subprocess.Popen does not close pipe in an error case

[Steven K. Wong]

Below, I have a Python script that launches 2 child programs, prog1
and prog2, with prog1's stdout connected to prog2's stdin via a pipe.
(It's like executing "prog1 | prog2" in the shell.)

If both child programs exit with 0, then the script runs to
completion. But if prog2 exits with non-0, prog1 does not exit and the
script hangs (i.e. prog1.poll() always returns None) -- unless I
uncomment the 2 lines marked by XXX to close prog1.stdout.

I was expecting that I don't have to explicitly close prog1.stdout,
whether prog2 succeeds or fails. Is the current behavior a bug in the
subprocess module or is it expected? Or am I doing something wrong?

Thanks.

import subprocess
import time

# prog1: a program that writes lots of data to the pipe
cmd = ['zcat', '--force', 'a_large_file']
prog1 = subprocess.Popen(cmd, bufsize=-1, stdout=subprocess.PIPE)

# prog2: a program that fails without reading much data from the pipe
cmd = ['python', '-c', 'import time; time.sleep(10); asdf']
prog2 = subprocess.Popen(cmd, bufsize=-1, stdin=prog1.stdout,
        stdout=open('popen.out', 'w'))
print 'waiting for a while'

retCodeProg2 = prog2.wait()
print 'prog2 returns', retCodeProg2
# XXX
# if retCodeProg2 != 0:
#     prog1.stdout.close()
while prog1.poll() is None:
    print 'sleep a bit'
    time.sleep(1)
retCodeProg1 = prog1.poll()
print 'prog1 returns', retCodeProg1