CODEWITHSUNDEEP
pythonsubprocess
Chris Lieb
Chris Lieb

Reputation: 3836

Reliably terminating subprocess on Python 2.5

I have a script that repeatedly runs an Ant buildfile and scrapes output into a parsable format. When I create the subprocess using Popen, there is a small time window where hitting Ctrl+C will kill the script, but will not kill the subprocess running Ant, leaving a zombie that is printing output to the console that can only be killed using Task Manager. Once Ant has started printing output, hitting Ctrl+C will always kill my script as well as Ant. Is there a way to make it so that hitting Ctrl+C will always kill the subprocess running Ant without leaving a zombie behind?

Also of note: I have a handler for SIGINT that performs a few cleanup operations before calling exit(0). If I manually kill the subprocess in the handler using os.kill(p.pid, signal.SIGTERM) (not SIGINT), then I can successfully kill the subprocess in situations where it would normally zombify. However, when you hit Ctrl+C once Ant has started producing output, you get a stacktrace from subprocess where it is unable to kill the subprocess itself as I have already killed it.

EDIT: My code looked something like:

p = Popen('ls')
def handle_sig_int(signum, stack_frame):
    # perform cleanup
    os.kill(p.pid, signal.SIGTERM)
    exit(0)
signal.signal(signal.SIGINT, handle_sig_int)

p.wait()

Which would produce the following stacktrace when triggered incorrectly:

File "****.py", line ***, in run_test
  p.wait()
File "/usr/lib/python2.5/subprocess.py", line 1122, in wait
  pid, sts = os.waitpid(self.pid, 0)
File "****.py", line ***, in handle_sig_int
  os.kill(p.pid, signal.SIGTERM)

I fixed it by catching the OSError raised by p.wait and exiting:

try:
    p.wait()
except OSError:
    exit('The operation was interrupted by the user')

This seems to work in the vast majority of my test runs. I occasionally get a uname: write error: Broken pipe, though I don't know what causes it. It seems to happen if I time the Ctrl+C just right before the child process can start displaying output.

Upvotes: 1

Views: 1611

Answers (1)

Aaron Digulla
Aaron Digulla

Reputation: 328566

Call p.terminate() in your SIGTERM handler:

 if p.poll() is None: # Child still around?
     p.terminate() # kill it

[EDIT] Since you're stuck with Python 2.5, use os.kill(p.pid, signal.SIGTERM) instead of p.terminate(). The check should make sure you don't get an exception (or reduce the number of times you get one).

To make it even better, you can catch the exception and check the message. If it means "child process not found", then ignore the exception. Otherwise, rethrow it with raise (no arguments).

Upvotes: 2

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