fix multithread related crashes in skpdiff

tools/skpdiff/SkDiffContext.h

Index: tools/skpdiff/SkDiffContext.h
diff --git a/tools/skpdiff/SkDiffContext.h b/tools/skpdiff/SkDiffContext.h
index 20193b38514248e77ba6e7a34752cf299a413d14..42282d2f670e32cc4b2515d2cb5db13de80dab17 100644
--- a/tools/skpdiff/SkDiffContext.h
+++ b/tools/skpdiff/SkDiffContext.h
@@ -8,9 +8,11 @@
 #ifndef SkDiffContext_DEFINED
 #define SkDiffContext_DEFINED
 
+#include "SkThread.h"
 #include "SkString.h"
 #include "SkTArray.h"
 #include "SkTDArray.h"
+#include "SkTLList.h"
 
 class SkWStream;
 class SkImageDiffer;
@@ -110,13 +112,22 @@ private:
         SkString           fBaselinePath;
         SkString               fTestPath;
         SkTArray<DiffData>        fDiffs;
-        DiffRecord*                fNext;
     };
 
+    // This is needed to work around a bug in the multithreaded case where the
+    // image decoders are crashing when large numbers of threads are invoking
+    // the decoder at the same time.
+    // see https://code.google.com/p/skia/issues/detail?id=1803
+    SkMutex fImageMutex;
+
+    // Used to protect access to fRecords and ensure only one thread is
+    // adding new entries and/or iterating over the existing entries at a time
+    SkMutex fRecordMutex;

[mtklein, 2013/11/06 14:24:36]

You don't seem to grab this mutex when iterating over fRecords.  Is that only
done from one thread?

[djsollen, 2013/11/06 15:01:58]

I just forgot to clean up that comment after updating the behavior at Leon's
suggestion.

+
     // We use linked list for the records so that their pointers remain stable. A resizable array
     // might change its pointers, which would make it harder for async diffs to record their
     // results.
-    DiffRecord * fRecords;
+    SkTLList<DiffRecord> fRecords;
 
     SkImageDiffer** fDiffers;
     int fDifferCount;