ffmpeg解码流程解析（3）-HLS解析

本篇就针对HLS协议案例本身做一个整体分析，从客户端的角度分析下整体流程

客户端需要做什么

客户端负责选择合适的请求资源，下载器资源，然后解码显示（整成播放器的功能）。 客户端从获取索引文件开始，通常使用给定的URL来识别该流的信息。这个索引文件一般给出了可用媒体文件、解密密钥和其他可选流的位置。客户端选定流之后，就开始顺序下载每个可用的媒体文件。每个文件中包含特定流的连续分片。只要客户端下载到足够的数据，就可以开始解码数据并显示了。 如果需要，客户端负责读取所有解密密钥、认证或为用户提供用于认证或解密的接口。 客户端可以一直持续这个过程，直到它遇到索引文件中的*#EXT-X-ENDLIST*标签；若不存在该标签，则表示该索引文件是一个直播源，客户端需要定期更新索引文件，重复上述过程。

较为常用的HLS系统中，使用硬编码器将输入的音频编码为AAC、将输入的视频编码为h264，并将二者复用到MPEG-TS中，之后使用分片工具将其切分为一系列小的ts文件；这些文件将可以放到web服务器上。分片工具同时会创建并维护一个索引文件（HLS中称为*.M3U8*），其中包含可用媒体文件的列表。索引文件的URL会在web服务器上发布。客户端可以读取该索引文件，然后顺序请求列出的媒体文件，这些分片可以无缝播放。

hls.c

前面两章讲到了avformat_open_input，会根据文件路径推测出对应的AVInputFormat，然后去读header，去read_packet，回顾一下，大致流程如下。

avformat_open_input(http.xxx.m3u8)
    init_input(s, filename, &tmp))
        //提供的文件名信息不能探测格式
        av_probe_input_format2(&pd, 0, &score)))
        io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
            io_open_default
                ffio_open_whitelist
                    ffurl_alloc
                        //探测是HTTP协议 URLProtocol ff_http_protocol
                        url_find_protocol(filename);  
                    ffurl_connect    //发送HTTP报文头，下载http.xxx.m3u8文件
        //读m3u8文件探测解复用是AVInputFormat *iformat ="hls,applehttp"
        *fmt = av_probe_input_buffer2(s->pb, &s->iformat, filename, s, 0, s->format_probesize);
        s->iformat->read_header(s);  //iformat iformat  hls.c-->hls_read_header

hls.c/read_header

static int hls_read_header(AVFormatContext *s)
{
 
	....................	
    //解析playlist，也就是m3u8文件
    if ((ret = parse_playlist(c, s->url, NULL, s->pb)) < 0)
        goto fail;

    if (c->n_variants == 0) {
        av_log(s, AV_LOG_WARNING, "Empty playlist\n");
        ret = AVERROR_EOF;
        goto fail;
    }
    /* If the playlist only contained playlists (Master Playlist),
     * parse each individual playlist. */
   	
   	//如果解析出来是 Master Playlist，则针对每个再次进行解析
    if (c->n_playlists > 1 || c->playlists[0]->n_segments == 0) {
        for (i = 0; i < c->n_playlists; i++) {
            struct playlist *pls = c->playlists[i];
            pls->m3u8_hold_counters = 0;
            if ((ret = parse_playlist(c, pls->url, pls, NULL)) < 0) {
                av_log(s, AV_LOG_WARNING, "parse_playlist error %s [%s]\n", av_err2str(ret), pls->url);
                pls->broken = 1;
                if (c->n_playlists > 1)
                    continue;
                goto fail;
            }
        }
    }

    for (i = 0; i < c->n_variants; i++) {
        if (c->variants[i]->playlists[0]->n_segments == 0) {
            av_log(s, AV_LOG_WARNING, "Empty segment [%s]\n", c->variants[i]->playlists[0]->url);
            c->variants[i]->playlists[0]->broken = 1;
        }
    }

    /* If this isn't a live stream, calculate the total duration of the
     * stream. */
    //如果有finished这个标志，则说明这个不是一个直播流，那么计算下整个时长duration
    
    if (c->variants[0]->playlists[0]->finished) {
        int64_t duration = 0;
        for (i = 0; i < c->variants[0]->playlists[0]->n_segments; i++)
            duration += c->variants[0]->playlists[0]->segments[i]->duration;
        s->duration = duration;
    }

    /* Associate renditions with variants */
    for (i = 0; i < c->n_variants; i++) {
        struct variant *var = c->variants[i];

        if (var->audio_group[0])
            add_renditions_to_variant(c, var, AVMEDIA_TYPE_AUDIO, var->audio_group);
        if (var->video_group[0])
            add_renditions_to_variant(c, var, AVMEDIA_TYPE_VIDEO, var->video_group);
        if (var->subtitles_group[0])
            add_renditions_to_variant(c, var, AVMEDIA_TYPE_SUBTITLE, var->subtitles_group);
    }

    /* Create a program for each variant */
    for (i = 0; i < c->n_variants; i++) {
        struct variant *v = c->variants[i];
        AVProgram *program;

        program = av_new_program(s, i);
        if (!program)
            goto fail;
        av_dict_set_int(&program->metadata, "variant_bitrate", v->bandwidth, 0);
    }

    /* Select the starting segments */
    for (i = 0; i < c->n_playlists; i++) {
        struct playlist *pls = c->playlists[i];

        if (pls->n_segments == 0)
            continue;

        pls->cur_seq_no = select_cur_seq_no(c, pls);
        highest_cur_seq_no = FFMAX(highest_cur_seq_no, pls->cur_seq_no);
    }

