Add GetSystemTime query option

GetSystemTime is a callback used to override the default method of
obtaining the local system time during time synchronization.

Author: chrisceder

ntp.go

@@ -249,6 +249,11 @@ type QueryOptions struct {
 	// DEPRECATED. Use Dialer instead.
 	Dial func(laddr string, lport int, raddr string, rport int) (net.Conn, error)
 
+	// GetSystemTime is a callback used to override the default method of
+	// obtaining the local system time during time synchronization. If not
+	// specified, time.Now is used.
+	GetSystemTime func() time.Time
+
 	// Port indicates the port used to reach the remote NTP server.
 	//
 	// DEPRECATED. Embed the port number in the query address string instead.
@@ -495,6 +500,9 @@ func getTime(address string, opt *QueryOptions) (*header, ntpTime, error) {
 	if opt.Dialer == nil {
 		opt.Dialer = defaultDialer
 	}
+	if opt.GetSystemTime == nil {
+		opt.GetSystemTime = time.Now
+	}
 
 	// Compose a conforming host:port remote address string if the address
 	// string doesn't already contain a port.
@@ -566,7 +574,7 @@ func getTime(address string, opt *QueryOptions) (*header, ntpTime, error) {
 	appendMAC(&xmitBuf, opt.Auth, authKey)
 
 	// Transmit the query and keep track of when it was transmitted.
-	xmitTime := time.Now()
+	xmitTime := opt.GetSystemTime()
 	_, err = con.Write(xmitBuf.Bytes())
 	if err != nil {
 		return nil, 0, err
@@ -581,11 +589,10 @@ func getTime(address string, opt *QueryOptions) (*header, ntpTime, error) {
 	// Keep track of the time the response was received. As of go 1.9, the
 	// time package uses a monotonic clock, so delta will never be less than
 	// zero for go version 1.9 or higher.
-	delta := time.Since(xmitTime)
-	if delta < 0 {
-		delta = 0
+	recvTime := opt.GetSystemTime()
+	if recvTime.Sub(xmitTime) < 0 {
+		recvTime = xmitTime
 	}
-	recvTime := xmitTime.Add(delta)
 
 	// Parse the response header.
 	recvBuf = recvBuf[:recvBytes]

ntp_test.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"os"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -154,6 +155,47 @@ func TestOnlineTTL(t *testing.T) {
 	assert.NotNil(t, err)
 }
 
+func TestOnlineCustomGetSystemTime(t *testing.T) {
+	if host == "localhost" {
+		t.Skip("Timeout test not available with localhost NTP server.")
+		return
+	}
+
+	var simuTime atomic.Value
+	simuTime.Store(time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC))
+
+	const timerInterval = 1 * time.Millisecond
+	ctx := t.Context()
+
+	// Start a simulated clock independent of the system wall clock,
+	// initialized at 2020-01-01T00:00:00, advancing in 1 ms increments.
+	go func() {
+		ticker := time.NewTicker(timerInterval)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				current := simuTime.Load().(time.Time)
+				simuTime.Store(current.Add(timerInterval))
+			}
+		}
+	}()
+
+	r, err := QueryWithOptions(host, QueryOptions{
+		GetSystemTime: func() time.Time { return simuTime.Load().(time.Time) },
+	})
+	if isNil(t, host, err) {
+		tm := simuTime.Load().(time.Time)
+		trueTime := tm.Add(r.ClockOffset)
+		t.Logf(" Custom Time: %s\n", tm.Format(timeFormat))
+		t.Logf("  ~True Time: %s\n", trueTime.Format(timeFormat))
+		t.Logf("~ClockOffset: %v\n", trueTime.Sub(tm))
+	}
+}
+
 func TestOfflineConvertLong(t *testing.T) {
 	ts := []ntpTime{0x0, 0xff800000, 0x1ff800000, 0x80000000ff800000, 0xffffffffff800000}
 	for _, v := range ts {