config = session.config
+
+        # When we disable terminalreporter in worker mode we still need a
+        # minimal reporter so assertion introspection can render diffs.
+        if self.config.pluginmanager.get_plugin("terminalreporter") is None:
+            self.config.pluginmanager.unblock("terminalreporter")
+            self.config.pluginmanager.register(
+                _NullTerminalReporter(), "terminalreporter"
+            )
+
         return (yield)
 
     @pytest.hookimpl