decoder/ffmpeg: use AVSEEK_FLAG_BACKWARD for seeking

Ask FFmpeg to seek to the next packet boundary *before* the seek position, so we don't miss audio data. Now we get too much, but we'll solve that in the next commit.

decoder/ffmpeg: use AVSEEK_FLAG_BACKWARD for seeking
d11e2724 · Max Kellermann · f768ca3a · d11e2724
Commit d11e2724 authored Jun 19, 2015 by Max Kellermann
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FfmpegDecoderPlugin.cxx src/decoder/plugins/FfmpegDecoderPlugin.cxx +5 -1

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--- a/src/decoder/plugins/FfmpegDecoderPlugin.cxx
+++ b/src/decoder/plugins/FfmpegDecoderPlugin.cxx
@@ -597,8 +597,12 @@ ffmpeg_decode(Decoder &decoder, InputStream &input)
 					       av_stream->time_base) +
 				start_time_fallback(*av_stream);

+			/* AVSEEK_FLAG_BACKWARD asks FFmpeg to seek to
+			   the packet boundary before the seek time
+			   stamp, not after */
+
 			if (av_seek_frame(format_context, audio_stream, where,
-					  AVSEEK_FLAG_ANY) < 0)
+					  AVSEEK_FLAG_ANY|AVSEEK_FLAG_BACKWARD) < 0)
 				decoder_seek_error(decoder);
 			else {
 				avcodec_flush_buffers(codec_context);