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/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