Make SkCodec support peek() and read()

src/codec/SkWebpCodec.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodecPriv.h"
 #include "SkWebpCodec.h"
 #include "SkTemplates.h"
 
 // A WebP decoder on top of (subset of) libwebp
 // For more information on WebP image format, and libwebp library, see:
 //   https://code.google.com/speed/webp/
 //   http://www.webmproject.org/code/#libwebp-webp-image-library
 //   https://chromium.googlesource.com/webm/libwebp
 
 // If moving libwebp out of skia source tree, path for webp headers must be
 // updated accordingly. Here, we enforce using local copy in webp sub-directory.
 #include "webp/decode.h"
 #include "webp/encode.h"
 
-bool SkWebpCodec::IsWebp(SkStream* stream) {
+bool SkWebpCodec::IsWebp(const char* bytes, size_t bytesRead) {
     // WEBP starts with the following:
     // RIFFXXXXWEBPVP
     // Where XXXX is unspecified.
-    const char LENGTH = 14;
-    char bytes[LENGTH];
-    if (stream->read(&bytes, LENGTH) != LENGTH) {
-        return false;
-    }
-    return !memcmp(bytes, "RIFF", 4) && !memcmp(&bytes[8], "WEBPVP", 6);
+    return bytesRead >= 14 && !memcmp(bytes, "RIFF", 4) && !memcmp(&bytes[8], "WEBPVP", 6);
 }
 
-static const size_t WEBP_VP8_HEADER_SIZE = 30;
-
 // Parse headers of RIFF container, and check for valid Webp (VP8) content.
 // NOTE: This calls peek instead of read, since onGetPixels will need these
 // bytes again.
 static bool webp_parse_header(SkStream* stream, SkImageInfo* info) {
     unsigned char buffer[WEBP_VP8_HEADER_SIZE];
-    if (!stream->peek(buffer, WEBP_VP8_HEADER_SIZE)) {
+    SkASSERT(WEBP_VP8_HEADER_SIZE <= SkCodec::BufferedBytesNeeded());
+
+    const size_t bytesPeeked = stream->peek(buffer, WEBP_VP8_HEADER_SIZE);
+    if (bytesPeeked != WEBP_VP8_HEADER_SIZE) {
+        // Use read + rewind as a backup
+        if (stream->read(buffer, WEBP_VP8_HEADER_SIZE) != WEBP_VP8_HEADER_SIZE
+            || !stream->rewind())
         return false;
     }
 
     WebPBitstreamFeatures features;
     VP8StatusCode status = WebPGetFeatures(buffer, WEBP_VP8_HEADER_SIZE, &features);
     if (VP8_STATUS_OK != status) {
         return false; // Invalid WebP file.
     }
 
     // sanity check for image size that's about to be decoded.
@@ skipping 193 matching lines @@
                 // Break out of the switch statement. Continue the loop.
                 break;
             default:
                 return kInvalidInput;
         }
     }
 }
 
 SkWebpCodec::SkWebpCodec(const SkImageInfo& info, SkStream* stream)
     : INHERITED(info, stream) {}