Fix for operation on closed file in faulthandler teardown (#11584)

Author: bnomis

changelog/11572.bugfix.rst

@@ -0,0 +1 @@
+Handle an edge case where :data:`sys.stderr` and :data:`sys.__stderr__` might already be closed when :ref:`faulthandler` is tearing down.

src/_pytest/faulthandler.py

@@ -9,8 +9,8 @@
 from _pytest.stash import StashKey
 
 
+fault_handler_original_stderr_fd_key = StashKey[int]()
 fault_handler_stderr_fd_key = StashKey[int]()
-fault_handler_originally_enabled_key = StashKey[bool]()
 
 
 def pytest_addoption(parser: Parser) -> None:
@@ -24,8 +24,15 @@ def pytest_addoption(parser: Parser) -> None:
 def pytest_configure(config: Config) -> None:
     import faulthandler
 
-    config.stash[fault_handler_stderr_fd_key] = os.dup(get_stderr_fileno())
-    config.stash[fault_handler_originally_enabled_key] = faulthandler.is_enabled()
+    # at teardown we want to restore the original faulthandler fileno
+    # but faulthandler has no api to return the original fileno
+    # so here we stash the stderr fileno to be used at teardown
+    # sys.stderr and sys.__stderr__ may be closed or patched during the session
+    # so we can't rely on their values being good at that point (#11572).
+    stderr_fileno = get_stderr_fileno()
+    if faulthandler.is_enabled():
+        config.stash[fault_handler_original_stderr_fd_key] = stderr_fileno
+    config.stash[fault_handler_stderr_fd_key] = os.dup(stderr_fileno)
     faulthandler.enable(file=config.stash[fault_handler_stderr_fd_key])
 
 
@@ -37,9 +44,10 @@ def pytest_unconfigure(config: Config) -> None:
     if fault_handler_stderr_fd_key in config.stash:
         os.close(config.stash[fault_handler_stderr_fd_key])
         del config.stash[fault_handler_stderr_fd_key]
-    if config.stash.get(fault_handler_originally_enabled_key, False):
-        # Re-enable the faulthandler if it was originally enabled.
-        faulthandler.enable(file=get_stderr_fileno())
+    # Re-enable the faulthandler if it was originally enabled.
+    if fault_handler_original_stderr_fd_key in config.stash:
+        faulthandler.enable(config.stash[fault_handler_original_stderr_fd_key])
+        del config.stash[fault_handler_original_stderr_fd_key]
 
 
 def get_stderr_fileno() -> int: