Fix caching bug in JPEGImageDecoder

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

Index: Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
diff --git a/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp b/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
index baf4bd09222f76f2f765c980953084c19d8b515b..f4ba504991c6da6878cff5cdfd7b0e96fe96c296 100644
--- a/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
+++ b/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
@@ -392,20 +392,14 @@ public:
         if (m_needsRestart) {
             m_needsRestart = false;
             m_nextReadPosition = m_restartPosition;
-        } else if (m_lastSetByte != m_info.src->next_input_byte) {
-            // next_input_byte was updated by jpeg, meaning that it found a restart position.
-            m_restartPosition = m_nextReadPosition - m_info.src->bytes_in_buffer;
+        } else {
+            updateRestartPosition();
         }
 
         const char* segment;
         const unsigned bytes = m_data->getSomeData(segment, m_nextReadPosition);
         if (bytes == 0) {
-            // We had to suspend. When we resume, we will need to start from the restart position.
-            m_needsRestart = true;
-            // Let libjpeg know that the buffer needs to be refilled.
-            m_info.src->bytes_in_buffer = 0;
-            m_info.src->next_input_byte = nullptr;
-            m_lastSetByte = nullptr;
+            suspend();
             return false;
         }
 
@@ -531,6 +525,8 @@ public:
             }
 
             if (onlySize) {
+                updateRestartPosition();

[Peter Kasting, 2015/09/19 00:51:34]

You should probably have comments here about why you need to do these.

[scroggo_chromium, 2015/09/21 18:21:28]

Done.

(My instinct is include lots of comments, but previous Blink code reviews have
encouraged me to remove many of them. I seem to have corrected too far in the
other direction.)

+                suspend();
                 return true;
             }
         // FALL THROUGH
@@ -682,6 +678,24 @@ public:
 #endif
 
 private:
+    void updateRestartPosition()
+    {
+        if (m_lastSetByte != m_info.src->next_input_byte) {
+            // next_input_byte was updated by jpeg, meaning that it found a restart position.
+            m_restartPosition = m_nextReadPosition - m_info.src->bytes_in_buffer;
+        }
+    }
+
+    void suspend()
+    {
+        // We had to suspend. When we resume, we will need to start from the restart position.
+        m_needsRestart = true;
+        // Let libjpeg know that the buffer needs to be refilled.
+        m_info.src->bytes_in_buffer = 0;
+        m_info.src->next_input_byte = nullptr;
+        m_lastSetByte = nullptr;
+    }
+
     RefPtr<SharedBuffer> m_data;
     JPEGImageDecoder* m_decoder;
     // True if we need to back up to m_restartPosition.