log: logging an empty string should still print a line

Print("") was printing a header but no line.

Fixes #9665.

Change-Id: Iac783187786065e1389ad6e8d7ef02c579ed7bd8
Reviewed-on: https://go-review.googlesource.com/8665
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/log/log.go

@@ -156,7 +156,7 @@ func (l *Logger) Output(calldepth int, s string) error {
 	l.buf = l.buf[:0]
 	l.formatHeader(&l.buf, now, file, line)
 	l.buf = append(l.buf, s...)
-	if len(s) > 0 && s[len(s)-1] != '\n' {
+	if len(s) == 0 || s[len(s)-1] != '\n' {
 		l.buf = append(l.buf, '\n')
 	}
 	_, err := l.out.Write(l.buf)

src/log/log_test.go

@@ -10,6 +10,7 @@ import (
 	"bytes"
 	"os"
 	"regexp"
+	"strings"
 	"testing"
 )
 
@@ -17,7 +18,7 @@ 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         = `(54|56):` // must update if the calls to l.Printf / l.Print below move
+	Rline         = `(55|57):` // 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
 )
@@ -118,6 +119,20 @@ func TestFlagAndPrefixSetting(t *testing.T) {
 	}
 }
 
+func TestEmptyPrintCreatesLine(t *testing.T) {
+	var b bytes.Buffer
+	l := New(&b, "Header:", LstdFlags)
+	l.Print()
+	l.Println("non-empty")
+	output := b.String()
+	if n := strings.Count(output, "Header"); n != 2 {
+		t.Errorf("expected 2 headers, got %d", n)
+	}
+	if n := strings.Count(output, "\n"); n != 2 {
+		t.Errorf("expected 2 lines, got %d", n)
+	}
+}
+
 func BenchmarkItoa(b *testing.B) {
 	dst := make([]byte, 0, 64)
 	for i := 0; i < b.N; i++ {