Add session recovery with automatic playback resume

[Arneball]

When the TCP connection to Spotify access points drops due to network instability, the keep-alive timeout fires after ~80s and invalidates the session. The old spirc shuts down and a new Session + Spirc is created. However, three things went wrong:

1. Session ID mismatch: The new Session generated a fresh random UUID, so Spotify could not match it to the previous playback session. The automatic transfer in handle_connection_id_update failed because the cluster session_id did not match the new session_id.

2. Volume reset: ConnectConfig.initial_volume was set once at startup and reused on every reconnection. The new SpircTask initialized the mixer to this stale value instead of the users current volume, causing a jarring volume jump on reconnect.

3. Playback not resuming: The transfer state from Spotify always has is_paused=true after a disconnect, since the device went offline. handle_transfer used this to set start_playing=false, so the track loaded paused even though the user was actively listening.

Fixes:

• Preserve session_id across reconnections for automatic transfer
• Preserve mixer volume on reconnect, only override after first connect
• Track user play intent via shared Arc was_playing flag
• Add force_play parameter to handle_transfer for reconnection resume
• Add shutdown reason strings to session.shutdown for debugging
• Fast-fail audio key requests when session is invalid
• Use RwLock in player for session hot-swap

Testing librespot session recovery via iptables

1. Find the AP connection

ss -tnp | grep librearmv7 | grep 4070

Example output:

ESTAB 0 0  192.168.0.14:57572  34.158.1.133:4070  users:(("librearmv7",pid=3429,fd=17))

Note the destination IP (34.158.1.133) and source port (57572).

2. Block the connection

sudo iptables -A OUTPUT -d <IP> -p tcp --sport <SPORT> --dport 4070 -j DROP
sudo iptables -A INPUT -s <IP> -p tcp --dport <SPORT> --sport 4070 -j DROP

Using the example values:

sudo iptables -A OUTPUT -d 34.158.1.133 -p tcp --sport 57572 --dport 4070 -j DROP
sudo iptables -A INPUT -s 34.158.1.133 -p tcp --dport 57572 --sport 4070 -j DROP

Blocking the specific source port is important — it kills the existing connection while allowing librespot to establish a new one on a different port.

3. Wait for reconnection

The keep-alive timeout fires after ~80 seconds. After that, the session is invalidated and librespot creates a new session + spirc. The audio buffer may sustain playback for another 1-2 minutes before the reconnection completes.

Monitor with:

tail -f /tmp/librespot.log

Look for:

• Shutdown: Invalidating session: keep-alive timeout
• Preserved session_id across reconnection
• start_playing=true (if music was playing)

4. Clean up iptables rules

sudo iptables -F