}

先看一部分代码，主要是通过parse_playlist这个方法，解析出相关的节目信息，并赋值给HLSContext

parse_playlist

static int parse_playlist(HLSContext *c, const char *url,
                          struct playlist *pls, AVIOContext *in)
{

    //解析m3u8文件的playerlist文件
    int ret = 0, is_segment = 0, is_variant = 0;
    int64_t duration = 0;
    enum KeyType key_type = KEY_NONE;
    uint8_t iv[16] = "";
    int has_iv = 0;
    char key[MAX_URL_SIZE] = "";
    char line[MAX_URL_SIZE];
    const char *ptr;
    int close_in = 0;
    int64_t seg_offset = 0;
    int64_t seg_size = -1;
    uint8_t *new_url = NULL;
    struct variant_info variant_info;
    char tmp_str[MAX_URL_SIZE];
    struct segment *cur_init_section = NULL;
    int is_http = av_strstart(url, "http", NULL);
    struct segment **prev_segments = NULL;
    int prev_n_segments = 0;
    int64_t prev_start_seq_no = -1;

    if (is_http && !in && c->http_persistent && c->playlist_pb) {
        in = c->playlist_pb;
        ret = open_url_keepalive(c->ctx, &c->playlist_pb, url, NULL);
        if (ret == AVERROR_EXIT) {
            return ret;
        } else if (ret < 0) {
            if (ret != AVERROR_EOF)
                av_log(c->ctx, AV_LOG_WARNING,
                    "keepalive request failed for '%s' with error: '%s' when parsing playlist\n",
                    url, av_err2str(ret));
            in = NULL;
        }
    }

    if (!in) {
        AVDictionary *opts = NULL;
        av_dict_copy(&opts, c->avio_opts, 0);

        if (c->http_persistent)
            av_dict_set(&opts, "multiple_requests", "1", 0);

        ret = c->ctx->io_open(c->ctx, &in, url, AVIO_FLAG_READ, &opts);
        av_dict_free(&opts);
        if (ret < 0)
            return ret;

        if (is_http && c->http_persistent)
            c->playlist_pb = in;
        else
            close_in = 1;
    }

    if (av_opt_get(in, "location", AV_OPT_SEARCH_CHILDREN, &new_url) >= 0)
        url = new_url;

    ff_get_chomp_line(in, line, sizeof(line));
    if (strcmp(line, "#EXTM3U")) {
        ret = AVERROR_INVALIDDATA;
        goto fail;
    }

    if (pls) {
        prev_start_seq_no = pls->start_seq_no;
        prev_segments = pls->segments;
        prev_n_segments = pls->n_segments;
        pls->segments = NULL;
        pls->n_segments = 0;

        pls->finished = 0;
        pls->type = PLS_TYPE_UNSPECIFIED;
    }
    while (!avio_feof(in)) {
        ff_get_chomp_line(in, line, sizeof(line));
        if (av_strstart(line, "#EXT-X-STREAM-INF:", &ptr)) {
            //表示master playlist，描述了variant的诸多信息，比如下载带宽、音视频编码信息、视频分辨率等等
            is_variant = 1;
            memset(&variant_info, 0, sizeof(variant_info));
            ff_parse_key_value(ptr, (ff_parse_key_val_cb) handle_variant_args,
                               &variant_info);
        } else if (av_strstart(line, "#EXT-X-KEY:", &ptr)) {
            //这个字段表示是加密playlist
            struct key_info info = {{0}};
            ff_parse_key_value(ptr, (ff_parse_key_val_cb) handle_key_args,
                               &info);
            key_type = KEY_NONE;
            has_iv = 0;
            if (!strcmp(info.method, "AES-128"))
                key_type = KEY_AES_128;
            if (!strcmp(info.method, "SAMPLE-AES"))
                key_type = KEY_SAMPLE_AES;
            if (!strncmp(info.iv, "0x", 2) || !strncmp(info.iv, "0X", 2)) {
                ff_hex_to_data(iv, info.iv + 2);
                has_iv = 1;
            }
            av_strlcpy(key, info.uri, sizeof(key));
        } else if (av_strstart(line, "#EXT-X-MEDIA:", &ptr)) {
            //alternate media为HLS提供了一种外挂音频、视频、字幕的格式，可以在不改动已生成的HLS分片信息的情况下，
            // 为客户端提供新的可选媒体信息。为此，在STREAM-INF添加了AUIOD、VIDEO属性，以实现关联
            //#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="bipbop_audio",LANGUAGE="eng",NAME="BipBop Audio 1",AUTOSELECT=YES,DEFAULT=YES
            struct rendition_info info = {{0}};
            ff_parse_key_value(ptr, (ff_parse_key_val_cb) handle_rendition_args,
                               &info);
            new_rendition(c, &info, url);
        } else if (av_strstart(line, "#EXT-X-TARGETDURATION:", &ptr)) {
            //EXT-X-TARGETDURATION 切片时长
            ret = ensure_playlist(c, &pls, url);
            if (ret < 0)
                goto fail;
            pls->target_duration = strtoll(ptr, NULL, 10) * AV_TIME_BASE;
        } else if (av_strstart(line, "#EXT-X-MEDIA-SEQUENCE:", &ptr)) {
            //序号
            uint64_t seq_no;
            ret = ensure_playlist(c, &pls, url);
            if (ret < 0)
                goto fail;
            seq_no = strtoull(ptr, NULL, 10);
            if (seq_no > INT64_MAX) {
                av_log(c->ctx, AV_LOG_DEBUG, "MEDIA-SEQUENCE higher than "
                        "INT64_MAX, mask out the highest bit\n");
                seq_no &= INT64_MAX;
            }
            pls->start_seq_no = seq_no;
        } else if (av_strstart(line, "#EXT-X-PLAYLIST-TYPE:", &ptr)) {
            //playerlist类型 vod:点播 EVENT:Event playlist
            ret = ensure_playlist(c, &pls, url);
            if (ret < 0)
                goto fail;
            if (!strcmp(ptr, "EVENT"))
                pls->type = PLS_TYPE_EVENT;
            else if (!strcmp(ptr, "VOD"))
                pls->type = PLS_TYPE_VOD;
        } else if (av_strstart(line, "#EXT-X-MAP:", &ptr)) {
            //为'#EXT-X-MAP' 将一个完整视频的头单独存放, 将其他部分存在另一个视频文件中, 这样就是你们抓到了其中一个视频, 也播放不了,
            // 这是防止盗链的一种手段, "将鸡蛋放在不同的篮子里"
            struct init_section_info info = {{0}};
            ret = ensure_playlist(c, &pls, url);
            if (ret < 0)
                goto fail;
            ff_parse_key_value(ptr, (ff_parse_key_val_cb) handle_init_section_args,
                               &info);
            cur_init_section = new_init_section(pls, &info, url);
            cur_init_section->key_type = key_type;
            if (has_iv) {
                memcpy(cur_init_section->iv, iv, sizeof(iv));
            } else {
                int64_t seq = pls->start_seq_no + pls->n_segments;
                memset(cur_init_section->iv, 0, sizeof(cur_init_section->iv));
                AV_WB64(cur_init_section->iv + 8, seq);
            }

