From 9ba1821e9121c3ee49a68b4863cd7ee50de4a5a5 Mon Sep 17 00:00:00 2001
From: Andreas Motl <andreas@getkotori.org>
Date: Sun, 17 Jan 2021 19:23:57 +0100
Subject: [PATCH] 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                      |  1 +
 changelog/8249.bugfix.rst    |  1 +
 src/_pytest/faulthandler.py  |  7 ++++++-
 testing/test_faulthandler.py | 25 +++++++++++++++++++++++++
 4 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 changelog/8249.bugfix.rst

diff --git a/AUTHORS b/AUTHORS
index e75baa8cd..0c721a496 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -21,6 +21,7 @@ Anders Hovmöller
 Andras Mitzki
 Andras Tim
 Andrea Cimatoribus
+Andreas Motl
 Andreas Zeidler
 Andrey Paramonov
 Andrzej Klajnert
diff --git a/changelog/8249.bugfix.rst b/changelog/8249.bugfix.rst
new file mode 100644
index 000000000..aa084c757
--- /dev/null
+++ b/changelog/8249.bugfix.rst
@@ -0,0 +1 @@
+Fix the ``faulthandler`` plugin for occasions when running with ``twisted.logger`` and using ``pytest --capture=no``.
diff --git a/src/_pytest/faulthandler.py b/src/_pytest/faulthandler.py
index d0cc0430c..ff673b5b1 100644
--- a/src/_pytest/faulthandler.py
+++ b/src/_pytest/faulthandler.py
@@ -69,7 +69,12 @@ class FaultHandlerHooks:
     @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
diff --git a/testing/test_faulthandler.py b/testing/test_faulthandler.py
index caf39813c..370084c12 100644
--- a/testing/test_faulthandler.py
+++ b/testing/test_faulthandler.py
@@ -1,3 +1,4 @@
+import io
 import sys
 
 import pytest
@@ -135,3 +136,27 @@ def test_already_initialized(faulthandler_timeout: int, pytester: Pytester) -> N
         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