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POIO#read blocks on non-blocking socket?
 

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  1. POIO#read blocks on non-blocking socket?
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    <p>Ruby 1.8.7. I'm calling read on a socket which has been opened and connected with: </p> <pre><code>socket = Socket.new(AF_INET, SOCK_STREAM, 0) sockaddr = Socket.sockaddr_in(mp.port, mp.ip_address.ip) begin socket.connect_nonblock(sockaddr) [...] </code></pre> <p>The connection is confirmed by calling select() and then connecting a second time looking for Errno::EISCONN.</p> <p>I then call select again with a 0 timeout and if the return is not nil I read from the socket, first confirming that it has O_NONBLOCK set:</p> <pre><code> rc = select([socket], nil, nil, 0) puts " select returned: #{rc.pretty_inspect}" if rc begin puts " reading: #{socket} nonblock: #{socket.fcntl(Fcntl::F_GETFL) &amp; Fcntl::O_NONBLOCK}" response = socket.read puts " done reading" [...] </code></pre> <p>This all happens in a loop once per minute. The output the first time through the loop is:</p> <pre><code>select returned: [[#&lt;Socket:0xb6e0dcb8&gt;], [], []] reading: #&lt;Socket:0xb6e0dcb8&gt; nonblock: 2048 done reading </code></pre> <p>However the second time through the loop hangs here:</p> <pre><code>select returned: [[#&lt;Socket:0xb6e0dcb8&gt;], [], []] reading: #&lt;Socket:0xb6e0dcb8&gt; nonblock: 2048 </code></pre> <p>Attaching gdb to the process shows this backtrace:</p> <p>0 0xffffe410 in __kernel_vsyscall ()<br> 1 0xb7e5539d in select () from /lib/tls/i686/cmov/libc.so.6<br> 2 0x08064368 in rb_thread_schedule () at eval.c:11020<br> 3 0x080785bb in io_fread ( </p> <p>Replacing the call to read with a call to rcvfrom_nonblock works, and interestingly it does not get EAGAIN, it actually reads data (as you would expect given the return from select).</p> <p>Any ideas?</p> <p>steve</p>
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    1. USSteveRawlinson
    1. USSteveRawlinson
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      1. USSirupsen
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      1. VTUpMod
    1. COI don't see in your code where the `mp` is set after the `select(2)`. Are you sure it's indexing to the same socket that's been signaled readable?
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      1. POIO#read blocks on non-blocking socket?
      1. USNikolai Fetissov
    2. COSorry that's an error that crept in while I was simplifying the code for the question. As you can see from the output (which is cut & paste) it's definitely the same socket
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      1. USSteveRawlinson
    3. COFrom the `gdb` trace it looks like a race in the Ruby interpreter itself.
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      1. POIO#read blocks on non-blocking socket?
      1. USNikolai Fetissov
 

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