            if (key_type != KEY_NONE) {
                ff_make_absolute_url(tmp_str, sizeof(tmp_str), url, key);
                if (!tmp_str[0]) {
                    av_free(cur_init_section);
                    ret = AVERROR_INVALIDDATA;
                    goto fail;
                }
                cur_init_section->key = av_strdup(tmp_str);
                if (!cur_init_section->key) {
                    av_free(cur_init_section);
                    ret = AVERROR(ENOMEM);
                    goto fail;
                }
            } else {
                cur_init_section->key = NULL;
            }

        } else if (av_strstart(line, "#EXT-X-ENDLIST", &ptr)) {
            //有结束标志，则不是直播类型playerlist
            if (pls)
                pls->finished = 1;
        } else if (av_strstart(line, "#EXTINF:", &ptr)) {
            //EXTINF  ts片时长
            is_segment = 1;
            duration   = atof(ptr) * AV_TIME_BASE;
        } else if (av_strstart(line, "#EXT-X-BYTERANGE:", &ptr)) {
            //使用该字段，可以不用在server上存储大量的小文件，而仅通过一个文件
            // ，通过偏移量和长度来支持分片。与点播playlist中的m3u8等价的playlist内容如下：
            //#EXT-X-BYTERANGE:75232@0

            seg_size = strtoll(ptr, NULL, 10);
            ptr = strchr(ptr, '@');
            if (ptr)
                seg_offset = strtoll(ptr+1, NULL, 10);
        } else if (av_strstart(line, "#", NULL)) {
            av_log(c->ctx, AV_LOG_INFO, "Skip ('%s')\n", line);
            continue;
        } else if (line[0]) {
            //如果是包含master playerlist的，记录下，拼接url
            if (is_variant) {
                if (!new_variant(c, &variant_info, line, url)) {
                    ret = AVERROR(ENOMEM);
                    goto fail;
                }
                is_variant = 0;
            }
            //ts segment的url
            if (is_segment) {
                struct segment *seg;
                ret = ensure_playlist(c, &pls, url);
                if (ret < 0)
                    goto fail;
                seg = av_malloc(sizeof(struct segment));
                if (!seg) {
                    ret = AVERROR(ENOMEM);
                    goto fail;
                }
                if (has_iv) {
                    memcpy(seg->iv, iv, sizeof(iv));
                } else {
                    int64_t seq = pls->start_seq_no + pls->n_segments;
                    memset(seg->iv, 0, sizeof(seg->iv));
                    AV_WB64(seg->iv + 8, seq);
                }

                if (key_type != KEY_NONE) {
                    ff_make_absolute_url(tmp_str, sizeof(tmp_str), url, key);
                    if (!tmp_str[0]) {
                        ret = AVERROR_INVALIDDATA;
                        av_free(seg);
                        goto fail;
                    }
                    seg->key = av_strdup(tmp_str);
                    if (!seg->key) {
                        av_free(seg);
                        ret = AVERROR(ENOMEM);
                        goto fail;
                    }
                } else {
                    seg->key = NULL;
                }

                ff_make_absolute_url(tmp_str, sizeof(tmp_str), url, line);
                if (!tmp_str[0]) {
                    ret = AVERROR_INVALIDDATA;
                    if (seg->key)
                        av_free(seg->key);
                    av_free(seg);
                    goto fail;
                }
                seg->url = av_strdup(tmp_str);
                if (!seg->url) {
                    av_free(seg->key);
                    av_free(seg);
                    ret = AVERROR(ENOMEM);
                    goto fail;
                }

                if (duration < 0.001 * AV_TIME_BASE) {
                    av_log(c->ctx, AV_LOG_WARNING, "Cannot get correct #EXTINF value of segment %s,"
                                    " set to default value to 1ms.\n", seg->url);
                    duration = 0.001 * AV_TIME_BASE;
                }
                seg->duration = duration;
                seg->key_type = key_type;
                dynarray_add(&pls->segments, &pls->n_segments, seg);
                is_segment = 0;

