net/ktimeout: add a package to set TCP user timeout

Setting a user timeout will be a more practical tuning knob for a number
of endpoints, this provides a way to set it.

Updates tailscale/corp#17587

Signed-off-by: James Tucker <james@tailscale.com>

net/ktimeout/ktimeout_linux_test.go

@@ -0,0 +1,46 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ktimeout
+
+import (
+	"net"
+	"testing"
+	"time"
+
+	"golang.org/x/net/nettest"
+	"golang.org/x/sys/unix"
+	"tailscale.com/util/must"
+)
+
+func TestSetUserTimeout(t *testing.T) {
+	l := must.Get(nettest.NewLocalListener("tcp"))
+	defer l.Close()
+
+	var err error
+	if e := must.Get(l.(*net.TCPListener).SyscallConn()).Control(func(fd uintptr) {
+		err = SetUserTimeout(fd, 0)
+	}); e != nil {
+		t.Fatal(e)
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	v := must.Get(unix.GetsockoptInt(int(must.Get(l.(*net.TCPListener).File()).Fd()), unix.SOL_TCP, unix.TCP_USER_TIMEOUT))
+	if v != 0 {
+		t.Errorf("TCP_USER_TIMEOUT: got %v; want 0", v)
+	}
+
+	if e := must.Get(l.(*net.TCPListener).SyscallConn()).Control(func(fd uintptr) {
+		err = SetUserTimeout(fd, 30*time.Second)
+	}); e != nil {
+		t.Fatal(e)
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	v = must.Get(unix.GetsockoptInt(int(must.Get(l.(*net.TCPListener).File()).Fd()), unix.SOL_TCP, unix.TCP_USER_TIMEOUT))
+	if v != 30000 {
+		t.Errorf("TCP_USER_TIMEOUT: got %v; want 30000", v)
+	}
+}