Fix faulthandler for Twisted Logger when used with "--capture=no"

The Twisted Logger will return an invalid file descriptor since it is
not backed by an FD. So, let's also forward this to the same code path
as with `pytest-xdist`.

AUTHORS

@@ -21,6 +21,7 @@ Anders Hovmöller
 Andras Mitzki
 Andras Tim
 Andrea Cimatoribus
+Andreas Motl
 Andreas Zeidler
 Andrey Paramonov
 Andrzej Klajnert

changelog/8249.bugfix.rst

@@ -0,0 +1 @@
+Fix the ``faulthandler`` plugin for occasions when running with ``twisted.logger`` and using ``pytest --capture=no``.

src/_pytest/faulthandler.py

@@ -69,7 +69,12 @@ def pytest_unconfigure(self, config: Config) -> None:
     @staticmethod
     def _get_stderr_fileno():
         try:
-            return sys.stderr.fileno()
+            fileno = sys.stderr.fileno()
+            # The Twisted Logger will return an invalid file descriptor since it is not backed
+            # by an FD. So, let's also forward this to the same code path as with pytest-xdist.
+            if fileno == -1:
+                raise AttributeError()
+            return fileno
         except (AttributeError, io.UnsupportedOperation):
             # pytest-xdist monkeypatches sys.stderr with an object that is not an actual file.
             # https://docs.python.org/3/library/faulthandler.html#issue-with-file-descriptors

testing/test_faulthandler.py

@@ -1,3 +1,4 @@
+import io
 import sys
 
 import pytest
@@ -135,3 +136,27 @@ def test():
         result.stdout.no_fnmatch_line(warning_line)
     result.stdout.fnmatch_lines("*1 passed*")
     assert result.ret == 0
+
+
+def test_get_stderr_fileno_invalid_fd() -> None:
+    """Test for faulthandler being able to handle invalid file descriptors for stderr (#8249)."""
+    from _pytest.faulthandler import FaultHandlerHooks
+
+    class StdErrWrapper(io.StringIO):
+        """
+        Mimic ``twisted.logger.LoggingFile`` to simulate returning an invalid file descriptor.
+
+        https://github.com/twisted/twisted/blob/twisted-20.3.0/src/twisted/logger/_io.py#L132-L139
+        """
+
+        def fileno(self):
+            return -1
+
+    wrapper = StdErrWrapper()
+
+    with pytest.MonkeyPatch.context() as mp:
+        mp.setattr("sys.stderr", wrapper)
+
+        # Even when the stderr wrapper signals an invalid file descriptor,
+        # ``_get_stderr_fileno()`` should return the real one.
+        assert FaultHandlerHooks._get_stderr_fileno() == 2