                seg->size = seg_size;
                if (seg_size >= 0) {
                    seg->url_offset = seg_offset;
                    seg_offset += seg_size;
                    seg_size = -1;
                } else {
                    seg->url_offset = 0;
                    seg_offset = 0;
                }

                seg->init_section = cur_init_section;
            }
        }
    }
    ......................................
    if (pls)
        pls->last_load_time = av_gettime_relative();

fail:
    av_free(new_url);
    if (close_in)
        ff_format_io_close(c->ctx, &in);
    c->ctx->ctx_flags = c->ctx->ctx_flags & ~(unsigned)AVFMTCTX_UNSEEKABLE;
    if (!c->n_variants || !c->variants[0]->n_playlists ||
        !(c->variants[0]->playlists[0]->finished ||
          c->variants[0]->playlists[0]->type == PLS_TYPE_EVENT))
        c->ctx->ctx_flags |= AVFMTCTX_UNSEEKABLE;
    return ret;
}

如果对于playlist不太了解的话，可能看的有点懵，这里简单介绍下几种常用的playlist，了解之后再去对照代码中的字段解析，就比较清晰明了了。

• 点播（VOD，Video On Demand）playlist
• 直播playlist
• 秘钥playlist
• master playlist

点播 playlist

HLS中点播playlist是静态的文件，生成之后一般不允许修改，server端可以预先生成切片文件。其格式如下：

#EXTM3U
#EXT-X-PLAYLIST-TYPE:VOD
#EXT-X-TARGETDURATION:10
#EXT-X-VERSION:3
#EXTINF:9.009,
http://media.example.com/first.ts
#EXTINF:9.009,
http://media.example.com/second.ts
#EXTINF:3.003,
http://media.example.com/third.ts
#EXT-X-ENDLIST

这类playlist比较典型的特征是

• playlist中包含#EXT-X-ENDLIST字段，表示结束标志
• EXT-X-PLAYLIST-TYPE字段为VOD（可选）

直播playlist

对于直播playlist，最简单的特征就是没有EXT-X-ENDLIST标签，并且不存在EXT-X-PLAYLIST-TYPE标签，直播playlist是一个典型的滑动窗口，server端会对源格式做实时转码（存在一点延时），并定期清理已发布的分片信息。典型的滑动窗口大小为3-5个分片。其m3u8格式如下

#EXTM3U
#EXT-X-TARGETDURATION:10
#EXT-X-VERSION:3
#EXT-X-MEDIA-SEQUENCE:1
#EXTINF:10,
fileSequence1.ts
#EXTINF:10,
fileSequence2.ts
#EXTINF:10,
fileSequence3.ts

秘钥playlist

这里的密钥是由EXT-X-KEY字段指定的，其中给出了加密方法，和密钥的存储路径。

#EXTM3U
#EXT-X-VERSION:3
#EXT-X-MEDIA-SEQUENCE:7794
#EXT-X-TARGETDURATION:15
#EXT-X-KEY:METHOD=AES-128,URI="https://priv.example.com/key.php?r=52"
#EXTINF:2.833,
http://media.example.com/fileSequence52-A.ts
#EXTINF:15.0,
http://media.example.com/fileSequence52-B.ts
#EXTINF:13.333,
http://media.example.com/fileSequence52-C.ts
#EXT-X-KEY:METHOD=AES-128,URI="https://priv.example.com/key.php?r=53"
#EXTINF:15.0,
http://media.example.com/fileSequence53-A.ts

master playlist

master playlist其中并不包含分片有关信息，而是描述了同一个源的不同编码格式，在HLS中称之为variant。master playlist中描述了variant的诸多信息，比如下载带宽、音视频编码信息、视频分辨率等等。典型的示例如下：

#EXTM3U
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=150000,RESOLUTION=416x234, \
CODECS="avc1.42e00a,mp4a.40.2"
http://example.com/low/index.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=240000,RESOLUTION=416x234, \
CODECS="avc1.42e00a,mp4a.40.2"
http://example.com/lo_mid/index.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=640000,RESOLUTION=640x360, \
CODECS="avc1.42e00a,mp4a.40.2"
http://example.com/high/index.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=64000,CODECS="mp4a.40.5"
http://example.com/audio/index.m3u8

当然，还有一些类型playlist就不在这里一一赘述了，有兴趣可以自己额外学习下。

ffio_init_context

/* Open the demuxer for each playlist */
    for (i = 0; i < c->n_playlists; i++) {
        struct playlist *pls = c->playlists[i];
        char *url;
        ff_const59 AVInputFormat *in_fmt = NULL;

        if (!(pls->ctx = avformat_alloc_context())) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }

        if (pls->n_segments == 0)
            continue;

        pls->index  = i;
        pls->needed = 1;
        pls->parent = s;

