Fixes for SkRWBuffer

include/core/SkRWBuffer.h

Index: include/core/SkRWBuffer.h
diff --git a/include/core/SkRWBuffer.h b/include/core/SkRWBuffer.h
index 106e572587d8dcaa13d84e792bef40444e207696..7cf0b9b82c203bac09a4dd7db7d1461a5601393e 100644
--- a/include/core/SkRWBuffer.h
+++ b/include/core/SkRWBuffer.h
@@ -53,22 +53,30 @@ public:
     private:
         const SkBufferBlock* fBlock;
         size_t               fRemaining;
+        const SkROBuffer*    fBuffer;

[scroggo, 2016/04/08 16:15:28]

SkROBuffer is a const class, so should I remove this "const" here, as we have
done with SkPicture, etc?

     };
 
 private:
-    SkROBuffer(const SkBufferHead* head, size_t used);
+    SkROBuffer(const SkBufferHead* head, size_t used, const SkBufferBlock* tail);
     virtual ~SkROBuffer();
 
-    const SkBufferHead* fHead;
-    const size_t        fUsed;
+    const SkBufferHead*  fHead;
+    const size_t         fUsed;
+
+    // Final block that existed when this object was created. It is the only block that may
+    // have more bytes in it than this object knows about. Store its fUsed amount at creation
+    // time, so it can be read later without racing with a writer updating it.
+    const SkBufferBlock* fTail;
+    const size_t         fTailUsed;

[scroggo, 2016/04/08 16:15:29]

I'm not a huge fan of this name, but I don't have a better one. This is the
value of fUsed when this ROBuffer was created; fUsed may have increased since
then. It's more like fTailKnownUsed or something like that. Suggestions?

 
     friend class SkRWBuffer;
 };
 
 /**
  *  Accumulates bytes of memory that are "appended" to it, growing internal storage as needed.
- *  The growth is done such that at any time, a RBuffer or StreamAsset can be snapped off, which
- *  can see the previously stored bytes, but which will be unaware of any future writes.
+ *  The growth is done such that at any time in the writer's thread, an RBuffer or StreamAsset
+ *  can be snapped off (and safely passed to another thread). The RBuffer/StreamAsset snapshot
+ *  can see the previously stored bytes, but will be unaware of any future writes.
  */
 class SK_API SkRWBuffer {
 public: