net/sockopts,wgengine/magicsock: export socket buffer sizing logic (#16909)

For eventual use by net/udprelay.Server

Updates tailscale/corp#31164

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

net/sockopts/sockopts.go

@@ -0,0 +1,37 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package sockopts contains logic for applying socket options.
+package sockopts
+
+import (
+	"net"
+	"runtime"
+
+	"tailscale.com/types/nettype"
+)
+
+// BufferDirection represents either the read/receive or write/send direction
+// of a socket buffer.
+type BufferDirection string
+
+const (
+	ReadDirection  BufferDirection = "read"
+	WriteDirection BufferDirection = "write"
+)
+
+func portableSetBufferSize(pconn nettype.PacketConn, direction BufferDirection, size int) error {
+	if runtime.GOOS == "plan9" {
+		// Not supported. Don't try. Avoid logspam.
+		return nil
+	}
+	var err error
+	if c, ok := pconn.(*net.UDPConn); ok {
+		if direction == WriteDirection {
+			err = c.SetWriteBuffer(size)
+		} else {
+			err = c.SetReadBuffer(size)
+		}
+	}
+	return err
+}