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POWhy does a background task block the response in SimpleHTTPServer?
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copied!<p>I'm writing a simple browser-based front end that should be able to launch a background task and then get progress from it. I want the browser to receive a response saying whether the task launched successfully, and then poll to determine when it is done. However, the presence of a background task seems to be stopping the XMLHttpRequest response from being sent immediately, so I can't report the success of launching the process. Consider the following (simplified) code:</p> <pre><code>import SocketServer import SimpleHTTPServer import multiprocessing import time class MyProc(multiprocessing.Process): def run(self): print 'Starting long process..' for i in range(100): time.sleep(1) print 'Done long process' class Page(SimpleHTTPServer.SimpleHTTPRequestHandler): def do_GET(self): if self.path == '/': print >>self.wfile, "<html><body><a href='/run'>Run</a></body></html>" if self.path == '/run': self.proc = MyProc() print 'Starting..' self.proc.start() print 'After start.' print >>self.wfile, "Process started." httpd = SocketServer.TCPServer(('', 8000), Page) httpd.serve_forever() </code></pre> <p>When I run this, and browse to <a href="http://localhost:8000" rel="nofollow">http://localhost:8000</a>, I get a button named "Run". When I click on it, the terminal displays:</p> <pre><code>Starting.. After start. </code></pre> <p>However the browser view does not change.. in fact the cursor is spinning. Only when I press Ctrl-C in the terminal to interrupt the program, then the browser is update with the message <code>Process started.</code></p> <p>The message <code>After start</code> is clearly being printed. Therefore I can assume that <code>do_GET</code> is returning after starting the process. Yet, the browser doesn't get a response until after I interrupt the long-running process. I have to conclude there is something blocking between <code>do_GET</code> and the response being sent, which is inside <code>SimpleHTTPServer</code>.</p> <p>I've also tried this with threads and subprocess.Popen but ran into similar problems. Any ideas?</p>

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