Treat a path with AS 0 anywhere as a withdrawal (closes #141)

An AS path that holds AS 0 anywhere, a member of an AS set included, now
decodes as an empty path. RFC 7607 makes such a path malformed and its
announcements withdrawals. An empty path already gets that handling:
the prefix handler deletes the peer's route for the prefix, and the AS
and peering handlers record nothing from it. Before, a path such as
[64496, 0, 64497] stored the route with origin AS64497, recorded both
ASes, and recorded the peerings between the path's other ASes.

Model: opus-5-5

internal/routewatch/ashandler_internal_test.go

@@ -1,6 +1,8 @@
 package routewatch
 
 import (
+	"maps"
+	"slices"
 	"testing"
 
 	"git.eeqj.de/sneak/routewatch/internal/logger"
@@ -42,3 +44,25 @@ func TestASHandlerStoresTheEarliestAndLatestTimesOfANewAS(t *testing.T) {
 			asn.LastSeen, second.ParsedTimestamp)
 	}
 }
+
+// TestASHandlerRecordsNothingFromAPathWithASZero handles an update whose AS
+// path holds AS 0 between two other ASes. RFC 7607 makes such a path
+// malformed, so neither AS is recorded.
+func TestASHandlerRecordsNothingFromAPathWithASZero(t *testing.T) {
+	t.Parallel()
+
+	store := newMockStore()
+
+	// Built without NewASHandler's flush goroutine, so the test flushes itself.
+	h := &ASHandler{db: store, logger: logger.New()}
+
+	msg := peerAnnouncement(testPeerIP, testV4Prefix)
+	msg.Path = decodePath(t, testASNA, 0, testASNB)
+	h.HandleMessage(msg)
+	h.flushBatch()
+
+	if len(store.ASNs) > 0 {
+		t.Errorf("ASes recorded = %v, want none",
+			slices.Sorted(maps.Keys(store.ASNs)))
+	}
+}

internal/routewatch/peeringhandler_internal_test.go

@@ -153,6 +153,43 @@ func TestPeeringHandlerLeavesATrailingASSetOutOfThePeerings(t *testing.T) {
 	}
 }
 
+// TestPeeringHandlerRecordsNothingFromAPathWithASZero handles AS paths that
+// hold AS 0. RFC 7607 makes such a path malformed, so no peering is recorded
+// from it, not even one between two other ASes of the path.
+func TestPeeringHandlerRecordsNothingFromAPathWithASZero(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		path []any
+	}{
+		{name: "AS 0 between two ASes", path: []any{testASNA, 0, testASNB}},
+		{
+			name: "AS 0 after two ASes",
+			path: []any{testASNA, testASNB, 0, testASNC},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			store := newMockStore()
+			h := newTestHandler(store)
+			h.HandleMessage(&ristypes.RISMessage{
+				Path:            decodePath(t, tt.path...),
+				ParsedTimestamp: time.Now().UTC(),
+			})
+			h.ProcessPeeringsNow()
+
+			if len(store.Peerings) > 0 {
+				t.Errorf("peerings recorded = %v, want none",
+					slices.Sorted(maps.Keys(store.Peerings)))
+			}
+		})
+	}
+}
+
 // TestPeeringHandlerProcessesEachRunAndEmpties verifies that a processing run
 // empties the path map (the swap) and that a path seen again after a run is
 // recorded in the next run too.

internal/routewatch/prefixhandler.go

@@ -45,8 +45,8 @@ const (
 
 // PrefixHandler tracks BGP prefixes and maintains a live routing table in the
 // 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.
+// announcement whose AS path is empty or holds AS 0 anywhere, and all routes of
+// a peer are deleted when its BGP session goes down.
 type PrefixHandler struct {
 	db      database.Store
 	logger  *logger.Logger
@@ -373,10 +373,10 @@ func (h *PrefixHandler) flushBatch() {
 		// 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). 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).
+		// prefix: a withdrawal, or an announcement whose path is empty or
+		// holds AS 0 anywhere (the decoder empties such a path). 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

internal/routewatch/prefixhandler_internal_test.go

@@ -381,11 +381,12 @@ func TestPrefixHandlerDoesNotStoreAnAnnouncementWithAnEmptyPath(t *testing.T) {
 // 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.
+// an AS path that gives no origin: one whose last AS is 0, an empty one, or one
+// that holds AS 0 before its end, which decodes as an empty path. 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,
 ) {
@@ -421,6 +422,12 @@ func TestPrefixHandlerRemovesTheRouteOnAnAnnouncementWithoutAnOrigin(
 			name:  "an empty path",
 			batch: []*ristypes.RISMessage{noOrigin(ristypes.ASPath{})},
 		},
+		{
+			name: "a path with AS 0 before its end",
+			batch: []*ristypes.RISMessage{
+				noOrigin(decodePath(t, testASNA, 0, testASNB)),
+			},
+		},
 		{
 			name:  "a withdrawal and then a path whose last AS is 0",
 			batch: []*ristypes.RISMessage{withdrawal, noOrigin(endsInZero)},