python subprocess.Popen hanging

Question

            child = subprocess.Popen(command,
                         shell=True,
                         env=environment,
                         close_fds=True,
                         stdout=subprocess.PIPE,
                         stderr=subprocess.STDOUT,
                         bufsize=1,
                                     )

            subout = ""
            with child.stdout:
                for line in iter(child.stdout.readline, b''):
                    subout += line
            logging.info(subout)
            rc = child.wait()

some times (intermittently) this hangs forever. not sure if it hangs on iter(child.stdout.readline) or child.wait()

i ps -ef for the process it Popens and that process no longer exists

my guess is that it has do with bufsize so that child.stdout.readline is going on forever but i have no idea how to test it and as this happens intermittently

I could implement alarm but i m not sure if that's appropriate as i cant really tell whether the popen'd process is just slow or hanging

let's assume that either child.stdout.readline or wait() hangs forever, what actions could i take besides alarm ?

Answer 1

Add close_fds=True to the call to subprocess.Popen() as mentioned here.

Answer 2

You're likely hitting the deadlock that's explained in the documentation:

Popen.wait():

Wait for child process to terminate. Set and return returncode attribute.

Warning: 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.

The solution is to use Popen.communicate().