net/udprelay: use batching.Conn (#16866)

This significantly improves throughput of a peer relay server on Linux.

Server.packetReadLoop no longer passes sockets down the stack. Instead,
packet handling methods return a netip.AddrPort and []byte, which
packetReadLoop gathers together for eventual batched writes on the
appropriate socket(s).

Updates tailscale/corp#31164

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

net/batching/conn.go

@@ -32,7 +32,6 @@ type Conn interface {
 	// 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.
 	//

net/batching/conn_default.go

@@ -19,3 +19,5 @@ var controlMessageSize = 0
 func MinControlMessageSize() int {
 	return controlMessageSize
 }
+
+const IdealBatchSize = 1

net/batching/conn_linux.go

@@ -384,7 +384,7 @@ func setGSOSizeInControl(control *[]byte, gsoSize uint16) {
 }
 
 // TryUpgradeToConn probes the capabilities of the OS and pconn, and upgrades
-// pconn to a [Conn] if appropriate. A batch size of MinReadBatchMsgsLen() is
+// pconn to a [Conn] if appropriate. A batch size of [IdealBatchSize] is
 // suggested for the best performance.
 func TryUpgradeToConn(pconn nettype.PacketConn, network string, batchSize int) nettype.PacketConn {
 	if runtime.GOOS != "linux" {
@@ -457,6 +457,4 @@ func MinControlMessageSize() int {
 	return controlMessageSize
 }
 
-func MinReadBatchMsgsLen() int {
-	return 128
-}
+const IdealBatchSize = 128

net/batching/conn_linux_test.go

@@ -310,7 +310,7 @@ func TestMinReadBatchMsgsLen(t *testing.T) {
 	// So long as magicsock uses [Conn], and [wireguard-go/conn.Bind] API is
 	// shaped for wireguard-go to control packet memory, these values should be
 	// aligned.
-	if MinReadBatchMsgsLen() != conn.IdealBatchSize {
-		t.Fatalf("MinReadBatchMsgsLen():%d != conn.IdealBatchSize(): %d", MinReadBatchMsgsLen(), conn.IdealBatchSize)
+	if IdealBatchSize != conn.IdealBatchSize {
+		t.Fatalf("IdealBatchSize: %d != conn.IdealBatchSize(): %d", IdealBatchSize, conn.IdealBatchSize)
 	}
 }