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_test.go

@@ -0,0 +1,64 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tcpinfo
+
+import (
+	"bytes"
+	"io"
+	"net"
+	"runtime"
+	"testing"
+)
+
+func TestRTT(t *testing.T) {
+	switch runtime.GOOS {
+	case "linux", "darwin":
+	default:
+		t.Skipf("not currently supported on %s", runtime.GOOS)
+	}
+
+	ln, err := net.Listen("tcp4", "localhost:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+
+	go func() {
+		for {
+			c, err := ln.Accept()
+			if err != nil {
+				return
+			}
+			t.Cleanup(func() { c.Close() })
+
+			// Copy from the client to nowhere
+			go io.Copy(io.Discard, c)
+		}
+	}()
+
+	conn, err := net.Dial("tcp4", ln.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Write a bunch of data to the conn to force TCP session establishment
+	// and a few packets.
+	junkData := bytes.Repeat([]byte("hello world\n"), 1024*1024)
+	for i := 0; i < 10; i++ {
+		if _, err := conn.Write(junkData); err != nil {
+			t.Fatalf("error writing junk data [%d]: %v", i, err)
+		}
+	}
+
+	// Get the RTT now
+	rtt, err := RTT(conn)
+	if err != nil {
+		t.Fatalf("error getting RTT: %v", err)
+	}
+	if rtt == 0 {
+		t.Errorf("expected RTT > 0")
+	}
+
+	t.Logf("TCP rtt: %v", rtt)
+}