Issue 7448: when piping output between subprocesses some fd is left open blocking forever (original) (raw)

Suppose I want to simulate the following shell pipe using the subprocess module:

grep -v not | cut -c 1-10

The documentation example here http://docs.python.org/library/subprocess.html#replacing-shell-pipeline Implies that I want to run this:

grep_process = Popen(["grep", "-v", "not"], stdin=PIPE, stdout=PIPE) cut_process = Popen(["cut", "-c", "1-10"], stdin=p1.stdout, stdout=PIPE) grep_process.stdin.write('Hello World\n') grep_process.stdin.close() result = cut_process.stdout.read() # blocks forever here assert result == "Hello Worl\n"

When grep_prcoess starts, two file descriptors are created, one for each end of the pipe. Lets call those grep-stdin-w and grep-stdout-r. When I run cut_process, grep-stdout-r gets passed as cut_process sdtin. Since close_fds=False by default, The effect of that is that cut_process also inherits grep-stdin-w. So grep can't die even if I explicty run grep_process.stdin.close() because grep-stdin-w stdin is still inside cut_process (cut ignores the extra open fd).

Passing close_fds=True to the second Popen call makes the code work perfectly. close_fds=True should be the default on unix systems, because it closes files descriptors that have nothing to do with the process, leaving stdin, stdout and stderr working, which is the intended behaviour of most code.