Find a refused packet's tunnel from its header, ask the lighthouse when probing, and address review

overlay/batch/tx_batch.go

@@ -4,16 +4,20 @@ import "net/netip"
 
 const SendBatchCap = 128
 
+// FailureHeadLen is how much of a refused packet a Failure keeps, nebula's header, which says whose packet it was
+const FailureHeadLen = 16
+
 // batchWriter is the minimal subset of udp.Conn needed by SendBatch to flush.
 type batchWriter interface {
 	WriteBatch(bufs [][]byte, addrs []netip.AddrPort, failed func(i int, err error)) (int, error)
 }
 
-// Failure is a packet the underlay would not send, with the tag it was committed under
+// Failure is a packet the underlay would not send
 type Failure struct {
-	Tag uint32
-	Dst netip.AddrPort
-	Err error
+	// Head is the start of the packet, zero padded if the packet was shorter
+	Head [FailureHeadLen]byte
+	Dst  netip.AddrPort
+	Err  error
 }
 
 // SendBatch accumulates encrypted UDP packets and flushes them via WriteBatch.
@@ -23,7 +27,6 @@ type SendBatch struct {
 	out   batchWriter
 	bufs  [][]byte
 	dsts  []netip.AddrPort
-	tags  []uint32
 	arena *Arena
 
 	failures []Failure
@@ -37,7 +40,6 @@ func NewSendBatch(out batchWriter, batchCap, arenaSize int) *SendBatch {
 		out:   out,
 		bufs:  make([][]byte, 0, batchCap),
 		dsts:  make([]netip.AddrPort, 0, batchCap),
-		tags:  make([]uint32, 0, batchCap),
 		arena: NewArena(arenaSize),
 	}
 	b.recordFailure = b.failed
@@ -45,7 +47,9 @@ func NewSendBatch(out batchWriter, batchCap, arenaSize int) *SendBatch {
 }
 
 func (b *SendBatch) failed(i int, err error) {
-	b.failures = append(b.failures, Failure{Tag: b.tags[i], Dst: b.dsts[i], Err: err})
+	f := Failure{Dst: b.dsts[i], Err: err}
+	copy(f.Head[:], b.bufs[i])
+	b.failures = append(b.failures, f)
 }
 
 func (b *SendBatch) Reserve(sz int) []byte {
@@ -57,12 +61,9 @@ func (b *SendBatch) Reserve(sz int) []byte {
 // bounding how long the first packet of a large read batch waits.
 func (b *SendBatch) Len() int { return len(b.bufs) }
 
-// Commit queues pkt for dst. tag comes back in Failures if the underlay refuses the packet, so the caller can tell
-// whose packet it was.
-func (b *SendBatch) Commit(pkt []byte, dst netip.AddrPort, tag uint32) {
+func (b *SendBatch) Commit(pkt []byte, dst netip.AddrPort) {
 	b.bufs = append(b.bufs, pkt)
 	b.dsts = append(b.dsts, dst)
-	b.tags = append(b.tags, tag)
 }
 
 // Flush writes every queued packet and reports how many actually went out. A short count means some destinations
@@ -79,7 +80,6 @@ func (b *SendBatch) Flush() (int, error) {
 	clear(b.bufs)
 	b.bufs = b.bufs[:0]
 	b.dsts = b.dsts[:0]
-	b.tags = b.tags[:0]
 	b.arena.Reset()
 	return written, err
 }