net/tcpinfo: add package to allow fetching TCP information

This package contains platform-independent abstractions for fetching
information about an open TCP connection.

Updates #8413

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I236657b1060d7e6a45efc7a2f6aacf474547a2fe

net/tcpinfo/tcpinfo_linux.go

@@ -0,0 +1,33 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tcpinfo
+
+import (
+	"net"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+func rttImpl(conn *net.TCPConn) (time.Duration, error) {
+	rawConn, err := conn.SyscallConn()
+	if err != nil {
+		return 0, err
+	}
+
+	var (
+		tcpInfo *unix.TCPInfo
+		sysErr  error
+	)
+	err = rawConn.Control(func(fd uintptr) {
+		tcpInfo, sysErr = unix.GetsockoptTCPInfo(int(fd), unix.IPPROTO_TCP, unix.TCP_INFO)
+	})
+	if err != nil {
+		return 0, err
+	} else if sysErr != nil {
+		return 0, sysErr
+	}
+
+	return time.Duration(tcpInfo.Rtt) * time.Microsecond, nil
+}