SkRWBuffer for thread-safe 'stream' sharing

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
+
+#include "SkRefCnt.h"
+
+struct SkBufferBlock;
+struct SkBufferHead;
+class SkData;
+class SkRWBuffer;
+class SkStreamAsset;
+
+/**
+ *  Contains a read-only, thread-sharable block of memory. To access the memory, the caller must
+ *  instantiate a local iterator, as the memory is stored in 1 or more contiguous blocks.
+ */
+class SkRBuffer : public SkRefCnt {
+public:
+    /**
+     *  Return the logical length of the data owned/shared by this buffer. It may be stored in
+     *  multiple contiguous blocks, accessible via the iterator.
+     */
+    size_t size() const { return fUsed; }
+    
+    class Iter {
+    public:
+        Iter(const SkRBuffer*);
+
+        void reset(const SkRBuffer*);
+
+        /**
+         *  Return the current continuous block of memory, or NULL if the iterator is exhausted
+         */
+        const void* data() const;
+
+        /**
+         *  Returns the number of bytes in the current continguous block of memory, or 0 if the
+         *  iterator is exhausted.
+         */
+        size_t size() const;
+
+        /**
+         *  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;
+    };
+
+private:
+    SkRBuffer(const SkBufferHead* head, size_t used);
+    virtual ~SkRBuffer();
+    
+    const SkBufferHead* fHead;
+    const size_t        fUsed;
+    
+    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.
+ */
+class SkRWBuffer {
+public:
+    SkRWBuffer(size_t initialCapacity = 0);
+    ~SkRWBuffer();
+    
+    size_t size() const { return fTotalUsed; }
+    void append(const void* buffer, size_t length);
+
+    SkData* newDataSnapshot() const;

[bungeman-skia, 2015/04/27 14:52:42]

I would be very, very careful about adding this, as it appears that the only way
to implement this is to make a copy. Blink's SharedBuffer has a "data()" method
which, though it has lots of warnings about what a bad idea it is to call it, is
the most used method on SharedBuffer. If allowed, this *will* be over/misused,
to the point where there is no point in having the complexity of this class.
(Much as there is little point these days to SharredBuffer, since it requires
*every user* to use it correctly or it becomes no better than just making copies
all the time.)

[reed2, 2015/04/28 02:25:00]

will remove for now

+    SkRBuffer* 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