testing: fix memory blowup when formatting many lines.

Fixes #3830.

R=golang-dev, r
CC=golang-dev, remy
https://golang.org/cl/6373047

diff --git a/src/pkg/testing/testing.go b/src/pkg/testing/testing.go
index f59ce8ed6f7300d66ce21786dbf1225e8dc6360e..6a25cb31052fa210e68f9164e32544599b55f6ec 100644 (file)
--- a/src/pkg/testing/testing.go
+++ b/src/pkg/testing/testing.go
@@ -79,6 +79,7 @@
 package testing
 
 import (
+       "bytes"
        "flag"
        "fmt"
        "os"
@@ -128,37 +129,42 @@ func Short() bool {
        return *short
 }
 
-// decorate inserts the final newline if needed and indentation tabs for formatting.
-// If addFileLine is true, it also prefixes the string with the file and line of the call site.
-func decorate(s string, addFileLine bool) string {
-       if addFileLine {
-               _, file, line, ok := runtime.Caller(3) // decorate + log + public function.
-               if ok {
-                       // Truncate file name at last file name separator.
-                       if index := strings.LastIndex(file, "/"); index >= 0 {
-                               file = file[index+1:]
-                       } else if index = strings.LastIndex(file, "\\"); index >= 0 {
-                               file = file[index+1:]
-                       }
-               } else {
-                       file = "???"
-                       line = 1
+// decorate prefixes the string with the file and line of the call site
+// and inserts the final newline if needed and indentation tabs for formatting.
+func decorate(s string) string {
+       _, file, line, ok := runtime.Caller(3) // decorate + log + public function.
+       if ok {
+               // Truncate file name at last file name separator.
+               if index := strings.LastIndex(file, "/"); index >= 0 {
+                       file = file[index+1:]
+               } else if index = strings.LastIndex(file, "\\"); index >= 0 {
+                       file = file[index+1:]
                }
-               s = fmt.Sprintf("%s:%d: %s", file, line, s)
-       }
-       s = "\t" + s // Every line is indented at least one tab.
-       n := len(s)
-       if n > 0 && s[n-1] != '\n' {
-               s += "\n"
-               n++
+       } else {
+               file = "???"
+               line = 1
        }
-       for i := 0; i < n-1; i++ { // -1 to avoid final newline
-               if s[i] == '\n' {
+       buf := new(bytes.Buffer)
+       fmt.Fprintf(buf, "%s:%d: ", file, line)
+
+       lines := strings.Split(s, "\n")
+       for i, line := range lines {
+               if i > 0 {
+                       buf.WriteByte('\n')
+               }
+               // Every line is indented at least one tab.
+               buf.WriteByte('\t')
+               if i > 0 {
                        // Second and subsequent lines are indented an extra tab.
-                       return s[0:i+1] + "\t" + decorate(s[i+1:n], false)
+                       buf.WriteByte('\t')
                }
+               buf.WriteString(line)
+       }
+       if l := len(s); l > 0 && s[len(s)-1] != '\n' {
+               // Add final new line if needed.
+               buf.WriteByte('\n')
        }
-       return s
+       return buf.String()
 }
 
 // T is a type passed to Test functions to manage test state and support formatted test logs.
@@ -204,7 +210,7 @@ func (c *common) FailNow() {
 
 // log generates the output. It's always at the same stack depth.
 func (c *common) log(s string) {
-       c.output = append(c.output, decorate(s, true)...)
+       c.output = append(c.output, decorate(s)...)
 }
 
 // Log formats its arguments using default formatting, analogous to Println(),