        /*
         * If this is a live stream and this playlist looks like it is one segment
         * behind, try to sync it up so that every substream starts at the same
         * time position (so e.g. avformat_find_stream_info() will see packets from
         * all active streams within the first few seconds). This is not very generic,
         * though, as the sequence numbers are technically independent.
         */
        if (!pls->finished && pls->cur_seq_no == highest_cur_seq_no - 1 &&
            highest_cur_seq_no < pls->start_seq_no + pls->n_segments) {
            pls->cur_seq_no = highest_cur_seq_no;
        }

        pls->read_buffer = av_malloc(INITIAL_BUFFER_SIZE);
        if (!pls->read_buffer){
            ret = AVERROR(ENOMEM);
            avformat_free_context(pls->ctx);
            pls->ctx = NULL;
            goto fail;
        }
        ffio_init_context(&pls->pb, pls->read_buffer, INITIAL_BUFFER_SIZE, 0, pls,
                          read_data, NULL, NULL);
        pls->ctx->probesize = s->probesize > 0 ? s->probesize : 1024 * 4;
        pls->ctx->max_analyze_duration = s->max_analyze_duration > 0 ? s->max_analyze_duration : 4 * AV_TIME_BASE;
        pls->ctx->interrupt_callback = s->interrupt_callback;
        url = av_strdup(pls->segments[0]->url);
        ret = av_probe_input_buffer(&pls->pb, &in_fmt, url, NULL, 0, 0);
        if (ret < 0) {
            /* Free the ctx - it isn't initialized properly at this point,
             * so avformat_close_input shouldn't be called. If
             * avformat_open_input fails below, it frees and zeros the
             * context, so it doesn't need any special treatment like this. */
            av_log(s, AV_LOG_ERROR, "Error when loading first segment '%s'\n", url);
            avformat_free_context(pls->ctx);
            pls->ctx = NULL;
            av_free(url);
            goto fail;
        }
        av_free(url);
        pls->ctx->pb       = &pls->pb;
        pls->ctx->io_open  = nested_io_open;
        pls->ctx->flags   |= s->flags & ~AVFMT_FLAG_CUSTOM_IO;

        if ((ret = ff_copy_whiteblacklists(pls->ctx, s)) < 0)
            goto fail;
		
        //打开第0个ts片
        ret = avformat_open_input(&pls->ctx, pls->segments[0]->url, in_fmt, NULL);
        if (ret < 0)
            goto fail;

        if (pls->id3_deferred_extra && pls->ctx->nb_streams == 1) {
            ff_id3v2_parse_apic(pls->ctx, pls->id3_deferred_extra);
            avformat_queue_attached_pictures(pls->ctx);
            ff_id3v2_parse_priv(pls->ctx, pls->id3_deferred_extra);
            ff_id3v2_free_extra_meta(&pls->id3_deferred_extra);
        }

        if (pls->is_id3_timestamped == -1)
            av_log(s, AV_LOG_WARNING, "No expected HTTP requests have been made\n");

        /*
         * For ID3 timestamped raw audio streams we need to detect the packet
         * durations to calculate timestamps in fill_timing_for_id3_timestamped_stream(),
         * but for other streams we can rely on our user calling avformat_find_stream_info()
         * on us if they want to.
         */
        if (pls->is_id3_timestamped || (pls->n_renditions > 0 && pls->renditions[0]->type == AVMEDIA_TYPE_AUDIO)) {
            ret = avformat_find_stream_info(pls->ctx, NULL);
            if (ret < 0)
                goto fail;
        }

        pls->has_noheader_flag = !!(pls->ctx->ctx_flags & AVFMTCTX_NOHEADER);

        /* Create new AVStreams for each stream in this playlist */
        ret = update_streams_from_subdemuxer(s, pls);
        if (ret < 0)
            goto fail;

        /*
         * Copy any metadata from playlist to main streams, but do not set
         * event flags.
         */
        if (pls->n_main_streams)
            av_dict_copy(&pls->main_streams[0]->metadata, pls->ctx->metadata, 0);

        add_metadata_from_renditions(s, pls, AVMEDIA_TYPE_AUDIO);
        add_metadata_from_renditions(s, pls, AVMEDIA_TYPE_VIDEO);
        add_metadata_from_renditions(s, pls, AVMEDIA_TYPE_SUBTITLE);
    }

    update_noheader_flag(s);

    return 0;

拿到相关节目信息之后，针对每个playlist进行解协议，ffio_init_context，初始化IOContext信息，注意这里面传入的**read_data**,这个函数参数针对读取网络数据，真正IO层做的事情

接下来的方法应该很熟悉了av_probe_input_buffer，来根据文件名的去推断AVInputformat(不熟悉的可以看看前面几章)，这里是ts文件，所以最终推断出来的是ff_mpegts_demuxer，在mpegts.c之中

AVInputFormat ff_mpegts_demuxer = {
    .name           = "mpegts",
    .long_name      = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
    .priv_data_size = sizeof(MpegTSContext),
    .read_probe     = mpegts_probe,
    .read_header    = mpegts_read_header，
    .read_packet    = mpegts_read_packet,
    .read_close     = mpegts_read_close,
    .read_timestamp = mpegts_get_dts,
    .flags          = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,
    .priv_class     = &mpegts_class,
};

read_packet

static int hls_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    HLSContext *c = s->priv_data;
    int ret, i, minplaylist = -1;

    recheck_discard_flags(s, c->first_packet);
    c->first_packet = 0;

    for (i = 0; i < c->n_playlists; i++) {
        struct playlist *pls = c->playlists[i];
        /* Make sure we've got one buffered packet from each open playlist
         * stream */
        /* 从每一个已打开的playlist上读取一个AVPacket */
        if (pls->needed && !pls->pkt->data) {
            while (1) {
                int64_t ts_diff;
                AVRational tb;
                ret = av_read_frame(pls->ctx, pls->pkt);
                if (ret < 0) {
                    if (!avio_feof(&pls->pb) && ret != AVERROR_EOF)
                        return ret;
                    break;
                } else {
                    /* stream_index check prevents matching picture attachments etc. */
                    if (pls->is_id3_timestamped && pls->pkt->stream_index == 0) {
                        /* audio elementary streams are id3 timestamped */
                        fill_timing_for_id3_timestamped_stream(pls);
                    }

