Skip unnecessary disconnect and wait for disconnect to finish in shutdown

[yyforyongyu]

Was debugging reconnection behavior in quiescence and found a few behaviors that can be improved:

• when peer is already disconnected, no need to disconnect again.
• fixed a case the peer may be stuck in WaitForDisconnect
• similar to this comment, we want to make sure we wait for goroutines to finish when shutting down.

[gemini-code-assist]

Summary of Changes

Hello @yyforyongyu, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the peer disconnection and shutdown mechanisms within the Brontide peer implementation and the server's peer management. The primary goal is to enhance robustness and efficiency by preventing redundant disconnect operations, resolving a potential deadlock in the WaitForDisconnect routine, and ensuring that all peer-related goroutines gracefully complete during shutdown.

Highlights

• Improved Disconnection Idempotency: The Disconnect method now explicitly checks if a peer is already in the process of disconnecting, preventing redundant calls and logging a warning if an unnecessary disconnect is attempted. Similarly, the Start method now logs a warning if called on an already started peer.
• Resolved WaitForDisconnect Deadlock: The WaitForDisconnect routine has been updated to correctly handle cases where the peer might quit prematurely, ensuring it doesn't get stuck waiting indefinitely by adding return statements in select cases for p.cg.Done().
• Ensured Goroutine Termination on Shutdown: The server's DisconnectPeer method now properly tracks the peer disconnection goroutine using a sync.WaitGroup, guaranteeing that the server waits for this operation to complete before fully shutting down.
• Enhanced Logging for Peer Lifecycle: Added more detailed trace and debug logging to various peer lifecycle methods (Start, WaitForDisconnect, Disconnect, readHandler, writeHandler) to provide better visibility into peer state transitions and disconnection flows.

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[gemini-code-assist]

Code Review

This pull request refactors the peer shutdown logic to be more robust and observable. Key changes include using CompareAndSwap for start-once logic, fixing a potential hang in WaitForDisconnect, adding more detailed logging, and ensuring the server waits for peer disconnection goroutines to complete on shutdown. The changes are well-reasoned and improve the overall stability of peer lifecycle management. I have one suggestion to make the readHandler's disconnect logic even more robust by providing a default error reason.

[lightninglabs-deploy]

@ziggie1984: review reminder
@yyforyongyu, remember to re-request review from reviewers when ready

[ziggie1984]

LGTM, thanks for the improvement

[ziggie1984]

Nit: what is this dash at the beginning ?

[ziggie1984]

why don't we want to record the peerExiting error anymore ?

[ziggie1984]

fixed a case the peer may be stuck in WaitForDisconnect

curious why this case was hit because normally we wait for all the peer goroutines to finish and if the Done() channel was closed normally this should finish as well, I don't really see the bug at first glance.