Handle SkStream::rewind properly.

include/core/SkStream.h

Index: include/core/SkStream.h
diff --git a/include/core/SkStream.h b/include/core/SkStream.h
index d7a105d3b84e1080fdc4157b1d40f52a1c68750c..3572d605bb3d1e15f5308f9031df145cdfe74b75 100644
--- a/include/core/SkStream.h
+++ b/include/core/SkStream.h
@@ -96,7 +96,9 @@ public:
     SkData* readData();
 
 //SkStreamRewindable
-    /** Rewinds to the beginning of the stream. If this cannot be done, return false. */
+    /** Rewinds to the beginning of the stream. If this cannot be done, return false.
+     *  Return true if the stream is already at the beginning.

[djsollen, 2013/08/22 19:59:18]

that is misleading for subclasses that don't override this method.

[bungeman-skia, 2013/08/22 20:19:00]

SkStreamRewindable makes this pure virtual so that such subclasses must override
this method. Though I see your point if no bytes have been read from the stream
yet then 'rewind' should return true. However, I'm not sure that all SkStreams
know this information. I suppose in some sense they must (in order to know what
to return from the next 'read').

I believe that there are currently no 'bare' SkStream implementations outside of
Android. I think this is implemented here and not pure virtual (like read) both
so that it could be added without breaking existing SkStream implementations and
because the thought was that if it did anything other than return false it would
be implemented fully. If all of the Android SkStreams get updated then this
could be made pure virtual, I suppose, with a note that SkStreams must
'partially' implement it. (ie, change this to 'Must return true if...')

[bungeman-skia, 2013/08/22 20:42:45]

Actually, an SkStream which is just wrapping stdin doesn't already know this
information. It would need an extra flag which would be set on the first read so
that it would know it was 'dirty'. I suppose requiring an extra flag on such
streams isn't all that bad.

[djsollen, 2013/08/23 12:15:13]

I wasn't proposing that we change the behavior in this CL, just that the comment
was misleading.

[scroggo, 2013/08/23 21:14:07]

New comment for your judgment:
Streams are expected to either return false always, if rewinding is not
supported, or rewind to the beginning and return true always, regardless of the
current position.

+     */
     virtual bool rewind() { return false; }
 
     /** Duplicates this stream. If this cannot be done, returns NULL.