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/ristypes/ris.go

@@ -4,6 +4,7 @@ package ristypes
 
 import (
 	"encoding/json"
+	"slices"
 	"time"
 )
 
@@ -24,7 +25,8 @@ const (
 )
 
 // ASPath is a BGP AS path as the feed writes it: an array of AS numbers in
-// which an AS set is an array of its members, such as [1, 2, [3, 4]].
+// which an AS set is an array of its members, such as [1, 2, [3, 4]]. A path
+// that holds AS 0 decodes as an empty one (UnmarshalJSON).
 type ASPath struct {
 	// Sequence is the path's AS numbers in order. A set of one member that
 	// ends the path counts as that AS and is its last element. The members of
@@ -48,23 +50,41 @@ func (p *ASPath) Origin() int {
 	return p.Sequence[len(p.Sequence)-1]
 }
 
-// UnmarshalJSON implements the json.Unmarshaler interface for ASPath.
+// UnmarshalJSON implements the json.Unmarshaler interface for ASPath. A path
+// that holds AS 0 anywhere, a member of an AS set included, decodes as an empty
+// path: RFC 7607 makes such a path malformed and its announcements
+// withdrawals. On an announcement with an empty path the prefix handler
+// deletes the peer's route, and the AS and peering handlers record nothing.
 func (p *ASPath) UnmarshalJSON(data []byte) error {
+	path, err := decodeASPath(data)
+	if err != nil {
+		return err
+	}
+
+	if slices.Contains(path.Sequence, 0) || slices.Contains(path.Set, 0) {
+		path = ASPath{}
+	}
+
+	*p = path
+
+	return nil
+}
+
+// decodeASPath decodes an AS path as the feed writes it.
+func decodeASPath(data []byte) (ASPath, error) {
 	// Most paths have no AS set and decode as a plain array of integers.
 	var sequence []int
 
 	err := json.Unmarshal(data, &sequence)
 	if err == nil {
-		*p = ASPath{Sequence: sequence}
-
-		return nil
+		return ASPath{Sequence: sequence}, nil
 	}
 
 	var raw []any
 
 	err = json.Unmarshal(data, &raw)
 	if err != nil {
-		return err
+		return ASPath{}, err
 	}
 
 	path := ASPath{Sequence: make([]int, 0, len(raw))}
@@ -90,9 +110,7 @@ func (p *ASPath) UnmarshalJSON(data []byte) error {
 		}
 	}
 
-	*p = path
-
-	return nil
+	return path, nil
 }
 
 // RISLiveMessage represents the outer wrapper message from the RIPE RIS Live stream.