fix multithread related crashes in skpdiff

tools/skpdiff/SkDifferentPixelsMetric_cpu.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <cstring>
 
 #include "SkBitmap.h"
 
 #include "SkDifferentPixelsMetric.h"
 #include "skpdiff_util.h"
 
 struct SkDifferentPixelsMetric::QueuedDiff {
     bool finished;
     double result;
     SkTDArray<SkIPoint>* poi;
 };
 
 const char* SkDifferentPixelsMetric::getName() {
     return "different_pixels";
 }
 
 int SkDifferentPixelsMetric::queueDiff(SkBitmap* baseline, SkBitmap* test) {
     double startTime = get_seconds();
+
+    // since we currently synchronously do the diff within queueDiff we only need
+    // to protect the createion of the ID and queuedDiff object.
+    SkAutoMutexAcquire queueLock(fQueueMutex);
     int diffID = fQueuedDiffs.count();
     QueuedDiff* diff = fQueuedDiffs.push();
+    queueLock.release();
+
     SkTDArray<SkIPoint>* poi = diff->poi = new SkTDArray<SkIPoint>();
 
     // If we never end up running the kernel, include some safe defaults in the result.
     diff->finished = false;
     diff->result = -1;
 
     // Ensure the images are comparable
     if (baseline->width() != test->width() || baseline->height() != test->height() ||
         baseline->width() <= 0 || baseline->height() <= 0 ||
         baseline->config() != test->config()) {
@@ skipping 47 matching lines @@
     return fQueuedDiffs[id].result;
 }
 
 int SkDifferentPixelsMetric::getPointsOfInterestCount(int id) {
     return fQueuedDiffs[id].poi->count();
 }
 
 SkIPoint* SkDifferentPixelsMetric::getPointsOfInterest(int id) {
     return fQueuedDiffs[id].poi->begin();
 }