the definitive tun offloads branch (#1704)

overlay/batch/passthrough.go

@@ -0,0 +1,38 @@
+package batch
+
+import (
+	"io"
+)
+
+// Passthrough is MultiCoalescer's verbatim lane: no batching, packets are written at Flush in the
+// order enqueued.
+type Passthrough struct {
+	out   io.Writer
+	slots [][]byte
+}
+
+func NewPassthrough(w io.Writer) *Passthrough {
+	return &Passthrough{
+		out:   w,
+		slots: make([][]byte, 0, 128),
+	}
+}
+
+// enqueue accepts one packet, already sorted into transmission order by dispatch.
+func (p *Passthrough) enqueue(pkt []byte) error {
+	p.slots = append(p.slots, pkt)
+	return nil
+}
+
+func (p *Passthrough) Flush() error {
+	var firstErr error
+	for _, s := range p.slots {
+		_, err := p.out.Write(s)
+		if err != nil && firstErr == nil {
+			firstErr = err
+		}
+	}
+	clear(p.slots)
+	p.slots = p.slots[:0]
+	return firstErr
+}