runtime: fix Stack

Fixes #8626.

LGTM=bradfitz
R=golang-codereviews
CC=bradfitz, golang-codereviews, iant, r
https://golang.org/cl/137050043

src/pkg/runtime/mprof.go

@@ -584,13 +584,13 @@ func Stack(buf []byte, all bool) int {
 
 	n := 0
 	if len(buf) > 0 {
-		gp.writebuf = buf
+		gp.writebuf = buf[0:0:len(buf)]
 		goroutineheader(gp)
 		traceback(pc, sp, 0, gp)
 		if all {
 			tracebackothers(gp)
 		}
-		n = len(buf) - len(gp.writebuf)
+		n = len(gp.writebuf)
 		gp.writebuf = nil
 	}
 

src/pkg/runtime/stack_test.go

@@ -6,6 +6,7 @@ package runtime_test
 
 import (
 	. "runtime"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -331,3 +332,12 @@ func TestStackCache(t *testing.T) {
 		}
 	}
 }
+
+func TestStackOutput(t *testing.T) {
+	b := make([]byte, 1024)
+	stk := string(b[:Stack(b, false)])
+	if !strings.HasPrefix(stk, "goroutine ") {
+		t.Errorf("Stack (len %d):\n%s", len(stk), stk)
+		t.Errorf("Stack output should begin with \"goroutine \"")
+	}
+}