[FFmpeg-devel] [PATCH] avformat/hlsenc: add hls_flag option to create segments atomically

[Bodecs Bela]

2017.01.31. 21:29 keltezéssel, Aman Gupta írta:
> From: Aman Gupta <aman at tmm1.net>
>
> Adds a `-hls_flags +temp_file` which will write segment data to
> filename.tmp, and then rename to filename when the segment is complete
> and before the file is added to the m3u8 playlist.
>
> This patch is similar in spirit to one used in Plex's ffmpeg fork, and
> allows a transcoding webserver to ensure incomplete segment files are
> never served up accidentally.
> ---
>   libavformat/hlsenc.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
>
> diff --git a/libavformat/hlsenc.c b/libavformat/hlsenc.c
> index bd1e684..23b9011 100644
> --- a/libavformat/hlsenc.c
> +++ b/libavformat/hlsenc.c
> @@ -76,6 +76,7 @@ typedef enum HLSFlags {
>       HLS_SECOND_LEVEL_SEGMENT_INDEX = (1 << 8), // include segment index in segment filenames when use_localtime  e.g.: %%03d
>       HLS_SECOND_LEVEL_SEGMENT_DURATION = (1 << 9), // include segment duration (microsec) in segment filenames when use_localtime  e.g.: %%09t
>       HLS_SECOND_LEVEL_SEGMENT_SIZE = (1 << 10), // include segment size (bytes) in segment filenames when use_localtime  e.g.: %%014s
> +    HLS_TEMP_FILE = (1 << 11),
>   } HLSFlags;
>   
>   typedef enum {
> @@ -915,6 +916,10 @@ static int hls_start(AVFormatContext *s)
>   
>       set_http_options(&options, c);
>   
> +    if (c->flags & HLS_TEMP_FILE) {
> +        av_strlcat(oc->filename, ".tmp", sizeof(oc->filename));
> +    }
> +
>       if (c->key_info_file) {
>           if ((err = hls_encryption_start(s)) < 0)
>               goto fail;
> @@ -1276,6 +1281,15 @@ static int hls_write_packet(AVFormatContext *s, AVPacket *pkt)
>   
>           av_write_frame(oc, NULL); /* Flush any buffered data */
>   
> +        if (hls->flags & HLS_TEMP_FILE) {
> +            size_t len = strlen(oc->filename);
> +            char final_filename[sizeof(oc->filename)];
> +            av_strlcpy(final_filename, oc->filename, len);
> +            final_filename[len-4] = '\0';
> +            ff_rename(oc->filename, final_filename, s);
> +            oc->filename[len-4] = '\0';
> +        }
> +
>           new_start_pos = avio_tell(hls->avf->pb);
>           hls->size = new_start_pos - hls->start_pos;
>           ret = hls_append_segment(s, hls, hls->duration, hls->start_pos, hls->size);
> @@ -1406,6 +1420,7 @@ static const AVOption options[] = {
>       {"hls_subtitle_path",     "set path of hls subtitles", OFFSET(subtitle_filename), AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,    E},
>       {"hls_flags",     "set flags affecting HLS playlist and media file generation", OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = 0 }, 0, UINT_MAX, E, "flags"},
>       {"single_file",   "generate a single media file indexed with byte ranges", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_SINGLE_FILE }, 0, UINT_MAX,   E, "flags"},
> +    {"temp_file", "write segment to temporary file and rename when complete", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_TEMP_FILE }, 0, UINT_MAX,   E, "flags"},
>       {"delete_segments", "delete segment files that are no longer part of the playlist", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_DELETE_SEGMENTS }, 0, UINT_MAX,   E, "flags"},
>       {"round_durations", "round durations in m3u8 to whole numbers", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_ROUND_DURATIONS }, 0, UINT_MAX,   E, "flags"},
>       {"discont_start", "start the playlist with a discontinuity tag", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_DISCONT_START }, 0, UINT_MAX,   E, "flags"},
I think the phrase  in your email subject "to create atomically" 
misleading because another renaming may occur afterwards your temp 
renaming if second_level_segment_size or second_level_segment_duration 
flag is on.
But the idea to write segment content into a temp file is good by me. To 
ensure the amoticity the two distinct renamings may be merged.

I have never thought of it but after your idea I concluded  that the 
segment file names are predictable.
In live streaming enviroment somebody predicting the segment name may 
see the program some seconds earlier than any other viewer by 
downloading the unpublished segment. (with one segment duration seconds 
earlier)
If the temp file name would be random name, then your idea may be seen 
as a security feature.
What is your opinion about this possibility?

bb