net/{batching,packet},wgengine/magicsock: export batchingConn (#16848)

For eventual use by net/udprelay.Server.

Updates tailscale/corp#31164

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/batching/conn.go

@@ -0,0 +1,48 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package batching implements a socket optimized for increased throughput.
+package batching
+
+import (
+	"net/netip"
+
+	"golang.org/x/net/ipv4"
+	"golang.org/x/net/ipv6"
+	"tailscale.com/net/packet"
+	"tailscale.com/types/nettype"
+)
+
+var (
+	// This acts as a compile-time check for our usage of ipv6.Message in
+	// [Conn] for both IPv6 and IPv4 operations.
+	_ ipv6.Message = ipv4.Message{}
+)
+
+// Conn is a nettype.PacketConn that provides batched i/o using
+// platform-specific optimizations, e.g. {recv,send}mmsg & UDP GSO/GRO.
+//
+// Conn originated from (and is still used by) magicsock where its API was
+// strongly influenced by [wireguard-go/conn.Bind] constraints, namely
+// wireguard-go's ownership of packet memory.
+type Conn interface {
+	nettype.PacketConn
+	// ReadBatch reads messages from [Conn] into msgs. It returns the number of
+	// messages the caller should evaluate for nonzero len, as a zero len
+	// message may fall on either side of a nonzero.
+	//
+	// Each [ipv6.Message.OOB] must be sized to at least MinControlMessageSize().
+	// len(msgs) must be at least MinReadBatchMsgsLen().
+	ReadBatch(msgs []ipv6.Message, flags int) (n int, err error)
+	// WriteBatchTo writes buffs to addr.
+	//
+	// If geneve.VNI.IsSet(), then geneve is encoded into the space preceding
+	// offset, and offset must equal [packet.GeneveFixedHeaderLength]. If
+	// !geneve.VNI.IsSet() then the space preceding offset is ignored.
+	//
+	// len(buffs) must be <= batchSize supplied in TryUpgradeToConn().
+	//
+	// WriteBatchTo may return a [neterror.ErrUDPGSODisabled] error if UDP GSO
+	// was disabled as a result of a send error.
+	WriteBatchTo(buffs [][]byte, addr netip.AddrPort, geneve packet.GeneveHeader, offset int) error
+}