Python3 Subprocess and Rsync Deadlock Strace Timeout

I recently came across a tough to debug issue where I was calling a shell script from python using the subprocess module, this shell script called rsync, no matter what I would always run into a timeout situation. I fired up strace and noticed that the process was in a timeout state.

select(4, NULL, [3], [3], {60, 0}) = 0 (Timeout)

I looked at the subprocess documentation and apparently using pipes will fill the system pipe buffer.


This will deadlock when using stdout=PIPE and/or stderr=PIPE and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use communicate() to avoid that.

I was baffled, I finally took the approach to eliminate stderr and stdout and just check the return status of the command using run(). Here is what I finally came up with, and all was well.

Hope you find this and it helps you.

One thought on “Python3 Subprocess and Rsync Deadlock Strace Timeout”

  1. run() is less flexible, it just does the following for you under the hood.

    proc = subprocess.Popen([
    ], stdout=subprocess.PIPE)
    proc_stdout, proc_stderr = proc.communicate()

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