python - ThreadPoolExecutor hangs when call futur result in a futur callback -


i using "requests-futures" package , call asynchronous get/post in asynchronous get/post result callback (add_done_callback on futur result). sometimes, code hangs. after many investigation hours, can reproduce lock minimal code: 

from concurrent.futures import threadpoolexecutor import time  pool = threadpoolexecutor(max_workers=10)  def f(_):     time.sleep(0.1) # try force context switch     x = pool.submit(lambda: none)     print "1"     x.result()     print "2"  def main():     x = pool.submit(lambda : none)     x.add_done_callback(f)     print "3"     x.result()     print "4"  print "===" main()

if run peace of code in bash loop:

$> while true; python code.py; done;

the program hangs every times "trace": 

(...) === 1 2 3 4 === 3 4 1

if break ctrl^c, have following stack trace:

^cerror in atexit._run_exitfuncs: traceback (most recent call last):   file "/usr/lib/python2.7/atexit.py", line 24, in _run_exitfuncs     func(*targs, **kargs)   file "/home/yienyien/angus/test/futur/env/local/lib/python2.7/site- packages/concurrent/futures/thread.py", line 46, in _python_exit     t.join(sys.maxint)   file "/usr/lib/python2.7/threading.py", line 951, in join     self.__block.wait(delay)   file "/usr/lib/python2.7/threading.py", line 359, in wait     _sleep(delay) keyboardinterrupt error in sys.exitfunc: traceback (most recent call last):   file "/usr/lib/python2.7/atexit.py", line 24, in _run_exitfuncs     func(*targs, **kargs)   file "/home/yienyien/angus/test/futur/env/local/lib/python2.7/site- packages/concurrent/futures/thread.py", line 46, in _python_exit     t.join(sys.maxint)  file "/usr/lib/python2.7/threading.py", line 951, in join     self.__block.wait(delay)   file "/usr/lib/python2.7/threading.py", line 359, in wait     _sleep(delay) keyboardinterrupt

somebody explain me happening ? check the possible deadlocks in concurrent.futures module, not think matches.

thank you.

tasks submitted fixed-sized thread pool may not call blocking operations future.result(). leads specific kind of deadlock, called "thread starvation". using time.sleep() switches thread off service , increases probability of thread starvation.


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