batch: move shared-arena Reset ownership from lanes to their owner

overlay/batch/passthrough.go

@@ -8,11 +8,11 @@ import (
 
 // Passthrough is a RxBatcher that doesn't batch anything, it just accumulates and then sends packets.
 type Passthrough struct {
-	out   io.Writer
-	slots [][]byte
-	// arena is injected; see TCPCoalescer.arena for the contract.
-	arena  *Arena
-	cursor int
+	out      io.Writer
+	slots    [][]byte
+	reserver Reserver
+	resetter Resetter
+	cursor   int
 }
 
 const passthroughBaseNumSlots = 128
@@ -21,16 +21,17 @@ const passthroughBaseNumSlots = 128
 // standalone Passthrough batcher: 128 slots × udp.MTU ≈ 1.1 MiB.
 const DefaultPassthroughArenaCap = passthroughBaseNumSlots * udp.MTU
 
-func NewPassthrough(w io.Writer, arena *Arena) *Passthrough {
+func NewPassthrough(w io.Writer, reserver Reserver, resetter Resetter) *Passthrough {
 	return &Passthrough{
-		out:   w,
-		slots: make([][]byte, 0, passthroughBaseNumSlots),
-		arena: arena,
+		out:      w,
+		slots:    make([][]byte, 0, passthroughBaseNumSlots),
+		reserver: reserver,
+		resetter: resetter,
 	}
 }
 
 func (p *Passthrough) Reserve(sz int) []byte {
-	return p.arena.Reserve(sz)
+	return p.reserver(sz)
 }
 
 func (p *Passthrough) Commit(pkt []byte) error {
@@ -38,7 +39,17 @@ func (p *Passthrough) Commit(pkt []byte) error {
 	return nil
 }
 
+// Flush drains every queued packet and calls the configured Resetter
 func (p *Passthrough) Flush() error {
+	firstErr := p.drain()
+	if p.resetter != nil {
+		p.resetter()
+	}
+	return firstErr
+}
+
+// drain writes out every queued packet and clears the slot list.
+func (p *Passthrough) drain() error {
 	var firstErr error
 	for _, s := range p.slots {
 		_, err := p.out.Write(s)
@@ -48,6 +59,5 @@ func (p *Passthrough) Flush() error {
 	}
 	clear(p.slots)
 	p.slots = p.slots[:0]
-	p.arena.Reset()
 	return firstErr
 }