fix multithread related crashes in skpdiff

tools/skpdiff/SkPMetric.cpp

 #include <cmath>
 
 #include "SkBitmap.h"
 #include "skpdiff_util.h"
 #include "SkPMetric.h"
 #include "SkPMetricUtil_generated.h"
 
 struct RGB {
     float r, g, b;
 };
@@ skipping 412 matching lines @@
     SkDELETE_ARRAY(contrastSensitivityTable);
     return 1.0 - (double)failures / (width * height);
 }
 
 const char* SkPMetric::getName() {
     return "perceptual";
 }
 
 int SkPMetric::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_back();
+    queueLock.release();
+
     diff.result = 0.0;
 
     // Ensure the images are comparable
     if (baseline->width() != test->width() || baseline->height() != test->height() ||
                     baseline->width() <= 0 || baseline->height() <= 0) {
         diff.finished = true;
         return diffID;
     }
 
     ImageLAB baselineLAB(baseline->width(), baseline->height());
     ImageLAB testLAB(baseline->width(), baseline->height());
 
     bitmap_to_cielab(baseline, &baselineLAB);
     bitmap_to_cielab(test, &testLAB);
 
     diff.result = pmetric(&baselineLAB, &testLAB, &diff.poi);
 
     SkDebugf("Time: %f\n", (get_seconds() - startTime));
 
     return diffID;
 }
 
 
 void SkPMetric::deleteDiff(int id) {
-
+    // clean up the POI array to save space
+    return fQueuedDiffs[id].poi.reset();
 }
 
 bool SkPMetric::isFinished(int id) {
     return fQueuedDiffs[id].finished;
 }
 
 double SkPMetric::getResult(int id) {
     return fQueuedDiffs[id].result;
 }
 
 int SkPMetric::getPointsOfInterestCount(int id) {
     return fQueuedDiffs[id].poi.count();
 }
 
 SkIPoint* SkPMetric::getPointsOfInterest(int id) {
     return fQueuedDiffs[id].poi.begin();
 }