FFMPEG Plugin: Implement video input buffer padding.

- Padding is required by FFMPEG for correct operation of all video decoders.
  FFMPEG performs some speed optimizations under the hood that may lead to
  reading over the end of the chunk buffer wouldn't there have been padding
  applied.
- Note: Padding is required for audio decoders, too. I will tackle this in some
  later commits. For the time being we have a degradation in code reuse, due to
  different memory ownership of chunk buffers in audio and video decoder path.
  Audio path must not care about freeing chunk buffers whereas video path must.
- Fix coding style and some typos.
- Update documentation accordingly.

src/add-ons/media/plugins/ffmpeg/AVCodecDecoder.cpp

@@ -106,6 +106,7 @@ AVCodecDecoder::AVCodecDecoder()
 	fOutputFrameSize(0),
 
 	fChunkBuffer(NULL),
+	fVideoChunkBuffer(NULL),
 	fChunkBufferOffset(0),
 	fChunkBufferSize(0),
 	fAudioDecodeError(false),
@@ -140,6 +141,9 @@ AVCodecDecoder::~AVCodecDecoder()
 	if (fCodecInitDone)
 		avcodec_close(fContext);
 
+	free(fVideoChunkBuffer);
+		// TODO: Replace with fChunkBuffer, once audio part is
+		// responsible for freeing the chunk buffer, too.
 	free(fDecodedData);
 
 	av_free(fPostProcessedDecodedPicture);
@@ -260,6 +264,9 @@ AVCodecDecoder::SeekedTo(int64 frame, bigtime_t time)
 	}
 
 	// Flush internal buffers as well.
+	free(fVideoChunkBuffer);
+		// TODO: Replace with fChunkBuffer, once audio part is
+		// responsible for freeing the chunk buffer, too.
 	fChunkBuffer = NULL;
 	fChunkBufferOffset = 0;
 	fChunkBufferSize = 0;
@@ -721,7 +728,10 @@ AVCodecDecoder::_DecodeVideo(void* outBuffer, int64* outFrameCount,
 	On first call, the member variables 	fSwsContext / fFormatConversionFunc
 	are initialized.
 
-	\return B_OK, when we successfully decoded one video frame
+	\returns B_OK when we successfully decoded one video frame
+	\returns B_LAST_BUFFER_ERROR when there are no more video frames available.
+	\returns B_NO_MEMORY when we have no memory left for correct operation.
+	\returns Other Errors
  */
 status_t
 AVCodecDecoder::_DecodeNextVideoFrame()
@@ -733,7 +743,8 @@ AVCodecDecoder::_DecodeNextVideoFrame()
 
 		if (fTempPacket.size == 0) {
 			// Our packet buffer is empty, so fill it now.
-			status_t getNextChunkStatus = GetNextChunk(&fChunkBuffer,
+			const void* chunkBuffer = NULL;
+			status_t getNextChunkStatus = GetNextChunk(&chunkBuffer,
 				&fChunkBufferSize, &chunkMediaHeader);
 			switch (getNextChunkStatus) {
 				case B_OK:
@@ -743,17 +754,31 @@ AVCodecDecoder::_DecodeNextVideoFrame()
 					return _FlushOneVideoFrameFromDecoderBuffer();
 
 				default:
-					TRACE("AVCodecDecoder::_DecodeNextVideoFrame(): error from "
+					TRACE("AVCodecDecoder::_DecodeNextVideoFrame(): error from"
-						"GetNextChunk(): %s\n", strerror(err));
+						" GetNextChunk(): %s\n", strerror(err));
 					return getNextChunkStatus;
 			}
 
