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_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.