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<h1 class="article-title">Graceful handling of <code>SIGINT</code> when using Python's <code>multiprocessing.Process</code></h1>
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<time class="article-timestamp" datetime="2015-05-05T22:03:39-07:00">May 5, 2015</time>
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<p>Today I learned something about Python's (at least CPython's) multiprocessing and signal handling, and I would like to share it here. Basically my situation was such (when developing <a href="https://github.com/zmwangx/zmwangx.github.io/blob/source/pyblog"><code>pydoc</code></a> that powers this blog):</p>
<ul>
<li>I would like to serve the blog with an HTTP server while auto-regenerating for changes;</li>
<li>The auto-regeneration is handled in the main process with a while loop, whereas the HTTP server (requiring little human intervention) is put in a <code>multiprocessing.Process</code> and launched with <code>http.server.HTTPServer.serve_forever()</code>;</li>
<li>Upon sending <code>SIGINT</code>, both processes need to clean up and quit; in particular, the server needs to exit its <code>serve_forever()</code> loop (which can be done via <code>shutdown()</code>, but how to invoke the method is a problem, since <code>serve_forever()</code> blocks);</li>
<li>Handling of <code>SIGINT</code> must be graceful in the main process — there might be an ongoing build that must not be interrupted until finishing.</li>
</ul>
<p>Given this context, I learned the following two critical concepts (at least true in the current version of CPython) through trial and error:</p>
<ol style="list-style-type: decimal">
<li><strong>A user-triggered <code>SIGINT</code> is sent to both processes</strong> — the main process and the <code>multiprocessing.Process</code> instance;</li>
<li><strong>Except for the defined interfaces, a <code>multiprocessing.Process</code> instance is almost completely separated from the main process, sharing as little resources as possible</strong>; by "defined interfaces" I mean the defined attributes and methods of a <code>Process</code> instance, as well as defined communication channels like <code>multiprocessing.Pipe</code> or <code>multiprocessing.Queue</code>. And to expand on resource sharing: yes, the two processes have their own copies of global variables, so using global variables as state registers is a no-go.</li>
</ol>
<p>Both concepts can be used to one's benefit or detriment. Below is how I solved my problem, using the two concepts. Observe that without a custom handler, Python translates a <code>SIGINT</code> to a <code>KeyboardInterrupt</code> exception; therefore, I use the default <code>KeyboardInterrupt</code> to interrupt the HTTP server in its own process (through handling the exception and calling <code>shutdown()</code>), but instead install a custom <code>SIGINT</code> handler in the main process that translates <code>SIGINT</code> to setting a <code>sigint_raised</code> flag that can be picked up by the while loop once the current build (if any) is finished. The proof of concept script is as follows (the production code is <a href="https://github.com/zmwangx/zmwangx.github.io/blob/a7a0b2073f30b1d0214c3152998d95e40a39b438/pyblog#L567-L635">here</a>):</p>
<div class="sourceCode"><pre class="sourceCode python"><code class="sourceCode python"><span class="co">#!/usr/bin/env python3</span>
<span class="im">import</span> http.server
<span class="im">import</span> multiprocessing
<span class="im">import</span> signal
<span class="im">import</span> sys
<span class="im">import</span> time
<span class="kw">class</span> HTTPServerProcess(multiprocessing.Process):
<span class="kw">def</span> run(<span class="va">self</span>):
httpd <span class="op">=</span> http.server.HTTPServer(
(<span class="st">""</span>, <span class="dv">8000</span>), http.server.SimpleHTTPRequestHandler)
<span class="cf">try</span>:
httpd.serve_forever()
<span class="cf">except</span> <span class="pp">KeyboardInterrupt</span>:
httpd.shutdown()
<span class="kw">def</span> do_things():
<span class="cf">for</span> i <span class="kw">in</span> <span class="bu">range</span>(<span class="dv">10</span>):
sys.stderr.write(<span class="st">"."</span>)
sys.stderr.flush()
time.sleep(<span class="dv">1</span>)
sys.stderr.write(<span class="st">"</span><span class="ch">\n</span><span class="st">"</span>)
<span class="kw">def</span> main():
server_process <span class="op">=</span> HTTPServerProcess()
server_process.start()
<span class="co"># define and install custom SIGINT handler</span>
sigint_raised <span class="op">=</span> <span class="va">False</span>
<span class="kw">def</span> sigint_mitigator(signum, frame):
<span class="kw">nonlocal</span> sigint_raised
sigint_raised <span class="op">=</span> <span class="va">True</span>
signal.signal(signal.SIGINT, sigint_mitigator)
<span class="cf">while</span> <span class="kw">not</span> sigint_raised:
do_things()
server_process.join()
<span class="cf">if</span> <span class="va">__name__</span> <span class="op">==</span> <span class="st">"__main__"</span>:
main()</code></pre></div>
<p>Beware that with this solution, if there are external programs or OS level operations happening in the main process, then the operation at the time of <code>SIGINT</code> will still be interrupted<a class="footnoteRef" href="#fn1" id="fnref1"><sup>1</sup></a> (for example, in the script above, the <code>time.sleep(1)</code> at the exact point of <code>SIGINT</code> is still interrupted, but otherwise <code>do_things</code> is carried on to its completion). I'm not sure how to explain this — maybe the handler isn't capturing the signal fast enough?<a class="footnoteRef" href="#fn2" id="fnref2"><sup>2</sup></a> Anyway, one single early interruption is at least more acceptable than a completely corrupted build<a class="footnoteRef" href="#fn3" id="fnref3"><sup>3</sup></a>, and certainly more graceful.</p>
<div class="footnotes">
<hr/>
<ol>
<li id="fn1"><p><a href="https://hg.python.org/cpython/file/1320ec1b24af/Modules/_multiprocessing">CPython's <code>multiprocessing</code> is written in C</a>, so the behavior might depend on the OS. I'm talking about OS X here. I haven't inspected and won't inspect the C source code.<a class="footnotes-backlink" href="#fnref1">↩︎</a></p></li>
<li id="fn2"><p>That's awfully naive and layman-sounding, I know, but I <em>am</em> almost a layman when it comes to system-level programming.<a class="footnotes-backlink" href="#fnref2">↩︎</a></p></li>
<li id="fn3"><p>That's assuming your build isn't interdependent in which any single failure corrupts everything. In that case, what can we do? I honestly see no way of injecting signal handling in <code>subprocess.Popen</code>.<a class="footnotes-backlink" href="#fnref3">↩︎</a></p></li>
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