Issue: Silent notification drop under backpressure in NotificationClient #217

Summary

The notification dispatcher inside (*NotificationClient).listen performs a non-blocking send to each subscriber's channel and silently discards the notification when the buffer is full or the receiver is gone. The drop path contains only the comment // message dropped , no log line, no metric, no error propagation. Because the txID is removed from the subscriber map before delivery is attempted , a dropped notification is unrecoverable: the caller waiting in WaitForEvent will time out as if the committer never responded.

Affected Code

File: tools/fxconfig/internal/client/notifications.go

// lines 223–228
for _, c := range notifications {
    select {
    case c.receiverQueue <- c.status:
    default:
        // message dropped
    }
}

Reproduction Conditions

The default branch is reached whenever any of the following holds at the moment of dispatch:

• Subscriber timed out early - WaitForEvent already returned via its ctx.WithTimeout; the channel was never drained.
• Late / duplicate status event - a second event for the same txID arrives after the first already filled the buffer slot.
• Abandoned subscriber - the caller subscribed but never called WaitForEvent (e.g., error on the submission path before the wait).
• Slow consumer - the subscriber goroutine is not scheduled fast enough to drain the channel before the dispatcher fires.

Root Cause

listen uses a non-blocking send to keep one slow subscriber from stalling
the entire dispatcher goroutine , that design intent is correct. However, the
fallthrough default branch has no instrumentation. The combination of:

1. pre-delivery subscriber map deletion.
2. A bare default with a comment.
   means any backpressure scenario silently loses state without any signal to
   operators or developers.