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gh-79366: Fix a race condition when removing a logging handler #154528

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gh-79366: Fix a race condition when removing a logging handler #154528

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6 changes: 5 additions & 1 deletion Lib/logging/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1709,7 +1709,11 @@ def removeHandler(self, hdlr):
"""
with _lock:
if hdlr in self.handlers:
self.handlers.remove(hdlr)
# Replace the list instead of mutating it in place, so that
# callHandlers() can iterate it without a lock (gh-79366).
handlers = self.handlers.copy()
handlers.remove(hdlr)
self.handlers = handlers

def hasHandlers(self):
"""
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17 changes: 17 additions & 0 deletions Lib/test/test_logging.py
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Original file line number Diff line number Diff line change
Expand Up @@ -814,6 +814,23 @@ def lock_holder_thread_fn():

support.wait_process(pid, exitcode=0)

def test_remove_handler_while_emitting(self):
# Removing a handler while callHandlers() iterates over the handlers
# should not cause the following handlers to be skipped (gh-79366).
logger = logging.Logger('test_remove_handler_while_emitting')
calls = []
class RemovingHandler(logging.Handler):
def emit(self, record):
calls.append('removing')
logger.removeHandler(self)
class CountingHandler(logging.Handler):
def emit(self, record):
calls.append('counting')
logger.addHandler(RemovingHandler())
logger.addHandler(CountingHandler())
logger.error('spam')
self.assertEqual(calls, ['removing', 'counting'])


class BadStream(object):
def write(self, data):
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-07-23-06-53-01.gh-issue-79366.3gMT7I.rst
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@@ -0,0 +1,3 @@
Fixed a race condition in :mod:`logging`:
if a handler was removed while a record was being emitted,
the following handlers of the same logger could be skipped.
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