[Bug 2150181] Re: [SRU] software-properties-gtk driver detection fails with Python 3.14 (cannot pickle SoftwarePropertiesGtk)

[Alessandro Astone]

** Changed in: software-properties (Ubuntu Stonking)
       Status: Confirmed => Fix Committed

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Title:
  [SRU] software-properties-gtk driver detection fails with Python 3.14
  (cannot pickle SoftwarePropertiesGtk)

Status in software-properties package in Ubuntu:
  Fix Committed
Status in software-properties source package in Resolute:
  Confirmed
Status in software-properties source package in Stonking:
  Fix Committed

Bug description:
  [ Impact ]

  The "Additional Drivers" tab in software-properties-gtk is completely
  broken on Ubuntu 26.04 (Resolute). When a user opens the Additional
  Drivers UI, it gets stuck on "Searching for available drivers..." and
  never completes. The underlying error is a TypeError in the driver
  detection thread.

  Python 3.14 changed the default multiprocessing start method from
  'fork' to 'forkserver'. The 'forkserver' method requires that the
  Process object, its target function, and all arguments be
  picklable. The detect_drivers code passes a bound method
  (self.wrapper_system_device_drivers) as the process target and
  pickling a bound method requires pickling the bound instance, which is
  a SoftwarePropertiesGtk object containing GTK widgets that cannot be
  pickled. Additionally, the apt_pkg.Cache argument is a C extension
  object that also cannot be pickled.

  The fix makes wrapper_system_device_drivers a @staticmethod (removing the
  unpicklable SoftwarePropertiesGtk instance from the serialization path)
  and moves apt cache creation into the subprocess itself rather than
  passing it as an argument.

  [ Test Plan ]

  1. Install software-properties-gtk on Ubuntu 26.04 (with Python 3.14).
  2. Launch Additional Drivers:
       software-properties-gtk --open-tab=4
     or:
       gio launch /usr/share/applications/software-properties-drivers.desktop
  3. Before the fix: the UI shows "Searching for available drivers..."
     indefinitely, and the terminal prints:
       TypeError: cannot pickle 'SoftwarePropertiesGtk' object
  4. After the fix: the driver search completes and available drivers are
     listed (or "No additional drivers available" is shown, depending on
     hardware).
  5. If drivers are listed, verify that selecting a different driver and
     applying the change still works correctly.
  6. Verify the Cancel button works during a search, and that Retry works
     after cancellation.

  [ Where problems could occur ]

  The subprocess now creates its own apt_pkg.Cache instead of reusing the
  parent's. If the apt cache state on disk is somehow inconsistent or
  changes between when the parent reads it and when the subprocess reads
  it, the subprocess could see a different set of packages. In practice
  this is not a meaningful risk since the cache is only used for driver
  detection, which is read-only, and the window between the two reads is
  negligible.

  The wrapper_system_device_drivers function now calls apt_pkg.init_config
  and apt_pkg.init_system in the subprocess. If these behave differently
  when called in a forkserver child versus a forked child (e.g. reading
  different configuration), driver detection results could differ. These
  functions are designed to be safe to call in fresh processes and are used
  this way elsewhere in the codebase (init_apt_cache).

  [ Other Info ]

  The Qt frontend (software-properties-qt) calls
  detect.system_device_drivers directly in a thread without using
  multiprocessing, so it is not affected by this bug.

  This is a compatibility fix for the Python 3.14 multiprocessing
  behaviour change. After the 3.14 migration, no one had tested this tab
  it seems (or had time to report a bug).

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