Track positions of dimensions in image files

third_party/WebKit/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp

Index: third_party/WebKit/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
diff --git a/third_party/WebKit/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp b/third_party/WebKit/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
index b940a89422b145bc6945c61eb426658016c51d68..c092b30aa72fd7ecd2debfdcc6c37c7790cab0dc 100644
--- a/third_party/WebKit/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
+++ b/third_party/WebKit/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
@@ -38,6 +38,9 @@
 #include "platform/image-decoders/jpeg/JPEGImageDecoder.h"
 
 #include "platform/PlatformInstrumentation.h"
+#include "public/platform/Platform.h"
+
+#include <stdint.h>
 
 extern "C" {
 #include <stdio.h> // jpeglib.h needs stdio FILE.
@@ -76,6 +79,38 @@ const int exifMarker = JPEG_APP0 + 1;
 // JPEG only supports a denominator of 8.
 const unsigned scaleDenominator = 8;
 
+
+// Returns a 16-bit integer with big-endian representation from a byte array.
+unsigned getUint16(const char* p)
+{
+    return ((*((unsigned char*)p)) << 8) + *((unsigned char*)(p + 1));
+}
+
+// Returns the number of bytes that must be fetched before the image dimensions can be determined.
+// Used for metrics only. Returns 0 if dimensions not found.
+size_t getDimensionsOffset(const RefPtr<blink::SharedBuffer>& m_data)
+{
+    const char* segment;
+    size_t pos = 0;
+    int height = 0, width = 0;
+    size_t length = m_data->getSomeData(segment, pos);
+    if (length < 2 || 0xFFD8 != getUint16(&segment[0]))
+        return 0;
+    pos += 2;
+    while ((length = m_data->getSomeData(segment, pos))) {
+        if (length < 2)
+            return 0;
+        int marker = getUint16(&segment[0]) & 0xfffc;
+        if (marker == 0xffc0)
+            break;
+        pos += 2 + getUint16(&segment[2]);
+    }
+    if (length < 8)
+        return 0;
+    // Height and width are two bytes each and are at bytes 5 and 7 of this segment, respectively.
+    return pos + 8;
+}
+
 } // namespace
 
 namespace blink {
@@ -470,6 +505,7 @@ public:
             if (m_info.scale_num != m_info.scale_denom)
                 overrideColorSpace = JCS_UNKNOWN;
             jpeg_calc_output_dimensions(&m_info);
+            m_decoder->setSizeOffset(m_nextReadPosition);
             m_decoder->setDecodedSize(m_info.output_width, m_info.output_height);
 
             m_decoder->setOrientation(readImageOrientation(info()));
@@ -771,7 +807,7 @@ void term_source(j_decompress_ptr jd)
 
 JPEGImageDecoder::JPEGImageDecoder(AlphaOption alphaOption, GammaAndColorProfileOption colorOptions, size_t maxDecodedBytes)
     : ImageDecoder(alphaOption, colorOptions, maxDecodedBytes)
-    , m_hasColorProfile(false)
+    , m_sizeOffset(0), m_sizeOffsetOptimized(0), m_hasColorProfile(false)
 {
 }
 
@@ -1070,6 +1106,13 @@ void JPEGImageDecoder::decode(bool onlySize)
     // has failed.
     if (!m_reader->decode(onlySize) && isAllDataReceived())
         setFailed();
+    // Decoding should happen only once.
+    if (!onlySize && isComplete(this, onlySize)) {
+        Platform::current()->histogramCustomCounts(
+            "Blink.Images.SizeOffsetOptimized_JPEG", getDimensionsOffset(m_data), 1, 1000000, 50);
+        Platform::current()->histogramCustomCounts(
+            "Blink.Images.SizeOffset_JPEG", m_sizeOffset, 1, 1000000, 50);
+    }
 
     // If decoding is done or failed, we don't need the JPEGImageReader anymore.
     if (isComplete(this, onlySize) || failed())