                    if (c->first_timestamp == AV_NOPTS_VALUE &&
                        pls->pkt->dts       != AV_NOPTS_VALUE)
                        c->first_timestamp = av_rescale_q(pls->pkt->dts,
                            get_timebase(pls), AV_TIME_BASE_Q);
                }
				
                if (pls->seek_timestamp == AV_NOPTS_VALUE)
                    break;
				//下面都是关于有seek的操作的处理
                if (pls->seek_stream_index < 0 ||
                    pls->seek_stream_index == pls->pkt->stream_index) {

                    if (pls->pkt->dts == AV_NOPTS_VALUE) {
                        pls->seek_timestamp = AV_NOPTS_VALUE;
                        break;
                    }

                    tb = get_timebase(pls);
                    ts_diff = av_rescale_rnd(pls->pkt->dts, AV_TIME_BASE,
                                            tb.den, AV_ROUND_DOWN) -
                            pls->seek_timestamp;
                    if (ts_diff >= 0 && (pls->seek_flags  & AVSEEK_FLAG_ANY ||
                                        pls->pkt->flags & AV_PKT_FLAG_KEY)) {
                        pls->seek_timestamp = AV_NOPTS_VALUE;
                        break;
                    }
                }
                /* 到此，说面当前AVPacket是需要丢弃的，重新读取 */
                av_packet_unref(pls->pkt);
            }
        }
        /* Check if this stream has the packet with the lowest dts */
        //找到最小的dts的packet，并记录其索引值
        if (pls->pkt->data) {
            struct playlist *minpls = minplaylist < 0 ?
                                     NULL : c->playlists[minplaylist];
            if (minplaylist < 0) {
                minplaylist = i;
            } else {
                int64_t dts     =    pls->pkt->dts;
                int64_t mindts  = minpls->pkt->dts;

                if (dts == AV_NOPTS_VALUE ||
                    (mindts != AV_NOPTS_VALUE && compare_ts_with_wrapdetect(dts, pls, mindts, minpls) < 0))
                    minplaylist = i;
            }
        }
    }

    /* If we got a packet, return it */
     /* 成功读取AVPacket，需要返回给上层调用者 */
    if (minplaylist >= 0) {
        struct playlist *pls = c->playlists[minplaylist];
        AVStream *ist;
        AVStream *st;

        ret = update_streams_from_subdemuxer(s, pls);
        if (ret < 0) {
            av_packet_unref(pls->pkt);
            return ret;
        }

        // If sub-demuxer reports updated metadata, copy it to the first stream
        // and set its AVSTREAM_EVENT_FLAG_METADATA_UPDATED flag.
        if (pls->ctx->event_flags & AVFMT_EVENT_FLAG_METADATA_UPDATED) {
            if (pls->n_main_streams) {
                st = pls->main_streams[0];
                av_dict_copy(&st->metadata, pls->ctx->metadata, 0);
                st->event_flags |= AVSTREAM_EVENT_FLAG_METADATA_UPDATED;
            }
            pls->ctx->event_flags &= ~AVFMT_EVENT_FLAG_METADATA_UPDATED;
        }

        /* check if noheader flag has been cleared by the subdemuxer */
        if (pls->has_noheader_flag && !(pls->ctx->ctx_flags & AVFMTCTX_NOHEADER)) {
            pls->has_noheader_flag = 0;
            update_noheader_flag(s);
        }

        if (pls->pkt->stream_index >= pls->n_main_streams) {
            av_log(s, AV_LOG_ERROR, "stream index inconsistency: index %d, %d main streams, %d subdemuxer streams\n",
                   pls->pkt->stream_index, pls->n_main_streams, pls->ctx->nb_streams);
            av_packet_unref(pls->pkt);
            return AVERROR_BUG;
        }

        ist = pls->ctx->streams[pls->pkt->stream_index];
        st = pls->main_streams[pls->pkt->stream_index];

        av_packet_move_ref(pkt, pls->pkt);
        pkt->stream_index = st->index;

        if (pkt->dts != AV_NOPTS_VALUE)
            c->cur_timestamp = av_rescale_q(pkt->dts,
                                            ist->time_base,
                                            AV_TIME_BASE_Q);

        /* There may be more situations where this would be useful, but this at least
         * handles newly probed codecs properly (i.e. request_probe by mpegts). */
        if (ist->codecpar->codec_id != st->codecpar->codec_id) {
            ret = set_stream_info_from_input_stream(st, pls, ist);
            if (ret < 0) {
                return ret;
            }
        }

        return 0;
    }
    return AVERROR_EOF;
}

这个代码调用了av_read_frame方法，但是实际上可以看到，整个函数只是做一些解析判断的工作，并没有是想象中的IO层的相关流程，真正的处理就在av_read_frame，最终会调用到read_data方法，也就是前面初始化IOContext的时候，传递进去的函数回调。

read_data

static int read_data(void *opaque, uint8_t *buf, int buf_size)
{
    struct playlist *v = opaque;
    HLSContext *c = v->parent->priv_data;
    int ret;
    int just_opened = 0;
    int reload_count = 0;
    struct segment *seg;
//使用了go语句
restart:
    if (!v->needed)
        return AVERROR_EOF;

    if (!v->input || (c->http_persistent && v->input_read_done)) {
        int64_t reload_interval;

