Desktop extension symlinks aren't updated on snap revert

[Lauren Brock]

The desktop extensions check whether or not to symlink libraries based on the contents of $SNAP_USER_DATA/.last_revision. Because $SNAP_USER_DATA is versioned, after a snap is reverted, the desktop-launch script will check .last_revision, but it will always match the (now) current revision. This leads to broken symlinks that point to a revision more recent than the current revision (since they are not updated after a revert).

This issue also affects the ubuntu/snapcraft-desktop-helpers repository.

[Lauren Brock (lauren-brock)]

I'm pretty certain that this is an issue that affects Snapcraft, rather than Snapd. The issue is the with the `desktop-launch` script that's generated from `extensions/desktop` in the Snapcraft repository. My naive solution with customers has been to replace `$SNAP_USER_DATA/.last_revision` with `$SNAP_USER_COMMON/.last_revision`, in `init` and `mark-and-exec`:

https://github.com/snapcore/snapcraft/blob/main/extensions/desktop/common/init#L31

https://github.com/snapcore/snapcraft/blob/main/extensions/desktop/common/mark-and-exec#L7

It's the `desktop-launch` script that's managing the symlinks that I'm talking about, and upon reverting a snap that uses `desktop-launch` in its command-chain (that also depends on one of the libraries that the script symlinks), the snap will fail to launch because those symlinks haven't been updated, since the $SNAP_DESKTOP_LAST_REVISION variable is set incorrectly.

Though my solution works, I don't want to prescribe a specific solution, since there may well be a better solution.