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POPython Catching Popen Output
 

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  1. POPython Catching Popen Output
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    <p>I'm experiencing some strange behavior with my Python script exiting without any error messages. Based on my debugging it is happening around a popen() call to scp a file to a server.</p> <p>The code was (I was not the original author):</p> <pre><code>logMessage(LEVEL_INFO, "copyto is " + copyto) pid = Popen(["scp", "-i", "/root/.ssh/id_rsa", "/usr/gpsw/gpslog" + self.node_addr, copyto], stdout=PIPE) __s = pid.communicate()[0] logMessage(LEVEL_INFO, "GPS log SCP complete") </code></pre> <p>In an attempt to debug I enhanced it to:</p> <pre><code>logMessage(LEVEL_INFO, "copyto is " + copyto) pid = Popen(["scp", "-i", "/root/.ssh/id_rsa", "/usr/gpsw/gpslog" + self.node_addr, copyto], stdout=PIPE, stderr=PIPE) out, err = pid.communicate() if out: print "[" + self.node_addr + "] stdout of pid: " + str(out) if err: print "[" + self.node_addr + "] stdout of pid: " + str(err) print "[" + self.node_addr + "] returncode of pid: " + str(pid.returncode) logMessage(LEVEL_INFO, "GPS log SCP complete") </code></pre> <p>Here is my console output: (The pattern should be 5dda77, 5dd9fa, 5dda0d repeatedly. This is based on physical events)</p> <pre><code>[5dda77] returncode of pid: 0 [5dd9fa] returncode of pid: 0 [5dda0d] returncode of pid: 0 [5dda77] returncode of pid: 0 [5dd9fa] returncode of pid: 0 [5dda0d] returncode of pid: 0 # (the script exited and I'm back at the prompt) </code></pre> <p>And here is my log output:</p> <pre><code>INFO copyto is root@192.168.20.1:/usr/scu/datafiles/gpslog5dda77 INFO GPS log SCP complete INFO copyto is root@192.168.20.1:/usr/scu/datafiles/gpslog5dd9fa INFO GPS log SCP complete INFO copyto is root@192.168.20.1:/usr/scu/datafiles/gpslog5dda0d INFO GPS log SCP complete INFO copyto is root@192.168.20.1:/usr/scu/datafiles/gpslog5dda77 INFO GPS log SCP complete INFO copyto is root@192.168.20.1:/usr/scu/datafiles/gpslog5dd9fa INFO GPS log SCP complete INFO copyto is root@192.168.20.1:/usr/scu/datafiles/gpslog5dda0d INFO GPS log SCP complete INFO copyto is root@192.168.20.1:/usr/scu/datafiles/gpslog5dda77 </code></pre> <p>So based on the log and output data. I believe something is going wrong during the SCP because the Python script crashes before it logs "GPS log SCP complete". What is interesting though is I see on the server end the file was copied entirely. So two questions:</p> <ol> <li>Am I using popen incorrectly?</li> <li>Why am I not seeing any error messages in stderr?</li> </ol> <p>Thanks </p> <p><strong>EDIT:</strong> This script should <em>never</em> exit. There is no "normal exit" of the process. It should run indefinitely servicing requests from the server to retrieve data from GPS nodes that are available. </p> <p>The main loop code is:</p> <pre><code>stop_flags = 0 ... (API for server and GPS nodes to interact with) def main(): ... (System initialization) while stop_flags == 0: # listen for messages xmlrpcserver.handle_request() comm.poll() if stop_flags == STOP_FLAG_RESTART: # suppress Pylint warning for reimport of Popen,PIPE # pylint: disable-msg=W0404 from subprocess import Popen, PIPE # use this instead of call to suppress output pid = Popen(["/etc/init.d/S999snap", "restart"], stdout=PIPE) __s = pid.communicate()[0] if stop_flags == STOP_FLAG_REBOOT: # suppress Pylint warning for reimport of Popen,PIPE # pylint: disable-msg=W0404 from subprocess import Popen, PIPE # use this instead of call to suppress output pid = Popen(["reboot"], stdout=PIPE) __s = pid.communicate()[0] if __name__ == '__main__': main() </code></pre>
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    1. COAre you sure there is any stderr to capture? Have you tried running the failing command manually from bash to see, or are you just assuming that there must be something there?
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    2. COMeanwhile, what makes you think it "crashes"? Is there a segfault? Or an exception traceback? Most likely, it exits just fine, and the only reason `logMessage` isn't showing the last message is that it's some custom code you wrote that never flushes closes the log file.
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      1. USabarnert
    3. COHow do you know python is crashing? You say there is no error messages and the scp was successful, so I'm curious why you think it is crashing.
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      1. USSethMMorton
 

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