[New-bugs-announce] [issue41503] Race between setTarget and flush in logging.handlers.MemoryHandler

[Irit Katriel]

New submission from Irit Katriel <iritkatriel at yahoo.com>:

The `logging.handlers.MemoryHandler.setTarget()` method does not acquire the lock, so it can change the target while flush is processing the buffer.

The script below causes flush to call target.handle when target is None, causing:

  File "C:\Users\User\src\cpython\lib\logging\handlers.py", line 1265, in emit
    self.flush()
  File "C:\Users\User\src\cpython\lib\logging\handlers.py", line 1348, in flush
    self.target.handle(record)
AttributeError: 'NoneType' object has no attribute 'handle'


------------------------------------------------
import io
import logging.handlers
import threading
import time

class SlowHandler:
    def __init__(self):
        self.stream = io.StringIO()
        self.streamHandler = logging.StreamHandler(self.stream)

    def handle(self, msg):
        time.sleep(1)
        self.streamHandler.handle(msg)

target = SlowHandler()
mem_hdlr = logging.handlers.MemoryHandler(10, logging.ERROR, target)
mem_logger = logging.getLogger('mem')
mem_logger.propagate = False
mem_logger.addHandler(mem_hdlr)

def toggleTarget():
    time.sleep(1)
    mem_hdlr.setTarget(None)

t = threading.Thread(target=toggleTarget, args=())
t.daemon = True
t.start()

while True:
    time.sleep(0.1)
    mem_logger.warning("warning not flushed")
    mem_logger.error("error is flushed")

------------------------------------------------

----------
components: Library (Lib)
messages: 375001
nosy: iritkatriel
priority: normal
severity: normal
status: open
title: Race between setTarget and flush in logging.handlers.MemoryHandler
type: behavior
versions: Python 3.10

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41503>
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