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POWhy is Pexpect intermittently hanging (not detecting EOF) after executing certain commands?
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copied!<p><strong>Context:</strong></p> <p>I have some code written using <code>pexpect</code>, whose job is to give "live" output of a command. I.e. print something out when a command generates some output, or soon after, rather than waiting until the command completes and then interacting with its output. </p> <p>All I'm doing is starting and stopping a service. I do this by <code>spawn</code>ing the process, and then outputting each line as it is printed, like so:</p> <pre><code>def watch(process): output = "" while True: try: line = process.read_nonblocking(timeout = -1) print(line, end ="") output += line except pexpect.EOF: break del process return output while True: print("IN 1") process = pexpect.spawn("service",["zend-server", "stop"], timeout = None) watch(process) print("OUT 1") print("IN 2") process = pexpect.spawn("service",["zend-server", "start"], timeout = None) watch(process) print("OUT 2") </code></pre> <p>This code should just loop the service: start it and stop it over and over, printing the output of the start/stop as it goes. It prints the output fine. However, it eventually hangs right before "OUT 2". I can view the output, and see the <code>service</code> call stop its execution. The <code>watch</code> function never raises an EOF and exits, however. </p> <p>This doesn't happen with every service. Some services loop indefinitely. <code>zend-server</code>, however, along with a few other unrelated commands, fail intermittently in the same way. </p> <p>By "eventually hangs", I mean that it starts/stops the service a few (variable on each run) times, and hangs. It usually gums up after 4-6, though never on the first call--always at least on the second (hence the <code>del</code> statement; I figured I'd play it safe). </p> <p>Python 2.6.6, CentOS (64) 6.3, Pexpect 2.3-6, FWIW</p> <p><strong>Question:</strong></p> <p>Why is <code>pexpect</code> hanging on certain commands? How should I resolve this issue? Using timeouts isn't a feasible solution, as some of these commands really can run for an arbitrarily long time. <code>service zend-server stop</code> is just the one I picked for an example because it doesn't take that long, and I can observe it finishing. </p> <p><strong>What I've Tried:</strong></p> <p>I've tried replacing the <code>watch</code> method with the following, which uses <code>expect('\n')</code>, but the results are the same: a variable number of restarts, and then an eventual hang. </p> <p>I can also add <code>pexpect.EOF</code> into the array that is <code>expect</code>ed along with <code>\n</code>, and handle the return value to break out of the loop, it still hangs in the same place. </p> <pre><code>def watch2(process): output = "" done = False while True: try: if not process.isalive(): line = process.readline() done = True else: process.expect(['\n']) line = process.before print(line) output += line if done: raise pexpect.EOF(0) except pexpect.EOF: break del process return output </code></pre>

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