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Subject: Re: [FFmpeg-devel] [PATCH] avformat/dhav: fix backward scanning for
get_duration and optimize seeking
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Derek Buitenhuis:
> From: Justin Ruggles <justinr@vimeo.com>
>
> The backwards scanning done for incomplete final packets should not
> assume a specific alignment at the end of the file. Truncated files
> result in hundreds of thousands of seeks if the final packet does not
> fall on a specific byte boundary, which can be extremely slow.
> For example, with HTTP, each backwards seek results in a separate
> HTTP request.
>
> This changes the scanning to check for the end tag 1 byte at a time
> and buffers the last 1 MiB using ffio_ensure_seekback to avoid additional
> seek operations.
>
> Signed-off-by: Justin Ruggles <justinr@vimeo.com>
> Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
> ---
> libavformat/dhav.c | 38 +++++++++++++++++++++++++++++---------
> 1 file changed, 29 insertions(+), 9 deletions(-)
>
> diff --git a/libavformat/dhav.c b/libavformat/dhav.c
> index b2ead99609..15a75b5033 100644
> --- a/libavformat/dhav.c
> +++ b/libavformat/dhav.c
> @@ -232,34 +232,54 @@ static void get_timeinfo(unsigned date, struct tm *timeinfo)
> timeinfo->tm_sec = sec;
> }
>
> +#define MAX_DURATION_BUFFER_SIZE (1024*1024)
> +
> static int64_t get_duration(AVFormatContext *s)
> {
> DHAVContext *dhav = s->priv_data;
> int64_t start_pos = avio_tell(s->pb);
> + int64_t end_pos = -1;
> int64_t start = 0, end = 0;
> struct tm timeinfo;
> - int max_interations = 100000;
> + uint8_t *end_buffer;
> + int64_t end_buffer_size;
> + int64_t end_buffer_pos;
>
> if (!s->pb->seekable)
> return 0;
>
> + end_buffer_size = FFMIN(MAX_DURATION_BUFFER_SIZE, avio_size(s->pb));
> + end_buffer = av_malloc(end_buffer_size);
> + if (!end_buffer)
> + return 0;
> + end_buffer_pos = avio_size(s->pb) - end_buffer_size;
> + avio_seek(s->pb, end_buffer_pos, SEEK_SET);
> + ffio_ensure_seekback(s->pb, end_buffer_size);
This can fail.
> + avio_read(s->pb, end_buffer, end_buffer_size);
> + av_freep(&end_buffer);
You read into a buffer just to ignore the read data? Why don't you just
rewrite the loop below to inspect the data directly?
> +
> avio_seek(s->pb, avio_size(s->pb) - 8, SEEK_SET);
> - while (avio_tell(s->pb) > 12 && max_interations--) {
> + while (avio_tell(s->pb) > end_buffer_pos) {
> if (avio_rl32(s->pb) == MKTAG('d','h','a','v')) {
> int64_t seek_back = avio_rl32(s->pb);
> -
> - avio_seek(s->pb, -seek_back, SEEK_CUR);
> - read_chunk(s);
> - get_timeinfo(dhav->date, &timeinfo);
> - end = av_timegm(&timeinfo) * 1000LL;
> + end_pos = avio_tell(s->pb) - seek_back;
> break;
> } else {
> - avio_seek(s->pb, -12, SEEK_CUR);
> + avio_seek(s->pb, -5, SEEK_CUR);
> }
> }
>
> - avio_seek(s->pb, start_pos, SEEK_SET);
> + if (end_pos < 0) {
> + avio_seek(s->pb, start_pos, SEEK_SET);
> + return 0;
> + }
> +
> + avio_seek(s->pb, end_pos, SEEK_SET);
> + read_chunk(s);
> + get_timeinfo(dhav->date, &timeinfo);
> + end = av_timegm(&timeinfo) * 1000LL;
>
> + avio_seek(s->pb, start_pos, SEEK_SET);
> read_chunk(s);
> get_timeinfo(dhav->date, &timeinfo);
> start = av_timegm(&timeinfo) * 1000LL;
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