Handle pipewire restarts

[cillian64]

If the pipewire daemon restarted while xdpw was running then our pipewire connection would drop and attempting to start a screencast wouldn't work until user manually restarted xdpw.

This commit adds a listener for fatal core connection errors from pipewire. In the handler it cleans up the connection and marks it as dead. Then, when the user attempts to start a screencast, we reconnect the pipewire core if needed.

To reproduce the problem:

• Start off with pipewire, xdg-desktop-portal, and xdg-desktop-portal-wlr running as normal in a working state
• systemctl --user restart pipewire
• Go to https://mozilla.github.io/webrtc-landing/gum_test.html in a recent release of Firefox and click "Screen capture"
• The screen capture does not start and xdg-desktop-portal-wlr hangs, consuming a full CPU core in a hard loop

With this change applied, the screen capture works after restarting pipewire (after testing the bad case, make sure to restart Firefox before testing with the fix applied).

Fixes #242, fixes #178

[emersion]

Can't we just exit when we loose the connection to PipeWire?

[cillian64]

I've just tested and that does work, assuming you're running xdpw from systemd or similar. I guess it's a little simpler than restarting the pipewire connection.

Would you like me to update this PR to do that? If so, do you think this is an abort() or exit(-1) situation?

[cillian64]

Actually, I just remembered one issue I had in testing was that if you try to reconnect to pipewire too soon after the error listener fires it's possible to reconnect to the old pipewire daemon before it has stopped (it seems like the server disconnects all the clients before it stops listening for new connections). I'd hope that this would just fire the error listener again but I'm not sure. In practice xdpw being restarted by systemd seems to take long enough to avoid this problem but it's potentially racey.

[emersion]

Yeah, I'd prefer to exit(1) when we get disconnected from PipeWire or Wayland. Trying to recreate our state doesn't seem worth the trouble.