Handle SkStream::rewind properly.

include/core/SkStream.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkStream_DEFINED
 #define SkStream_DEFINED
 
@@ skipping 78 matching lines @@
     SkScalar readScalar();
     size_t   readPackedUInt();
 
     /**
      *  Reconstitute an SkData object that was written to the stream
      *  using SkWStream::writeData().
      */
     SkData* readData();
 
 //SkStreamRewindable
-    /** Rewinds to the beginning of the stream. If this cannot be done, return false. */
+    /** Rewinds to the beginning of the stream. The current position is not affected
+     *  when false is returned.
+     *
+     *  Returns false iff after this call one of the following is true:
+     *      * The stream is known to be somewhere other than the beginning.
+     *      * It is unknown whether the stream is at the beginning.

[reed1, 2013/08/27 20:00:37]

Are both of these iff clauses true at the same times as "the position is not
affected" is true?

[scroggo, 2013/08/27 20:12:56]

No. An implementation could return false if the stream was already at the
beginning - this would satisfy the condition of the initial statement, but not
the iff clause.

Our decoders now fail if rewind returned false, so I want to make it clear to
anyone writing an implementation that they should not return false if the stream
was already at the beginning (which should not cause a failure to decode). The
previous comment leaves open to interpretation what it means to be able to
rewind, IMO.

+     */
     virtual bool rewind() { return false; }
 
     /** Duplicates this stream. If this cannot be done, returns NULL.
      *  The returned stream will be positioned at the beginning of its data.
      */
     virtual SkStreamRewindable* duplicate() const { return NULL; }
 
 //SkStreamSeekable
     /** Returns true if this stream can report it's current position. */
     virtual bool hasPosition() const { return false; }
@@ skipping 348 matching lines @@
     virtual void newline() SK_OVERRIDE;
 
 private:
     typedef SkWStream INHERITED;
 };
 
 // for now
 typedef SkFILEStream SkURLStream;
 
 #endif