stop using pthread_cancel() #2619
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I used much time to reproducer for #2615.
I saw many strange "defunct" (zombie) workers. The master called
waitpid(-1, ...)but it return 0 even there are some zombies. Finally, master sends KILL signal (MERCY) and worker is restarted.I believe this strange zombie was born from pthread_cancel. Subthreads calls pthread_cancel() for main thread and it cause strange process.
pthread_cancel() is very hard to use and debug. I can not even attach the strange zombie with
gdb --pid. I think it is not maintainable.