log: don't format if writing to io.Discard

Fixes #47164

Change-Id: Ied03842360be4c86f1d9ead816f12c057a1f8dad
Reviewed-on: https://go-review.googlesource.com/c/go/+/348741
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Valentin Deleplace <deleplace@google.com>

src/log/log.go

@@ -20,6 +20,7 @@ import (
 	"os"
 	"runtime"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -55,6 +56,7 @@ type Logger struct {
 	flag      int        // properties
 	out       io.Writer  // destination for output
 	buf       []byte     // for accumulating text to write
+	isDiscard int32      // atomic boolean: whether out == io.Discard
 }
 
 // New creates a new Logger. The out variable sets the
@@ -63,7 +65,11 @@ type Logger struct {
 // after the log header if the Lmsgprefix flag is provided.
 // The flag argument defines the logging properties.
 func New(out io.Writer, prefix string, flag int) *Logger {
-	return &Logger{out: out, prefix: prefix, flag: flag}
+	l := &Logger{out: out, prefix: prefix, flag: flag}
+	if out == io.Discard {
+		l.isDiscard = 1
+	}
+	return l
 }
 
 // SetOutput sets the output destination for the logger.
@@ -71,6 +77,11 @@ func (l *Logger) SetOutput(w io.Writer) {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 	l.out = w
+	isDiscard := int32(0)
+	if w == io.Discard {
+		isDiscard = 1
+	}
+	atomic.StoreInt32(&l.isDiscard, isDiscard)
 }
 
 var std = New(os.Stderr, "", LstdFlags)
@@ -188,16 +199,29 @@ func (l *Logger) Output(calldepth int, s string) error {
 // Printf calls l.Output to print to the logger.
 // Arguments are handled in the manner of fmt.Printf.
 func (l *Logger) Printf(format string, v ...interface{}) {
+	if atomic.LoadInt32(&l.isDiscard) != 0 {
+		return
+	}
 	l.Output(2, fmt.Sprintf(format, v...))
 }
 
 // Print calls l.Output to print to the logger.
 // Arguments are handled in the manner of fmt.Print.
-func (l *Logger) Print(v ...interface{}) { l.Output(2, fmt.Sprint(v...)) }
+func (l *Logger) Print(v ...interface{}) {
+	if atomic.LoadInt32(&l.isDiscard) != 0 {
+		return
+	}
+	l.Output(2, fmt.Sprint(v...))
+}
 
 // Println calls l.Output to print to the logger.
 // Arguments are handled in the manner of fmt.Println.
-func (l *Logger) Println(v ...interface{}) { l.Output(2, fmt.Sprintln(v...)) }
+func (l *Logger) Println(v ...interface{}) {
+	if atomic.LoadInt32(&l.isDiscard) != 0 {
+		return
+	}
+	l.Output(2, fmt.Sprintln(v...))
+}
 
 // Fatal is equivalent to l.Print() followed by a call to os.Exit(1).
 func (l *Logger) Fatal(v ...interface{}) {
@@ -277,9 +301,7 @@ func (l *Logger) Writer() io.Writer {
 
 // SetOutput sets the output destination for the standard logger.
 func SetOutput(w io.Writer) {
-	std.mu.Lock()
-	defer std.mu.Unlock()
-	std.out = w
+	std.SetOutput(w)
 }
 
 // Flags returns the output flags for the standard logger.
@@ -314,18 +336,27 @@ func Writer() io.Writer {
 // Print calls Output to print to the standard logger.
 // Arguments are handled in the manner of fmt.Print.
 func Print(v ...interface{}) {
+	if atomic.LoadInt32(&std.isDiscard) != 0 {
+		return
+	}
 	std.Output(2, fmt.Sprint(v...))
 }
 
 // Printf calls Output to print to the standard logger.
 // Arguments are handled in the manner of fmt.Printf.
 func Printf(format string, v ...interface{}) {
+	if atomic.LoadInt32(&std.isDiscard) != 0 {
+		return
+	}
 	std.Output(2, fmt.Sprintf(format, v...))
 }
 
 // Println calls Output to print to the standard logger.
 // Arguments are handled in the manner of fmt.Println.
 func Println(v ...interface{}) {
+	if atomic.LoadInt32(&std.isDiscard) != 0 {
+		return
+	}
 	std.Output(2, fmt.Sprintln(v...))
 }
 

src/log/log_test.go

@@ -9,6 +9,7 @@ package log
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"os"
 	"regexp"
 	"strings"
@@ -20,7 +21,7 @@ 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         = `(60|62):` // must update if the calls to l.Printf / l.Print below move
+	Rline         = `(61|63):` // 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
 )
@@ -179,6 +180,17 @@ func TestEmptyPrintCreatesLine(t *testing.T) {
 	}
 }
 
+func TestDiscard(t *testing.T) {
+	l := New(io.Discard, "", 0)
+	s := strings.Repeat("a", 102400)
+	c := testing.AllocsPerRun(100, func() { l.Printf("%s", s) })
+	// One allocation for slice passed to Printf,
+	// but none for formatting of long string.
+	if c > 1 {
+		t.Errorf("got %v allocs, want at most 1", c)
+	}
+}
+
 func BenchmarkItoa(b *testing.B) {
 	dst := make([]byte, 0, 64)
 	for i := 0; i < b.N; i++ {