[Python] How to package python app with multiprocessing as snap

I am trying to package our python app aTrain, a local audio transcription GUI app based on whisper, as a snap.

I actually got quite far, getting it to mostly run in devmode confinement. However, when I switched to strict confinement I got a problem with permissions for multiprocessing:

File "/usr/lib/python3.12/multiprocessing/synchronize.py", line 57, in __init__
sl = self._semlock = _multiprocessing.SemLock(
^^^^^^^^^^^^^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied


I tried to implement the fix of using private shared memory proposed here

https://forum.snapcraft.io/t/python-multiprocessing-sem-open-blocked-in-strict-mode/962

plugs:
shared-memory:
private: true


However, this breaks the python app, giving me the following error:

File "/snap/atrain/x1/lib/python3.12/site-packages/aTrain/routes.py", line 30, in home
languages = model_languages(default_model)
^^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'default_model' where it is not associated with a value


Any ideas on how to solve that would be greatly welcome.

Here are the two corresponding branches for the app and the backend.

https://github.com/JuergenFleiss/aTrain/tree/feature_snap

https://github.com/JuergenFleiss/atrain_core/tree/feature_snap

To reproduce, clone the aTrain repository and switch to the feature snap branch, then run snapcraft to build (warning: takes a few GB).

Below is also my snapcraft.yaml (also in the aTrain feature_snap branch)

name: atrain
summary: aTrain summary
description: |
A GUI tool for offline transcription of speech recordings, including speaker diarization, utilizing state-of-the-art machine learning models.
version: test
base: core24
confinement: strict

parts:
atrain:
plugin: python
source: .
python-packages:
- pyqt5
- pyqtwebengine
- pywebview
- pywebview[qt]
- pycairo
- PyGObject == 3.50.0
- lightning-utilities
- numpy == 1.26.4
- packaging
build-environment:
- PIP_EXTRA_INDEX_URL: https://download.pytorch.org/whl/cu121
build-packages:
- git
- build-essential
- libgl1-mesa-dev
- libcairo2
- libcairo2-dev
- libgirepository1.0-dev
- libglu1-mesa
- libwebkit2gtk-4.1-dev
stage-packages:
- libgl1
- libglx0
- libglvnd0
- libxau6
- libxdmcp6
- libx11-6
- libavfilter-dev
- libxcb-randr0-dev
- libxcb-xtest0-dev
- libxcb-xinerama0-dev
- libxcb-shape0-dev
- libxcb-xkb-dev
- libavcodec-dev
- libavcodec-extra
- libavutil-dev
- libavfilter-dev
- libavformat-dev
- libxcb-xinput0
- qtcreator
- qtbase5-dev
- qt5-qmake
- cmake

apps:
atrain:
command: bin/aTrain start
plugs:
- desktop
- desktop-legacy
- opengl
- x11
- wayland
- network
- network-bind
- network-status
- browser-support
extensions: [gnome]
environment:
LD_LIBRARY_PATH: $SNAP/usr/lib/x86_64-linux-gnu/webkit2gtk-4.1/:$LD_LIBRARY_PATH
# "PYTHONPATH": "$NAP/usr/lib/python3/dist-packages/"

hooks:
install:
plugs: [network]

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