        /* Check that the playlist is still needed before opening a new
         * segment. */
        //校验playlist合法性
        v->needed = playlist_needed(v);

        if (!v->needed) {
            av_log(v->parent, AV_LOG_INFO, "No longer receiving playlist %d ('%s')\n",
                   v->index, v->url);
            return AVERROR_EOF;
        }

        /* If this is a live stream and the reload interval has elapsed since
         * the last playlist reload, reload the playlists now. */
        //直播流的情况下，重新刷新的时长。
        reload_interval = default_reload_interval(v);

reload:
        reload_count++;
        if (reload_count > c->max_reload)
            return AVERROR_EOF;
        //如果是直播流的情况下，并且已经到达了reload的时机，重新去刷新playlist获取ts列表
        if (!v->finished &&
            av_gettime_relative() - v->last_load_time >= reload_interval) {
            if ((ret = parse_playlist(c, v->url, v, NULL)) < 0) {
                if (ret != AVERROR_EXIT)
                    av_log(v->parent, AV_LOG_WARNING, "Failed to reload playlist %d\n",
                           v->index);
                return ret;
            }
            /* If we need to reload the playlist again below (if
             * there's still no more segments), switch to a reload
             * interval of half the target duration. */
            // 按照HLS协议规定，如果还需要请求playlist，请求间隔可以设置为segment时长的一半
            reload_interval = v->target_duration / 2;
        }
        //如果当前的seqnumer，比起始的seq都要小，那么直接跳过中间segment并刷新到最新数据。
        if (v->cur_seq_no < v->start_seq_no) {
            av_log(v->parent, AV_LOG_WARNING,
                   "skipping %"PRId64" segments ahead, expired from playlists\n",
                   v->start_seq_no - v->cur_seq_no);
            v->cur_seq_no = v->start_seq_no;
        }
        if (v->cur_seq_no > v->last_seq_no) {
            v->last_seq_no = v->cur_seq_no;
            v->m3u8_hold_counters = 0;
        } else if (v->last_seq_no == v->cur_seq_no) {
            v->m3u8_hold_counters++;
            if (v->m3u8_hold_counters >= c->m3u8_hold_counters) {
                return AVERROR_EOF;
            }
        } else {
            av_log(v->parent, AV_LOG_WARNING, "maybe the m3u8 list sequence have been wraped.\n");
        }
        //如果当前的的序号>起始序号+总数ts片数量。
        if (v->cur_seq_no >= v->start_seq_no + v->n_segments) {
            //如果是点播，说明整个视频就到达了尾端，就结束了
            if (v->finished)
                return AVERROR_EOF;
            //如果是直播，如果还没有到达reload时间，那么就sleep一直等到reload时间
            while (av_gettime_relative() - v->last_load_time < reload_interval) {
                if (ff_check_interrupt(c->interrupt_callback))
                    return AVERROR_EXIT;
                av_usleep(100*1000);
            }
            /* Enough time has elapsed since the last reload */
            //到达reload时间，重新刷新playlist
            goto reload;
        }
		
        /* 所有的前提都满足，可以开始读segment了 */
        v->input_read_done = 0;
        seg = current_segment(v);

        /* load/update Media Initialization Section, if any */
        ret = update_init_section(v, seg);
        if (ret)
            return ret;

        if (c->http_multiple == 1 && v->input_next_requested) {
            FFSWAP(AVIOContext *, v->input, v->input_next);
            v->cur_seg_offset = 0;
            v->input_next_requested = 0;
            ret = 0;
        } else {
            /* 这里发起了的HTTP请求 */
            ret = open_input(c, v, seg, &v->input);
        }
        if (ret < 0) {
            if (ff_check_interrupt(c->interrupt_callback))
                return AVERROR_EXIT;
            //segment打开失败了，直接跳过当前segment，加载下一个 seq+1
            av_log(v->parent, AV_LOG_WARNING, "Failed to open segment %"PRId64" of playlist %d\n",
                   v->cur_seq_no,
                   v->index);
            v->cur_seq_no += 1;
            goto reload;
        }
        just_opened = 1;
    }

    if (c->http_multiple == -1) {
        uint8_t *http_version_opt = NULL;
        int r = av_opt_get(v->input, "http_version", AV_OPT_SEARCH_CHILDREN, &http_version_opt);
        if (r >= 0) {
            c->http_multiple = (!strncmp((const char *)http_version_opt, "1.1", 3) || !strncmp((const char *)http_version_opt, "2.0", 3));
            av_freep(&http_version_opt);
        }
    }

    seg = next_segment(v);
    if (c->http_multiple == 1 && !v->input_next_requested &&
        seg && seg->key_type == KEY_NONE && av_strstart(seg->url, "http", NULL)) {
        ret = open_input(c, v, seg, &v->input_next);
        if (ret < 0) {
            if (ff_check_interrupt(c->interrupt_callback))
                return AVERROR_EXIT;
            av_log(v->parent, AV_LOG_WARNING, "Failed to open segment %"PRId64" of playlist %d\n",
                   v->cur_seq_no + 1,
                   v->index);
        } else {
            v->input_next_requested = 1;
        }
    }

    if (v->init_sec_buf_read_offset < v->init_sec_data_len) {
        /* Push init section out first before first actual segment */
        int copy_size = FFMIN(v->init_sec_data_len - v->init_sec_buf_read_offset, buf_size);
        memcpy(buf, v->init_sec_buf, copy_size);
        v->init_sec_buf_read_offset += copy_size;
        return copy_size;
    }

