better and batched tun interface

overlay/batch/passthrough.go

@@ -0,0 +1,57 @@
+package batch
+
+import (
+	"io"
+
+	"github.com/slackhq/nebula/udp"
+)
+
+// Passthrough is a RxBatcher that doesn't batch anything, it just accumulates and then sends packets.
+type Passthrough struct {
+	out     io.Writer
+	slots   [][]byte
+	backing []byte
+	cursor  int
+}
+
+func NewPassthrough(w io.Writer) *Passthrough {
+	const baseNumSlots = 128
+	return &Passthrough{
+		out:     w,
+		slots:   make([][]byte, 0, baseNumSlots),
+		backing: make([]byte, 0, baseNumSlots*udp.MTU),
+	}
+}
+
+func (p *Passthrough) Reserve(sz int) []byte {
+	if len(p.backing)+sz > cap(p.backing) {
+		// Grow: allocate a fresh backing. Already-committed slices still
+		// reference the old array and remain valid until Flush drops them.
+		newCap := max(cap(p.backing)*2, sz)
+		p.backing = make([]byte, 0, newCap)
+	}
+	start := len(p.backing)
+	p.backing = p.backing[:start+sz]
+	return p.backing[start : start+sz : start+sz] //return zero length, sz-cap slice
+}
+
+func (p *Passthrough) Commit(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
+		}
+	}
+	for i := range p.slots {
+		p.slots[i] = nil
+	}
+	p.slots = p.slots[:0]
+	p.backing = p.backing[:0]
+	return firstErr
+}