How to detect the child's exit when waiting in select() for the anonymous pipe to become readable?

Question

My python program creates a pipe, forks, and then spawns another program from the child. The parent then sits and waits for the reader-side of the pipe to become readable.

    reader, writer = os.pipe()
    fcntl.fcntl(reader, fcntl.F_SETFL, os.O_NONBLOCK)
    child = os.fork()
    if child == 0:
        os.close(reader)
        os.execvp('program', ['program', '-o', '/dev/fd/%d' % writer])

    while True:
        if os.waitpid(child, os.WNOHANG) != (0, 0):
            break
        logger.debug('Going into select')
        r, w, x = select.select([reader], [], [])
        .....

For some reason, when the spawn child exits, the parent continues to wait in the select... Indefinitely... How is one supposed to detect this situation?

Did you try to close the writer in the parent (select might be deadlocked by parent's writer)? — jerry, May 13 '20 at 01:53
Yes, that was it... I came back here to report it and saw your comment. If you make it a proper answer, I'll accept it. Thanks! — Mikhail T., May 13 '20 at 01:59

score 1 · Accepted Answer · answered May 13 '20 at 02:02

1

There is a deadlock since parent process' writer is not closed before going to select. You could also close the writer in the parent process:

if child == 0:
    os.close(reader)
    ...
else:
    os.close(writer)

answered May 13 '20 at 02:02

jerry

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How to detect the child's exit when waiting in select() for the anonymous pipe to become readable?

1 Answers1