Fixes for SkRWBuffer

include/core/SkRWBuffer.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkRWBuffer_DEFINED
 #define SkRWBuffer_DEFINED
 
@@ skipping 35 matching lines @@
 
         /**
          *  Advance to the next contiguous block of memory, returning true if there is another
          *  block, or false if the iterator is exhausted.
          */
         bool next();
         
     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:
     SkRWBuffer(size_t initialCapacity = 0);
     ~SkRWBuffer();
 
     size_t size() const { return fTotalUsed; }
     void append(const void* buffer, size_t length);
     void* append(size_t length);
 
     SkROBuffer* newRBufferSnapshot() const;
     SkStreamAsset* newStreamSnapshot() const;
 
 #ifdef SK_DEBUG
     void validate() const;
 #else
     void validate() const {}
 #endif
     
 private:
     SkBufferHead*   fHead;
     SkBufferBlock*  fTail;
     size_t          fTotalUsed;
 };
 
 #endif