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Editorial Team
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Asked: 2026-05-25T18:47:07+00:00

I need to launch a pdftk process while serving a web request in Django,

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I need to launch a pdftk process while serving a web request in Django, and wait for it to finish. My current pdftk code looks like this:

proc = subprocess.Popen(["/usr/bin/pdftk", 
                         "/tmp/infile1.pdf", 
                         "/tmp/infile2.pdf", 
                         "cat", "output", "/tmp/outfile.pdf"])    
proc.communicate()

This works fine, as long as I’m executing under the dev server (running as user www-data). But as soon as I switch to mod_wsgi, changing nothing else, the code hangs at proc.communicate(), and “outfile.pdf” is left as an open file handle of zero length.

I’ve tried a several variants of the subprocess invocation (as well as plain old os.system) — setting stdin/stdout/stderr to PIPE or to various file handles changes nothing. Using “shell=True” prevents proc.communicate() from hanging, but then pdftk fails to create the output file, both under the devserver or mod_wsgi. This discussion seems to indicate there might be some deeper voodoo going on with OS signals and pdftk that I don’t understand.

Are there any workarounds to get a subprocess call like this to work properly under wsgi? I’m avoiding using PyPDF to combine pdf files, because I have to combine large enough numbers of files (several hundred) that it runs out of memory (PyPDF needs to keep every source pdf file open in memory while combining them).

I’m doing this under recent Ubuntu, pythons 2.6 and 2.7.

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1 Answer

  1. Editorial Team

    Try with absolute file system paths to input and output files. The current working directory under Apache will not be same directory as run server and could be anything.

    Second attempt after eliminating the obvious.

    The pdftk program is a Java program which is relying on being able to generate/receive SIGPWR signal to trigger garbage collection or perform other actions. Problem is that under Apache/mod_wsgi in daemon mode, signals are blocked within the request handler threads to ensure that they are only received by the main thread looking for process shutdown trigger events. When you are forking the process to run pdftk, it is unfortunately inheriting the blocked sigmask from the request handler thread. The consequence of this is that it impedes the operation of the Java garbage collection process and causes pdftk to fail in strange ways.

    The only solution for this is to use Celery and have the front end submit a job to the Celery queue for celeryd to then fork and execute pdftk. Because this is then done from a process created distinct from Apache, you will not have this issue.

    For more gory details Google for mod_wsgi and pdftk, in particular in Google Groups.

    http://groups.google.com/group/modwsgi/search?group=modwsgi&q=pdftk&qt_g=Search+this+group

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