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

README.md

@@ -190,10 +190,11 @@ routewatch/
     - **ASHandler**: Tracks all ASNs seen in AS paths
     - **PeerHandler**: Records BGP peer information
     - **PrefixHandler**: Maintains live routing table (upserts on announcement,
-      deletes on withdrawal, deletes all of a peer's routes when its BGP session
+      deletes on withdrawal or on an announcement whose AS path is empty or ends
-      goes down). Routes are kept per peer IP, so when one peer IP has sessions
+      in AS 0, deletes all of a peer's routes when its BGP session goes down).
-      on two collectors, a session going down on one also removes the routes the
+      Routes are kept per peer IP, so when one peer IP has sessions on two
-      other still carries, until they are announced again.
+      collectors, a session going down on one also removes the routes the other
+      still carries, until they are announced again.
     - **PeeringHandler**: Extracts AS peering relationships from AS paths
 
 5. **HTTP Request Flow**

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
+// database. Routes are added on announcement and deleted on withdrawal or on an
-// routes of a peer are deleted when its BGP session goes down.
+// 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
@@ -379,7 +384,7 @@ 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 {

internal/routewatch/prefixhandler_internal_test.go

@@ -378,6 +378,68 @@ func TestPrefixHandlerDoesNotStoreAnAnnouncementWithAnEmptyPath(t *testing.T) {
 	assertPeers(t, db, map[string][]string{testV4Prefix: nil})
 }
 
+// TestPrefixHandlerRemovesTheRouteOnAnAnnouncementWithoutAnOrigin runs, through
+// the prefix handler into a real database, a peer's announcement of a prefix
+// with origin AS testASNB and then the peer's announcement of the prefix with
+// an AS path that gives no origin: one whose last AS is 0, or an empty one. As
+// any later announcement from the peer does, the second replaces the peer's
+// route, here with none, whether it comes in a later batch or follows a
+// withdrawal in one batch. The in-memory route count and prefix distribution
+// must match the route table.
+func TestPrefixHandlerRemovesTheRouteOnAnAnnouncementWithoutAnOrigin(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	const prefixMaskLength = 24
+
+	noOrigin := func(path ristypes.ASPath) *ristypes.RISMessage {
+		msg := peerAnnouncement(testPeerIP, testV4Prefix)
+		msg.Path = path
+
+		return msg
+	}
+	endsInZero := ristypes.ASPath{Sequence: []int{testASNA, 0}}
+	withdrawal := &ristypes.RISMessage{
+		Type:            ristypes.MessageTypeUpdate,
+		Peer:            testPeerIP,
+		ParsedTimestamp: time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC),
+		Withdrawals:     []string{testV4Prefix},
+	}
+
+	tests := []struct {
+		name string
+		// Handled and flushed as one batch after the announcement with origin
+		// AS testASNB.
+		batch []*ristypes.RISMessage
+	}{
+		{
+			name:  "a path whose last AS is 0",
+			batch: []*ristypes.RISMessage{noOrigin(endsInZero)},
+		},
+		{
+			name:  "an empty path",
+			batch: []*ristypes.RISMessage{noOrigin(ristypes.ASPath{})},
+		},
+		{
+			name:  "a withdrawal and then a path whose last AS is 0",
+			batch: []*ristypes.RISMessage{withdrawal, noOrigin(endsInZero)},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			db := handleBatches(t, [][]*ristypes.RISMessage{
+				{peerAnnouncement(testPeerIP, testV4Prefix)},
+				tt.batch,
+			})
+			checkLivePeers(t, db, testV4Prefix, prefixMaskLength, nil)
+		})
+	}
+}
+
 // TestStateMessagesGoToThePrefixHandlerNotThePeerHandler checks that STATE
 // messages go to the prefix handler, which acts on a down, and not to the peer
 // handler, which would record a peer whose session never comes up as active.