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POAsynchronous KeyboardInterrupt and multithreading
 

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    <p>It seems that asynchronous signals in multithreaded programs are not correctly handled by Python. But, I thought I would check here to see if anyone can spot a place where I am violating some principle, or misunderstanding some concept.</p> <p>There are similar threads I've found here on SO, but none that seem to be quite the same.</p> <p>The scenario is: I have two threads, reader thread and writer thread (main thread). The writer thread writes to a pipe that the reader thread polls. The two threads are coordinated using a <code>threading.Event()</code> primitive (which I assume is implemented using <code>pthread_cond_wait</code>). The main thread waits on the <code>Event</code> while the reader thread eventually sets it.</p> <p>But, if I want to interrupt my program while the main thread is waiting on the <code>Event</code>, the KeyboardInterrupt is not handled asynchronously.</p> <p>Here is a small program to illustrate my point:</p> <pre><code>#!/usr/bin/python import os import sys import select import time import threading pfd_r = -1 pfd_w = -1 reader_ready = threading.Event() class Reader(threading.Thread): """Read data from pipe and echo to stdout.""" def run(self): global pfd_r while True: if select.select([pfd_r], [], [], 1)[0] == [pfd_r]: output = os.read(pfd_r, 1000) sys.stdout.write("R&gt; '%s'\n" % output) sys.stdout.flush() # Suppose there is some long-running processing happening: time.sleep(10) reader_ready.set() # Set up pipe. (pfd_r, pfd_w) = os.pipe() rt = Reader() rt.daemon = True rt.start() while True: reader_ready.clear() user_input = raw_input("&gt; ").strip() written = os.write(pfd_w, user_input) assert written == len(user_input) # Wait for reply -- Try to ^C here and it won't work immediately. reader_ready.wait() </code></pre> <p>Start the program with './bug.py' and enter some input at the prompt. Once you see the reader reply with the prefix 'R>', try to interrupt using <code>^C</code>.</p> <p>What I see (Ubuntu Linux 10.10, Python 2.6.6) is that the <code>^C</code> is not handled until after the blocking <code>reader_ready.wait()</code> returns. What I expected to see is that the <code>^C</code> is raised asynchronously, resulting in the program terminating (because I do not catch KeyboardInterrupt).</p> <p>This may seem like a contrived example, but I'm running into this in a real-world program where the <code>time.sleep(10)</code> is replaced by actual computation.</p> <p>Am I doing something obviously wrong, like misunderstanding what the expected result would be?</p> <p>Edit: I've also just tested with Python 3.1.1 and the same problem exists.</p>
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