Handle an announcement without an origin like a replacement (closes #113)

An announcement whose AS path is empty or ends in AS 0 now deletes the
peer's route for the prefix, as a withdrawal does. Before, it was neither
stored nor applied: the peer's old route stayed live, and within one
batch it cancelled the peer's earlier withdrawal of the prefix.

Model: opus-5-5

internal/routewatch/prefixhandler.go

@@ -44,8 +44,9 @@ const (
 )
 
 // PrefixHandler tracks BGP prefixes and maintains a live routing table in the
-// database. Routes are added on announcement and deleted on withdrawal, and all
-// routes of a peer are deleted when its BGP session goes down.
+// database. Routes are added on announcement and deleted on withdrawal or on an
+// announcement whose AS path is empty or ends in AS 0, and all routes of a peer
+// are deleted when its BGP session goes down.
 type PrefixHandler struct {
 	db      database.Store
 	logger  *logger.Logger
@@ -371,7 +372,11 @@ func (h *PrefixHandler) flushBatch() {
 	for _, update := range lastUpdates {
 		// An announcement is stored when its path gives an origin, or when it
 		// ends in an AS set of several members: that route is stored with
-		// origin 0 (none). An announcement with an empty path is not stored.
+		// origin 0 (none). Any other update deletes the peer's route for the
+		// prefix: a withdrawal, or an announcement whose path is empty or ends
+		// in AS 0. Such an announcement replaces the peer's route, as any
+		// announcement does, with none (RFC 7607 treats AS 0 in a path as a
+		// withdrawal).
 		if update.messageType == "announcement" &&
 			(update.originASN > 0 || update.endsInSet) {
 			// Create live route for batch upsert
@@ -379,11 +384,11 @@ func (h *PrefixHandler) flushBatch() {
 			if route != nil {
 				routesToUpsert = append(routesToUpsert, route)
 			}
-		} else if update.messageType == "withdrawal" {
+		} else {
 			// Parse CIDR to get IP version
 			_, ipVersion, err := parseCIDR(update.prefix)
 			if err != nil {
-				h.logger.Error("Failed to parse CIDR for withdrawal",
+				h.logger.Error("Failed to parse CIDR for route deletion",
 					"prefix", update.prefix, "error", err)
 
 				continue