On Nov 6, 2:54 pm, Thomas Christensen <[EMAIL PROTECTED]>
wrote:
> This issue has been raised a couple of times I am sure. But I have yet
> to find a satisfying answer.
>
> I am reading from a subprocess and this subprocess sometimes hang, in
> which case a call to read() call will block indefinite, keeping me from
> killing it.
>
> The folloing sample code illustrates the problem:
>
> proc = subprocess.Popen(['/usr/bin/foo', '/path/to/some/file'],
> stdout=subprocess.PIPE)
> output = StringIO.StringIO()
> while True:
> r = select.select([proc.stdout.fileno()], [], [], 5)[0]
> if r:
> # NOTE: This will block since it reads until EOF
> data = proc.stdout.read()
> if not data:
> break # EOF from process has been reached
> else:
> output.write(data)
> else:
> os.kill(proc.pid, signal.SIGKILL)
> proc.wait()
>
> <Process the output...>
>
> As the NOTE: comment above suggests the call to read() will block here.
>
> I see two solutions:
>
> 1. Read one byte at a time, meaning call read(1).
> 2. Read non-blocking.
>
> I think reading one byte at a time is a waste of CPU, but I cannot find
> a way to read non-blocking.
>
> Is there a way to read non-blocking? Or maybe event a better way in
> generel to handle this situation?
>From what I understand, you want a way to abort waiting on a blocking
read if the process is hung.
There are some challenges about how you decide if the process is hung
or just busy doing work without generating output for a while (or may
be the system is busy and the process didn't get enough CPU due to
other CPU hungry processes).
Assuming you have a valid way to figure this out, one option is to
have a timeout on the read.
If the timeout exceeds, you abort the read call. No, the read doesn't
provide a timeout, you can build one using alarm.
def alarm_handler(*args):
""" This signal stuff may not work in non unix env """
raise Exception("timeout")
signal.signal(signal.SIGALRM, alarm_handler)
try:
signal.alarm(timeout) # say timeout=60 for a max wait of 1
minute
data = proc.stdout.read()
except Exception, e:
if not str(e) == 'timeout': # something else went wrong ..
raise
# got the timeout exception from alarm .. proc is hung; kill it
Karthik
>
> Thanks
>
> Thomas
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