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    <p>Aha, subtle.</p> <p>There is a hidden, blocking <code>ensure</code> clause at the end of the IO#popen block in the second case. The Timeout::Error <em>is</em> raised raised timely, but you cannot <code>rescue</code> it until execution returns from that implicit <code>ensure</code> clause.</p> <p><a href="http://rxr.whitequark.org/mri/ident?v=1.9.3;i=rb_io_s_popen" rel="nofollow noreferrer">Under the hood</a>, <code>IO.popen(cmd) { |io| ... }</code> does something like this:</p> <pre><code>def my_illustrative_io_popen(cmd, &amp;block) begin pio = IO.popen(cmd) block.call(pio) # This *is* interrupted... ensure pio.close # ...but then control goes here, which blocks on cmd's termination end </code></pre> <p>and the IO#close call is really more-or-less a <code>pclose(3)</code>, which is blocking you in <code>waitpid(2)</code> until the sleeping child exits.</p> <p>You can verify this like so:</p> <pre><code>#!/usr/bin/env ruby require 'timeout' BEGIN { $BASETIME = Time.now.to_i } def xputs(msg) puts "%4.2f: %s" % [(Time.now.to_f - $BASETIME), msg] end begin Timeout.timeout(3) do begin xputs "popen(sleep 10)" pio = IO.popen("sleep 10") sleep 100 # or loop over pio.gets or whatever ensure xputs "Entering ensure block" #Process.kill 9, pio.pid # &lt;--- This would solve your problem! pio.close xputs "Leaving ensure block" end end rescue Timeout::Error =&gt; ex xputs "rescuing: #{ex}" end </code></pre> <p><strong>So, what can you do?</strong></p> <p>You'll have to do it the explicit way, since the interpreter doesn't expose a way to override the IO#popen <code>ensure</code> logic. You can use the above code as a starting template and uncomment the <code>kill()</code> line, for example.</p>
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    1. COI was staring for so long at io.c, looking just a few lines *above* the ensure, without seeing the ensure or considering it at all. Great answer, thanks so much.
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