gh-91555: disable logger while handling log record (GH-131812)

Prevent the possibility of re-entrancy leading to deadlock or infinite recursion (caused by logging triggered by logging), by disabling logging while the logger is handling log messages.

Lib/logging/__init__.py

@@ -1474,6 +1474,8 @@ class Logger(Filterer):
     level, and "input.csv", "input.xls" and "input.gnu" for the sub-levels.
     There is no arbitrary limit to the depth of nesting.
     """
+    _tls = threading.local()
+
     def __init__(self, name, level=NOTSET):
         """
         Initialize the logger with a name and an optional level.
@@ -1670,14 +1672,19 @@ class Logger(Filterer):
         This method is used for unpickled records received from a socket, as
         well as those created locally. Logger-level filtering is applied.
         """
-        if self.disabled:
+        if self._is_disabled():
             return
-        maybe_record = self.filter(record)
+
-        if not maybe_record:
+        self._tls.in_progress = True
-            return
+        try:
-        if isinstance(maybe_record, LogRecord):
+            maybe_record = self.filter(record)
-            record = maybe_record
+            if not maybe_record:
-        self.callHandlers(record)
+                return
+            if isinstance(maybe_record, LogRecord):
+                record = maybe_record
+            self.callHandlers(record)
+        finally:
+            self._tls.in_progress = False
 
     def addHandler(self, hdlr):
         """
@@ -1765,7 +1772,7 @@ class Logger(Filterer):
         """
         Is this logger enabled for level 'level'?
         """
-        if self.disabled:
+        if self._is_disabled():
             return False
 
         try:
@@ -1815,6 +1822,11 @@ class Logger(Filterer):
                        if isinstance(item, Logger) and item.parent is self and
                        _hierlevel(item) == 1 + _hierlevel(item.parent))
 
+    def _is_disabled(self):
+        # We need to use getattr as it will only be set the first time a log
+        # message is recorded on any given thread
+        return self.disabled or getattr(self._tls, 'in_progress', False)
+
     def __repr__(self):
         level = getLevelName(self.getEffectiveLevel())
         return '<%s %s (%s)>' % (self.__class__.__name__, self.name, level)

Lib/test/test_logging.py

@@ -4214,6 +4214,89 @@ class ConfigDictTest(BaseTest):
         handler = logging.getHandlerByName('custom')
         self.assertEqual(handler.custom_kwargs, custom_kwargs)
 
+    # See gh-91555 and gh-90321
+    @support.requires_subprocess()
+    def test_deadlock_in_queue(self):
+        queue = multiprocessing.Queue()
+        handler = logging.handlers.QueueHandler(queue)
+        logger = multiprocessing.get_logger()
+        level = logger.level
+        try:
+            logger.setLevel(logging.DEBUG)
+            logger.addHandler(handler)
+            logger.debug("deadlock")
+        finally:
+            logger.setLevel(level)
+            logger.removeHandler(handler)
+
+    def test_recursion_in_custom_handler(self):
+        class BadHandler(logging.Handler):
+            def __init__(self):
+                super().__init__()
+            def emit(self, record):
+                logger.debug("recurse")
+        logger = logging.getLogger("test_recursion_in_custom_handler")
+        logger.addHandler(BadHandler())
+        logger.setLevel(logging.DEBUG)
+        logger.debug("boom")
+
+    @threading_helper.requires_working_threading()
+    def test_thread_supression_noninterference(self):
+        lock = threading.Lock()
+        logger = logging.getLogger("test_thread_supression_noninterference")
+
+        # Block on the first call, allow others through
+        #
+        # NOTE: We need to bypass the base class's lock, otherwise that will
+        #       block multiple calls to the same handler itself.
+        class BlockOnceHandler(TestHandler):
+            def __init__(self, barrier):
+                super().__init__(support.Matcher())
+                self.barrier = barrier
+
+            def createLock(self):
+                self.lock = None
+
+            def handle(self, record):
+                self.emit(record)
+
+            def emit(self, record):
+                if self.barrier:
+                    barrier = self.barrier
+                    self.barrier = None
+                    barrier.wait()
+                    with lock:
+                        pass
+                super().emit(record)
+                logger.info("blow up if not supressed")
+
+        barrier = threading.Barrier(2)
+        handler = BlockOnceHandler(barrier)
+        logger.addHandler(handler)
+        logger.setLevel(logging.DEBUG)
+
+        t1 = threading.Thread(target=logger.debug, args=("1",))
+        with lock:
+
+            # Ensure first thread is blocked in the handler, hence supressing logging...
+            t1.start()
+            barrier.wait()
+
+            # ...but the second thread should still be able to log...
+            t2 = threading.Thread(target=logger.debug, args=("2",))
+            t2.start()
+            t2.join(timeout=3)
+
+            self.assertEqual(len(handler.buffer), 1)
+            self.assertTrue(handler.matches(levelno=logging.DEBUG, message='2'))
+
+            # The first thread should still be blocked here
+            self.assertTrue(t1.is_alive())
+
+        # Now the lock has been released the first thread should complete
+        t1.join()
+        self.assertEqual(len(handler.buffer), 2)
+        self.assertTrue(handler.matches(levelno=logging.DEBUG, message='1'))
 
 class ManagerTest(BaseTest):
     def test_manager_loggerclass(self):

Misc/NEWS.d/next/Library/2025-03-30-16-42-38.gh-issue-91555.ShVtwW.rst

@@ -0,0 +1,2 @@
+Ignore log messages generated during handling of log messages, to avoid
+deadlock or infinite recursion.