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.go

@@ -0,0 +1,51 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package tcpinfo provides platform-agnostic accessors to information about a
+// TCP connection (e.g. RTT, MSS, etc.).
+package tcpinfo
+
+import (
+	"errors"
+	"net"
+	"time"
+)
+
+var (
+	ErrNotTCP        = errors.New("tcpinfo: not a TCP conn")
+	ErrUnimplemented = errors.New("tcpinfo: unimplemented")
+)
+
+// RTT returns the RTT for the given net.Conn.
+//
+// If the net.Conn is not a *net.TCPConn and cannot be unwrapped into one, then
+// ErrNotTCP will be returned. If retrieving the RTT is not supported on the
+// current platform, ErrUnimplemented will be returned.
+func RTT(conn net.Conn) (time.Duration, error) {
+	tcpConn, err := unwrap(conn)
+	if err != nil {
+		return 0, err
+	}
+
+	return rttImpl(tcpConn)
+}
+
+// netConner is implemented by crypto/tls.Conn to unwrap into an underlying
+// net.Conn.
+type netConner interface {
+	NetConn() net.Conn
+}
+
+// unwrap attempts to unwrap a net.Conn into an underlying *net.TCPConn
+func unwrap(nc net.Conn) (*net.TCPConn, error) {
+	for {
+		switch v := nc.(type) {
+		case *net.TCPConn:
+			return v, nil
+		case netConner:
+			nc = v.NetConn()
+		default:
+			return nil, ErrNotTCP
+		}
+	}
+}