Context: You create a Popen(cmd, stdout=PIPE, stderr=PIPE), and you let it run (with a timeout); sometimes it completes successfully, which is cool, and sometimes it doesn't, in which case you read stdout and stderr and try to figure out what went wrong. This is all fine and dandy until one day you add a *little* bit more logging to the tool you're calling, and it suddenly wedges forever.
Explanation: Of course, this is because you've filled up some buffer, which you should have been periodically emptying. However, you can't just select() and read one byte at a time, because select doesn't work with pipes on Win32; you can't just read() what's there, because it blocks and stops the timeout from working; you don't want to spin off another thread to do your reading, because that involves tedious extra code and feels like killing a fly with a sledgehammer; and you don't want to screw around with readline() because that also involves tedious bookkeeping and extra code.
Solution: So, just do the following (warning, coded from memory):
from subprocess import Popen
from tempfile import TemporaryFile
from time import time, sleep
def assert_runs(cmd, timeout=10):
out = TemporaryFile()
err = TemporaryFile()
end_time = time() + timeout
process = Popen(cmd, stdout=out, stderr=err)
while process.poll() is None:
sleep(0.1)
if time() > end_time:
process.terminate()
if process.returncode != 0:
raise AssertionError('%s FAILED (%s)\nstdout:\n%s\nstderr:\n%s' % (
cmd, process.returncode, out.read(), err.read()))
Indeed, it's icky to fill up your hard disk rather than some internal buffer, but you can get a lot more done before you run out of HD space. Now, this surely feels nasty, but IMO it's slightly less nasty (or, at least, less code) than anything else I've tried. Hopefully you, gentle reader, have an infinitely superior solution that you will detail in the comments. Surprise me!
Please note that "Don't use Windows, har har", and variants thereof, fail to qualify as "surprising" ;-).
