Remove a peer's live routes when its session goes down (closes #56)

RIS sends no withdrawals for the routes of a peer whose BGP session goes
down, so routewatch kept them.

The streamer now hands STATE messages to the prefix handler. They are never
dropped for backpressure: one that finds the queue full is held and queued
first once there is room, and the handler's other messages are dropped
meanwhile, so none overtakes it. Nothing waits. On a down, the prefix
handler drops the peer's pending updates and its next flush deletes all the
peer's live routes before the batch's other writes, keeping the in-memory
counts exact. The live route tables are unique on peer and prefix, which
also finds a peer's routes.

Model: opus-5-5

internal/streamer/streamer.go

@@ -78,7 +78,8 @@ type MessageHandler interface {
 	// QueueCapacity returns the desired queue capacity for this handler.
 	// Handlers that process quickly can have larger queues to buffer bursts.
 	// When the queue fills up, messages will be dropped according to the
-	// backpressure algorithm.
+	// backpressure algorithm, except STATE messages, which are held until
+	// there is room.
 	QueueCapacity() int
 }
 
@@ -102,6 +103,9 @@ type handlerInfo struct {
 	handler MessageHandler
 	queue   chan *ristypes.RISMessage
 	metrics handlerMetrics
+	// heldState holds, in feed order, the STATE messages that found queue
+	// full (see queueMessage). Only the read loop uses it.
+	heldState []*ristypes.RISMessage
 }
 
 // Streamer manages a connection to the RIPE RIS Live streaming API for receiving
@@ -629,7 +633,7 @@ func (s *Streamer) stream(ctx context.Context) error {
 
 		// Process based on message type
 		switch msg.Type {
-		case "UPDATE":
+		case ristypes.MessageTypeUpdate:
 			// Process BGP UPDATE messages
 			// Will be dispatched to handlers
 		case "RIS_PEER_STATE":
@@ -668,9 +672,9 @@ func (s *Streamer) stream(ctx context.Context) error {
 			)
 
 			continue
-		case "STATE":
-			// Peer state changes - silently ignore
-			continue
+		case ristypes.MessageTypeState:
+			// The BGP session with the peer changed state, for example went
+			// down. Dispatched to the handlers that want it.
 		default:
 			s.logger.Warn("Unknown message type, skipping",
 				"type", msg.Type,
@@ -693,30 +697,20 @@ func (s *Streamer) stream(ctx context.Context) error {
 				continue
 			}
 
-			// Check if we should drop due to backpressure
-			if s.shouldDropForBackpressure(info) {
+			if !s.queueMessage(info, &msg) {
 				atomic.AddUint64(&info.metrics.droppedCount, 1)
 				atomic.AddUint64(&s.totalDropped, 1)
 
 				continue
 			}
 
-			// Try to queue the message
-			select {
-			case info.queue <- &msg:
-				// Message queued successfully
-				// Update high water mark if needed
-				queueLen := len(info.queue)
-				info.metrics.mu.Lock()
-				if queueLen > info.metrics.queueHighWaterMark {
-					info.metrics.queueHighWaterMark = queueLen
-				}
-				info.metrics.mu.Unlock()
-			default:
-				// Queue is full, drop the message
-				atomic.AddUint64(&info.metrics.droppedCount, 1)
-				atomic.AddUint64(&s.totalDropped, 1)
+			// Update high water mark if needed
+			queueLen := len(info.queue)
+			info.metrics.mu.Lock()
+			if queueLen > info.metrics.queueHighWaterMark {
+				info.metrics.queueHighWaterMark = queueLen
 			}
+			info.metrics.mu.Unlock()
 		}
 		s.mu.RUnlock()
 	}
@@ -736,6 +730,50 @@ func (s *Streamer) stream(ctx context.Context) error {
 	return errStreamEnded
 }
 
+// queueMessage puts msg on the handler's queue and reports false if it dropped
+// msg instead. It never waits for room, so a full queue holds up neither the
+// read loop nor Stop.
+//
+// A message is dropped while the queue is filling up (shouldDropForBackpressure)
+// or full, except a STATE message: one whose state is down is the only notice
+// that all of a peer's routes are gone. A STATE message skips the random drop,
+// and one that finds the queue full is held in info.heldState. Held messages
+// go into the queue, oldest first, before anything else for the handler, as
+// soon as there is room; while any is still held, the handler's other messages
+// are dropped, so none of them overtakes a held down.
+//
+// The held list stays small: a STATE message is a few hundred bytes and the
+// feed sends about ten a second, so a queue full for a minute holds about 600.
+func (s *Streamer) queueMessage(info *handlerInfo, msg *ristypes.RISMessage) bool {
+	for len(info.heldState) > 0 && trySend(info.queue, info.heldState[0]) {
+		info.heldState = info.heldState[1:]
+	}
+
+	if msg.Type == ristypes.MessageTypeState {
+		if len(info.heldState) > 0 || !trySend(info.queue, msg) {
+			info.heldState = append(info.heldState, msg)
+		}
+
+		return true
+	}
+
+	if len(info.heldState) > 0 || s.shouldDropForBackpressure(info) {
+		return false
+	}
+
+	return trySend(info.queue, msg)
+}
+
+// trySend puts msg on queue if there is room and reports whether it did.
+func trySend(queue chan *ristypes.RISMessage, msg *ristypes.RISMessage) bool {
+	select {
+	case queue <- msg:
+		return true
+	default:
+		return false
+	}
+}
+
 // shouldDropForBackpressure determines if a message should be dropped based on queue utilization
 func (s *Streamer) shouldDropForBackpressure(info *handlerInfo) bool {
 	// Calculate queue utilization