the definitive tun offloads branch (#1704)

overlay/batch/tx_batch.go

@@ -0,0 +1,59 @@
+package batch
+
+import "net/netip"
+
+const SendBatchCap = 128
+
+// batchWriter is the minimal subset of udp.Conn needed by SendBatch to flush.
+type batchWriter interface {
+	WriteBatch(bufs [][]byte, addrs []netip.AddrPort) (int, error)
+}
+
+// SendBatch accumulates encrypted UDP packets and flushes them via WriteBatch.
+// One SendBatch is owned by each listenIn goroutine; no locking is needed.
+// Slots are backed by an Arena (see its docs)
+type SendBatch struct {
+	out   batchWriter
+	bufs  [][]byte
+	dsts  []netip.AddrPort
+	arena *Arena
+}
+
+// NewSendBatch makes a SendBatch with batchCap slots and an arenaSize byte buffer for slices to back those slots
+func NewSendBatch(out batchWriter, batchCap, arenaSize int) *SendBatch {
+	return &SendBatch{
+		out:   out,
+		bufs:  make([][]byte, 0, batchCap),
+		dsts:  make([]netip.AddrPort, 0, batchCap),
+		arena: NewArena(arenaSize),
+	}
+}
+
+func (b *SendBatch) Reserve(sz int) []byte {
+	return b.arena.Reserve(sz)
+}
+
+// Len reports how many packets are queued for the next Flush. Callers use
+// it to flush incrementally once a full sendmmsg batch has accumulated,
+// bounding how long the first packet of a large read batch waits.
+func (b *SendBatch) Len() int { return len(b.bufs) }
+
+func (b *SendBatch) Commit(pkt []byte, dst netip.AddrPort) {
+	b.bufs = append(b.bufs, pkt)
+	b.dsts = append(b.dsts, dst)
+}
+
+// Flush writes every queued packet and reports how many actually went out. A short count means some destinations
+// were undeliverable; the batch is drained either way.
+func (b *SendBatch) Flush() (int, error) {
+	var err error
+	written := 0
+	if len(b.bufs) > 0 {
+		written, err = b.out.WriteBatch(b.bufs, b.dsts)
+	}
+	clear(b.bufs)
+	b.bufs = b.bufs[:0]
+	b.dsts = b.dsts[:0]
+	b.arena.Reset()
+	return written, err
+}