    seg = current_segment(v);
    /* 这是实际通过HTTP读取buf_size长度的数据，从server上获取的 */
    ret = read_from_url(v, seg, buf, buf_size);
    if (ret > 0) {
        if (just_opened && v->is_id3_timestamped != 0) {
            /* Intercept ID3 tags here, elementary audio streams are required
             * to convey timestamps using them in the beginning of each segment. */
            intercept_id3(v, buf, buf_size, &ret);
        }

        return ret;
    }
    /* 读取完成了，可以关闭segment所使用的HTTP资源了 */
    if (c->http_persistent &&
        seg->key_type == KEY_NONE && av_strstart(seg->url, "http", NULL)) {
        v->input_read_done = 1;
    } else {
        ff_format_io_close(v->parent, &v->input);
    }
  	//segment序号++，重新restart
    v->cur_seq_no++;

    c->cur_seq_no = v->cur_seq_no;

    goto restart;
}

这里可以发现read_data才是真正io层做的事情，从服务器读取数据，放到缓冲区去，真正的数据解析是由demuxer去做的。

read_seek

最后看一下，seek操作，基本思路就是按照给定的时间点(timestamp)找到对应的流的读取位置，然后继续读取数据。所以在执行seek之前，需要清理下之前缓存的数据

static int hls_read_seek(AVFormatContext *s, int stream_index,
                               int64_t timestamp, int flags)
{
    HLSContext *c = s->priv_data;
    struct playlist *seek_pls = NULL;
    int i, j;
    int stream_subdemuxer_index;
    int64_t first_timestamp, seek_timestamp, duration;
    int64_t seq_no;

    if ((flags & AVSEEK_FLAG_BYTE) || (c->ctx->ctx_flags & AVFMTCTX_UNSEEKABLE))
        return AVERROR(ENOSYS);
	
    first_timestamp = c->first_timestamp == AV_NOPTS_VALUE ?
                      0 : c->first_timestamp;
	//根据传入的timestamp，和index，转换最终的seek时间戳
    seek_timestamp = av_rescale_rnd(timestamp, AV_TIME_BASE,
                                    s->streams[stream_index]->time_base.den,
                                    flags & AVSEEK_FLAG_BACKWARD ?
                                    AV_ROUND_DOWN : AV_ROUND_UP);

    duration = s->duration == AV_NOPTS_VALUE ?
               0 : s->duration;
	/* 检查seek位置的有效性 */
    if (0 < duration && duration < seek_timestamp - first_timestamp)
        return AVERROR(EIO);

    /* find the playlist with the specified stream */
    /* 找到stream_index对应的playlist */
    for (i = 0; i < c->n_playlists; i++) {
        struct playlist *pls = c->playlists[i];
        for (j = 0; j < pls->n_main_streams; j++) {
            if (pls->main_streams[j] == s->streams[stream_index]) {
                seek_pls = pls;
                stream_subdemuxer_index = j;
                break;
            }
        }
    }
    /* check if the timestamp is valid for the playlist with the
     * specified stream index */
   	/* 检查给定的seek timestamp对指定的流是否有效 */
    if (!seek_pls || !find_timestamp_in_playlist(c, seek_pls, seek_timestamp, &seq_no))
        return AVERROR(EIO);

    /* set segment now so we do not need to search again below */
    seek_pls->cur_seq_no = seq_no;
    seek_pls->seek_stream_index = stream_subdemuxer_index;
	/* 下面是对正在读取的流的处理 */
    for (i = 0; i < c->n_playlists; i++) {
        /* Reset reading */
        struct playlist *pls = c->playlists[i];
        ff_format_io_close(pls->parent, &pls->input);
        pls->input_read_done = 0;
        ff_format_io_close(pls->parent, &pls->input_next);
        pls->input_next_requested = 0;
        av_packet_unref(pls->pkt);
        pls->pb.eof_reached = 0;
        /* Clear any buffered data */
        //清除缓冲区数据
        pls->pb.buf_end = pls->pb.buf_ptr = pls->pb.buffer;
        /* Reset the pos, to let the mpegts demuxer know we've seeked. */
        pls->pb.pos = 0;
        /* Flush the packet queue of the subdemuxer. */
        ff_read_frame_flush(pls->ctx);

        pls->seek_timestamp = seek_timestamp;
        pls->seek_flags = flags;
		
        /* 对于不是seek的playlist进行处理 */

        if (pls != seek_pls) {
            /* set closest segment seq_no for playlists not handled above */
            find_timestamp_in_playlist(c, pls, seek_timestamp, &pls->cur_seq_no);
            /* seek the playlist to the given position without taking
             * keyframes into account since this playlist does not have the
             * specified stream where we should look for the keyframes */
            pls->seek_stream_index = -1;
            pls->seek_flags |= AVSEEK_FLAG_ANY;
        }
    }

    c->cur_timestamp = seek_timestamp;

    return 0;
}

需要注意的是其中的第四个参数flag，表示seek到具体帧的方式。

#deflagfine AVSEEK_FLAG_BACKWARD    1 // 是seek到请求的timestamp之前最近的关键帧
#define AVSEEK_FLAG_BYTE            2 // 是基于字节位置的查找
#define AVSEEK_FLAG_ANY             4 //是可以seek到任意帧，注意不一定是关键帧，因此使用时可能会导致花屏 
#define AVSEEK_FLAG_FRAME           8 //是基于帧数量快进

小结

本文主要参考FFmpeg/libavformat/hls.c，对其代码逻辑做了简单收集及整理。整体来说，本文总结了ffmpeg中hls_demxuer的实现逻辑，希望对读者有所帮助，感谢收看~~