[QGIS-Developer] [processing] launching separate process/thread

[Matthias Kuhn]

Hi Andreas,

I recently had troubles with getting stdout / stderr from
subprocess.Popen on some Windows systems. Not sure about the reasons but
switching over to QProcess solved it.

Matthias


On 5/21/17 11:55 PM, Andreas Plesch wrote:
> Looking around some more, there is also os.spawn and its recommended
> update subprocess.Popen.
>
> So
>
> os.chdir(root_folder)
> pid = subprocess.Popen(['python', '-m', 'SimpleHTTPServer',
> str(port)]).pid
>
> and later
> os.kill(pid, 7)
>
> works robustly in my tests. This seems similar to
> QProcess.startDetached which may provide better platform coverage (?).
>
> -Andreas
>
>  
>
>
> On Sun, May 21, 2017 at 9:30 AM, Andreas Plesch
> <andreasplesch at gmail.com <mailto:andreasplesch at gmail.com>> wrote:
>
>     Hi Giovanni,
>
>     thanks for the pointer. When I use multiprocessing.Process instead
>     of threading.Thread I get:
>
>     PicklingError: Can't pickle <function webserver at 0x1622F330>:
>     it's not found as __main__.webserver
>
>     This is probably related to:
>
>     https://docs.python.org/2/library/multiprocessing.html#windows
>     <https://docs.python.org/2/library/multiprocessing.html#windows>
>
>     So I am not sure how one would use multiprocessing from a
>     processing script rather than from the __main__ python program. 
>
>     -Andreas
>
>
>     On Sun, May 21, 2017 at 8:45 AM, G. Allegri <giohappy at gmail.com
>     <mailto:giohappy at gmail.com>> wrote:
>
>         Side note: multiprocessing is part of the Python standard
>         library.
>         https://docs.python.org/2.7/library/multiprocessing.html
>         <https://docs.python.org/2.7/library/multiprocessing.html>
>
>         Giovanni
>
>         Il 21 mag 2017 14:18, "Andreas Plesch"
>         <andreasplesch at gmail.com <mailto:andreasplesch at gmail.com>> ha
>         scritto:
>
>             As a followup, here is the simple processing script which
>             is my attempt at launching a separate task:
>
>             https://github.com/andreasplesch/QGIS-X3D-Processing/blob/master/scripts/launch_webserver.py
>             <https://github.com/andreasplesch/QGIS-X3D-Processing/blob/master/scripts/launch_webserver.py>
>
>             qgis2web does not seem to need a real http server for the
>             preview as openlayers apparently can use the file protocol.
>
>             -Andreas
>
>             On Sat, May 20, 2017 at 5:29 PM, Andreas Plesch
>             <andreasplesch at gmail.com <mailto:andreasplesch at gmail.com>>
>             wrote:
>
>                 I would like to launch a minimal web server from a
>                 processing script to serve html properly (rather than
>                 using the file protocol).
>
>                 Since the web server needs to run and keep running
>                 separately from qgis or the qgis python interpreter, I
>                 investigated various ways to accomplish this using
>                 SimpleHTTPServer. On windows 2.18.7 as linux should be
>                 more robust.
>
>                 - threading.Threat : start() works until the server
>                 actually serves a file. Then qgis crashes with a
>                 minidump . For some reason, the crashing happens not
>                 always but more often than not.
>                 - multiprocessing : seems to be the recommended
>                 solution but I do not seem to have it with Qgis python (?)
>                 - QProcess.start() : works from the python terminal,
>                 seems robust but does not seem to work from a
>                 processing script (?). Would be the preferred solution.
>                 - QProcess.startDetached('python',(args),dir) : works
>                 reliably but requires killing and restarting the
>                 process to change the root directory served
>
>                 This may all change with Qgis 3 but I suspect 2.18
>                 will be around for a while, too.
>
>                 Any recommendations or ideas would be welcome.
>
>                 I will look into qgis2web to see what it does.
>
>                 -- 
>                 Andreas Plesch
>                 39 Barbara Rd.
>                 Waltham, MA 02453
>
>
>
>
>             -- 
>             Andreas Plesch
>             39 Barbara Rd.
>             Waltham, MA 02453
>
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>
>
>
>     -- 
>     Andreas Plesch
>     39 Barbara Rd.
>     Waltham, MA 02453
>
>
>
>
> -- 
> Andreas Plesch
> 39 Barbara Rd.
> Waltham, MA 02453
>
>
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