log: add flag LUTC, to use UTC time zone for time stamp

Issue 9483 suggests several approaches to correlating logs from
machines in different time zones. This approach is the simplest and
really should be sufficient: provide a way to clamp the time stamps
to UTC.

Fixes #9483.

Change-Id: If540b991d758c4d845a719779f8255ece7c452e7
Reviewed-on: https://go-review.googlesource.com/8761Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/log/log.go

@@ -32,11 +32,12 @@ const (
 	//	2009/01/23 01:23:23 message
 	// while flags Ldate | Ltime | Lmicroseconds | Llongfile produce,
 	//	2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message
-	Ldate         = 1 << iota     // the date: 2009/01/23
-	Ltime                         // the time: 01:23:23
+	Ldate         = 1 << iota     // the date in the local time zone: 2009/01/23
+	Ltime                         // the time in the local time zone: 01:23:23
 	Lmicroseconds                 // microsecond resolution: 01:23:23.123123.  assumes Ltime.
 	Llongfile                     // full file name and line number: /a/b/c/d.go:23
 	Lshortfile                    // final file name element and line number: d.go:23. overrides Llongfile
+	LUTC                          // if Ldate or Ltime is set, use UTC rather than the local time zone
 	LstdFlags     = Ldate | Ltime // initial values for the standard logger
 )
 
@@ -88,6 +89,9 @@ func itoa(buf *[]byte, i int, wid int) {
 
 func (l *Logger) formatHeader(buf *[]byte, t time.Time, file string, line int) {
 	*buf = append(*buf, l.prefix...)
+	if l.flag&LUTC != 0 {
+		t = t.UTC()
+	}
 	if l.flag&(Ldate|Ltime|Lmicroseconds) != 0 {
 		if l.flag&Ldate != 0 {
 			year, month, day := t.Date()

src/log/log_test.go

@@ -8,17 +8,19 @@ package log
 
 import (
 	"bytes"
+	"fmt"
 	"os"
 	"regexp"
 	"strings"
 	"testing"
+	"time"
 )
 
 const (
 	Rdate         = `[0-9][0-9][0-9][0-9]/[0-9][0-9]/[0-9][0-9]`
 	Rtime         = `[0-9][0-9]:[0-9][0-9]:[0-9][0-9]`
 	Rmicroseconds = `\.[0-9][0-9][0-9][0-9][0-9][0-9]`
-	Rline         = `(55|57):` // must update if the calls to l.Printf / l.Print below move
+	Rline         = `(57|59):` // must update if the calls to l.Printf / l.Print below move
 	Rlongfile     = `.*/[A-Za-z0-9_\-]+\.go:` + Rline
 	Rshortfile    = `[A-Za-z0-9_\-]+\.go:` + Rline
 )
@@ -119,6 +121,30 @@ func TestFlagAndPrefixSetting(t *testing.T) {
 	}
 }
 
+func TestUTCFlag(t *testing.T) {
+	var b bytes.Buffer
+	l := New(&b, "Test:", LstdFlags)
+	l.SetFlags(Ldate | Ltime | LUTC)
+	// Verify a log message looks right in the right time zone. Quantize to the second only.
+	now := time.Now().UTC()
+	l.Print("hello")
+	want := fmt.Sprintf("Test:%d/%.2d/%.2d %.2d:%.2d:%.2d hello\n",
+		now.Year(), now.Month(), now.Day(), now.Hour(), now.Minute(), now.Second())
+	got := b.String()
+	if got == want {
+		return
+	}
+	// It's possible we crossed a second boundary between getting now and logging,
+	// so add a second and try again. This should very nearly always work.
+	now.Add(time.Second)
+	want = fmt.Sprintf("Test:%d/%.2d/%.2d %.2d:%.2d:%.2d hello\n",
+		now.Year(), now.Month(), now.Day(), now.Hour(), now.Minute())
+	if got == want {
+		return
+	}
+	t.Errorf("got %q; want %q", got, want)
+}
+
 func TestEmptyPrintCreatesLine(t *testing.T) {
 	var b bytes.Buffer
 	l := New(&b, "Header:", LstdFlags)