mailbox: fix AwaitRPC lost-wakeup flake

[bhandras]

Summary

This fixes a flaky timeout in TestClient_ConcurrentInFlightDoesNotDrop by
eliminating a lost-wakeup race in AwaitRPC.

Root Cause

AwaitRPC previously did these steps separately:

1. Check pending for the response.
2. Register a waiter channel.

If a response arrived between steps 1 and 2, handleEnvelope would cache
the response and notify the current waiter set — which did not yet
include the new waiter. The newly registered waiter would then block until
context timeout.

Fix

Make the response check and waiter registration atomic under one lock via
popPendingOrAddWaiter:

• if pending response exists: return it immediately and skip waiter registration
• otherwise: register waiter before releasing the lock

This removes the race window where notifications can be missed.

CI Failure

Observed CI failure (PR #79, unit-cover):

• https://github.com/lightninglabs/darepo-client/actions/runs/22197814323/job/64202618296?pr=79

Failure signature:

• TestClient_ConcurrentInFlightDoesNotDrop
• context deadline exceeded

Validation

• Reproduced pre-fix flake locally with high repetition.
• Post-fix stress runs passed:
  • go test ./mailbox/client -run TestClient_ConcurrentInFlightDoesNotDrop -count=20000 -timeout=60m
  • go test -covermode=atomic -coverpkg=github.com/lightninglabs/darepo-client/... -tags='dev nolog' ./mailbox/client -run TestClient_ConcurrentInFlightDoesNotDrop -count=1000 -timeout=20m

[gemini-code-assist]

Summary of Changes

Hello @bhandras, 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 addresses a critical concurrency bug within the AwaitRPC mechanism of the mailbox client, specifically a lost-wakeup race condition. By introducing an atomic operation for checking pending responses and registering waiters, the client now reliably handles RPC responses, preventing scenarios where notifications could be missed. This fix significantly improves the stability of the client, resolving a previously observed flaky test in CI.

Highlights

• Atomic RPC Waiting: The AwaitRPC function was refactored to atomically check for pending responses and register waiters, eliminating a lost-wakeup race condition.
• Flaky Test Fix: This change resolves a context deadline exceeded flake in TestClient_ConcurrentInFlightDoesNotDrop by addressing the underlying concurrency bug.

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Changelog

• mailbox/client/client.go
  • AwaitRPC was modified to use the new popPendingOrAddWaiter function for handling RPC responses.
  • The popPending and addWaiter functions were removed, as their functionality was consolidated.
  • A new popPendingOrAddWaiter function was introduced to atomically manage pending responses and waiter registration.

Activity

• Observed CI failure in PR OOR client 3/4: durable actor, TLV codecs, signing context #79 (unit-cover) related to the flake.
• Reproduced the flake locally with high repetition before applying the fix.
• Conducted post-fix stress runs, including go test with high counts and timeouts, which passed successfully.

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[gemini-code-assist]

Code Review

The changes effectively address the lost-wakeup flake in AwaitRPC by making the response check and waiter registration atomic. The introduction of popPendingOrAddWaiter correctly synchronizes access to pending responses and waiter channels, resolving the race condition. The code is clear and the fix is well-explained in the PR description.

[Roasbeef]

Folded into the latest series.