BUG/MEDIUM: runtime: fix EventListener Close() deadlock and channel panic

Close() used to close the events channel directly while the internal
listen goroutine could still be sending on it, causing either a
deadlock (send blocked forever) or a panic (send on closed channel).

Fix by introducing a done channel: Close() signals via done and closes
the connection, while the listen goroutine is the sole owner of the
events channel and closes it on exit via defer. The send in listen()
now uses a select to also check the done channel.

Author: mjuraga

runtime/runtime_event_listener.go

@@ -59,6 +59,7 @@ type EventListener struct {
 	// Message delimiter. Either \n or 0 (zero).
 	delim     byte
 	events    chan Event
+	done      chan struct{}
 	lastError error
 	closed    atomic.Bool
 }
@@ -82,6 +83,7 @@ func NewEventListener(network, address, sink string, timeout time.Duration, flag
 		WriteTimeout: timeout,
 		delim:        '\n',
 		events:       make(chan Event),
+		done:         make(chan struct{}),
 	}
 
 	if slices.Contains(flags, "-0") {
@@ -121,10 +123,11 @@ func (l *EventListener) Listen(ctx context.Context) (Event, error) {
 	}
 }
 
-// Close the EventListener cleanly.
+// Close the EventListener cleanly. The events channel will be closed
+// asynchronously by the internal listen goroutine after it exits.
 func (l *EventListener) Close() error {
 	if l.closed.CompareAndSwap(false, true) {
-		defer close(l.events)
+		close(l.done)
 		if err := l.conn.Close(); err != nil {
 			return l.errorf("%w", err)
 		}
@@ -133,8 +136,11 @@ func (l *EventListener) Close() error {
 	return nil
 }
 
-// Listen for events and push them on the events channel.
+// listen reads events from the connection and pushes them on the events channel.
+// It is the sole owner of the events channel and closes it on exit.
 func (l *EventListener) listen() {
+	defer close(l.events)
+
 	if l.WriteTimeout > 0 {
 		_ = l.conn.SetWriteDeadline(time.Now().Add(l.WriteTimeout))
 	}
@@ -161,7 +167,11 @@ func (l *EventListener) listen() {
 			return
 		}
 
-		l.events <- event
+		select {
+		case l.events <- event:
+		case <-l.done:
+			return
+		}
 	}
 }