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")

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

New changeset 2353d77fad7ed9d11d8a4d66b5dd1306cdb94125 by Irit Katriel in branch 'master':
bpo-41503: Fix race between setTarget and flush in logging.handlers.MemoryHandler (GH-21765)
https://github.com/python/cpython/commit/2353d77fad7ed9d11d8a4d66b5dd1306cdb94125

New changeset 2c050e52f1ccf5db03819e4ed70690521d67e9fa by Miss Islington (bot) in branch '3.9':
[3.9] bpo-41503: Fix race between setTarget and flush in logging.handlers.MemoryHandler (GH-21765) (GH-21897)
https://github.com/python/cpython/commit/2c050e52f1ccf5db03819e4ed70690521d67e9fa

New changeset 08f0a2109297e8a64e8636d47dce737e5b7ccf2c by Miss Islington (bot) in branch '3.8':
[3.8] bpo-41503: Fix race between setTarget and flush in logging.handlers.MemoryHandler (GH-21765) (GH-21898)
https://github.com/python/cpython/commit/08f0a2109297e8a64e8636d47dce737e5b7ccf2c