-			fTempPacket.data = static_cast<uint8_t*>(const_cast<void*>(
+			free(fVideoChunkBuffer);
-				fChunkBuffer));
+				// Free any previously used chunk buffer first
+				// TODO: Replace with fChunkBuffer, once audio part is
+				// responsible for freeing the chunk buffer, too.
+			fVideoChunkBuffer
+				= static_cast<uint8_t*>(const_cast<void*>(chunkBuffer));
+				// TODO: Replace fVideoChunkBuffer with fChunkBuffer, once
+				// audio part is responsible for freeing the chunk buffer, too.
+			status_t chunkBufferPaddingStatus
+				= _AddInputBufferPaddingToVideoChunkBuffer();
+			if (chunkBufferPaddingStatus != B_OK)
+				return chunkBufferPaddingStatus;
+
+			fTempPacket.data = fVideoChunkBuffer;
+				// TODO: Replace with fChunkBuffer, once audio part is
+				// responsible for freeing the chunk buffer, too.
 			fTempPacket.size = fChunkBufferSize;
 
 			fContext->reordered_opaque = chunkMediaHeader.start_time;
-				// Let ffmpeg handle the relationship between start_time and
+				// Let FFMPEG handle the relationship between start_time and
 				// decoded video frame.
 				//
 				// Explanation:
@@ -779,7 +804,7 @@ AVCodecDecoder::_DecodeNextVideoFrame()
 
 #ifdef LOG_STREAM_TO_FILE
 			if (sDumpedPackets < 100) {
-				sStreamLogFile.Write(fChunkBuffer, fChunkBufferSize);
+				sStreamLogFile.Write(chunkBuffer, fChunkBufferSize);
 				printf("wrote %ld bytes\n", fChunkBufferSize);
 				sDumpedPackets++;
 			} else if (sDumpedPackets == 100)
@@ -857,6 +882,39 @@ AVCodecDecoder::_DecodeNextVideoFrame()
 }
 
 
+/*! \brief Adds a "safety net" of additional memory to fVideoChunkBuffer as
+		required by FFMPEG for input buffers to video decoders.
+
+	This is needed so that some decoders can read safely a predefined number of
+	bytes at a time for performance optimization purposes.
+
+	The additonal memory has a size of FF_INPUT_BUFFER_PADDING_SIZE as defined
+	in avcodec.h.
+
+	\returns B_OK Padding was successful. You are responsible for releasing the
+		allocated memory.
+	\returns B_NO_MEMORY Padding failed. The memory in fVideoChunkBuffer is not
+		freed. NULL is assigned to fVideoChunkBuffer.
+*/
+status_t
+AVCodecDecoder::_AddInputBufferPaddingToVideoChunkBuffer()
+{
+	// TODO: Rename fVideoChunkBuffer to fChunkBuffer, once the audio part is
+	// responsible for releasing the chunk buffer, too.
+	fVideoChunkBuffer
+		= static_cast<uint8_t*>(realloc(static_cast<void*>(fVideoChunkBuffer),
+			fChunkBufferSize + FF_INPUT_BUFFER_PADDING_SIZE));
+	if (fVideoChunkBuffer == NULL)
+		return B_NO_MEMORY;
+
+	memset(fVideoChunkBuffer + fChunkBufferSize, 0,
+		FF_INPUT_BUFFER_PADDING_SIZE);
+		// Establish safety net, by zero'ing the padding area.
+
+	return B_OK;
+}
+
+
 /*! \brief Executes all steps needed for a freshly decoded video frame.
 
 	\see _UpdateMediaHeaderForVideoFrame() and

src/add-ons/media/plugins/ffmpeg/AVCodecDecoder.h

@@ -66,6 +66,10 @@ private:
 									media_header* mediaHeader,
 									media_decode_info* info);
 			status_t			_DecodeNextVideoFrame();
+			status_t			_AddInputBufferPaddingToVideoChunkBuffer();
+									// TODO: Remove the "Video" word once
+									// the audio part is responsible for
+									// freeing the chunk buffer, too.
 			void				_HandleNewVideoFrameAndUpdateSystemState();
 			status_t			_FlushOneVideoFrameFromDecoderBuffer();
 			void				_UpdateMediaHeaderForVideoFrame();
@@ -106,6 +110,11 @@ private:
 									// sample size * channel count
 
 			const void*			fChunkBuffer;
+			uint8_t*			fVideoChunkBuffer;
+									// TODO: Remove and use fChunkBuffer again
+									// (with type uint8_t*) once the audio part is
+									// responsible for freeing the chunk buffer,
+									// too.
 			int32				fChunkBufferOffset;
 			size_t				fChunkBufferSize;
 			bool				fAudioDecodeError;