slayers: recycle path when decoding

In `slayers.SCION.DecodeFromBytes`, we allocate a new path for the
correct path type on every invocation. This negates the advantages of
recycling the layer object.

Introduce a "recycling bin" for paths that can be reused in the SCION
layer's DecodeFromBytes.
This is only used when explicitly enabled, as it only makes sense if the
layer is really reused for parsing. This opt-in also avoids issues with
tests that deep-compare the SCION layer with an expected value where
this recycling bin is empty..

Closes #4147

GitOrigin-RevId: cd7c81e4e062822bcc948eae4363c9544721f4d0

go/lib/slayers/path/path.go

@@ -104,6 +104,11 @@ func NewPath(pathType Type) (Path, error) {
 	return pm.New(), nil
 }
 
+// NewRawPath returns a new raw path that can hold any path type.
+func NewRawPath() Path {
+	return &rawPath{}
+}
+
 type rawPath struct {
 	raw      []byte
 	pathType Type

go/lib/slayers/scion.go

@@ -145,6 +145,9 @@ type SCION struct {
 
 	// Path is the path contained in the SCION header. It depends on the PathType field.
 	Path path.Path
+
+	pathPool    []path.Path
+	pathPoolRaw path.Path
 }
 
 func (s *SCION) LayerType() gopacket.LayerType {
@@ -244,7 +247,7 @@ func (s *SCION) DecodeFromBytes(data []byte, df gopacket.DecodeFeedback) error {
 		return serrors.New("provided buffer is too small", "expected", minLen, "actual", len(data))
 	}
 
-	s.Path, err = path.NewPath(s.PathType)
+	s.Path, err = s.getPath(s.PathType)
 	if err != nil {
 		return err
 	}
@@ -259,6 +262,34 @@ func (s *SCION) DecodeFromBytes(data []byte, df gopacket.DecodeFeedback) error {
 	return nil
 }
 
+// RecyclePaths enables recycling of paths used for DecodeFromBytes. This is
+// only useful if the layer itself is reused.
+// When this is enabled, the Path instance may be overwritten in
+// DecodeFromBytes. No references to Path should be kept in use between
+// invocations of DecodeFromBytes.
+func (s *SCION) RecyclePaths() {
+	if s.pathPool == nil {
+		s.pathPool = []path.Path{
+			empty.PathType:  empty.Path{},
+			onehop.PathType: &onehop.Path{},
+			scion.PathType:  &scion.Raw{},
+			epic.PathType:   &epic.Path{},
+		}
+		s.pathPoolRaw = path.NewRawPath()
+	}
+}
+
+// getPath returns a new or recycled path for pathType
+func (s *SCION) getPath(pathType path.Type) (path.Path, error) {
+	if s.pathPool == nil {
+		return path.NewPath(pathType)
+	}
+	if int(pathType) < len(s.pathPool) {
+		return s.pathPool[pathType], nil
+	}
+	return s.pathPoolRaw, nil
+}
+
 func decodeSCION(data []byte, pb gopacket.PacketBuilder) error {
 	scn := &SCION{}
 	err := scn.DecodeFromBytes(data, pb)

go/pkg/router/dataplane.go

@@ -553,7 +553,7 @@ type processResult struct {
 }
 
 func newPacketProcessor(d *DataPlane, ingressID uint16) *scionPacketProcessor {
-	return &scionPacketProcessor{
+	p := &scionPacketProcessor{
 		d:         d,
 		ingressID: ingressID,
 		buffer:    gopacket.NewSerializeBuffer(),
@@ -563,6 +563,8 @@ func newPacketProcessor(d *DataPlane, ingressID uint16) *scionPacketProcessor {
 			epicInput:  make([]byte, libepic.MACBufferSize),
 		},
 	}
+	p.scionLayer.RecyclePaths()
+	return p
 }
 
 func (p *scionPacketProcessor) reset() error {