Add a method to read a stream without advancing it.

include/core/SkStream.h

Index: include/core/SkStream.h
diff --git a/include/core/SkStream.h b/include/core/SkStream.h
index c227765b363624afbf7a3f98f39d480c7f05726d..7e89ef94095eafeb31d7b45164defb551e1b84bd 100644
--- a/include/core/SkStream.h
+++ b/include/core/SkStream.h
@@ -63,6 +63,34 @@ public:
         return this->read(NULL, size);
     }
 
+    /**
+     *  Return value for peek() if the stream does not support peeking, or if
+     *  it cannot from this offset.
+     */
+    static const size_t PEEK_UNSUPPORTED = (size_t) -1;

[bungeman-skia, 2015/03/31 22:04:48]

I think we tend toward enums with 'k' prefixes (though I'm fine with them not
being enums, I think we require 'enum foo : size_t' now). These all caps things
look like defines.

[scroggo, 2015/04/01 18:38:31]

I'm fine with renaming them to use 'k' prefixes (prefices?).
I was unfamiliar with that option. Sounds good to me.

Done.

+
+    /**
+     *  Return value for peek() if the stream encountered a fatal error. The
+     *  stream is no unreliable.
+     */
+    static const size_t PEEK_FAILURE     = (size_t) -2;
+
+    /**
+     *  Attempt to peek at size bytes.
+     *  If this stream supports peeking, copy up to size bytes into buffer. If
+     *  less than size is returned, the stream is less than size bytes from the
+     *  end or the stream has reached its limit for how many bytes it can peek.
+     *  If there is a fatal error, PEEK_FAILURE is returned, and the stream
+     *  cannot be used anymore.
+     *  After this call, the stream will have the same state it did beforehand,
+     *  unless there was an error (and PEEK_FAILURE was returned).
+     *  If peeking is unsupported, returns PEEK_UNSUPPORTED
+     *  @param buffer Must not be NULL. Destination to copy bytes.
+     *  @param size Number of bytes to copy.
+     *  @return the number of bytes actually read, or an error message.
+     */
+    virtual size_t peek(void* buffer, size_t size) { return PEEK_UNSUPPORTED; }

[bungeman-skia, 2015/03/31 22:04:48]

We already have the pattern of either returning bool (rewind, seek, move) or
having a bool capability checker (hasPosition, hasLength). I'm not sure what to
do about this one, since the root issue is that 'read' doesn't do a good job of
error reporting itself.

+
     /** Returns true when all the bytes in the stream have been read.
      *  This may return true early (when there are no more bytes to be read)
      *  or late (after the first unsuccessful read).
@@ -144,6 +172,16 @@ public:
     bool seek(size_t position) override = 0;
     bool move(long offset) override = 0;
     SkStreamSeekable* fork() const override = 0;
+    // Default implementation reads and then resets the position.
+    size_t peek(void* buffer, size_t size) override {
+        SkASSERT(buffer != NULL);
+        const size_t position = this->getPosition();
+        const size_t bytesRead = this->read(buffer, size);
+        if (!this->seek(position)) {
+            return PEEK_FAILURE;
+        }
+        return bytesRead;
+    }
 };
 
 /** SkStreamAsset is a SkStreamSeekable for which getLength is required. */