[时间：2018-05] [状态：Open]
[关键词：流媒体，stream，HLS, ffmpeg, demuxer，avformat]
0 引言
本文将关注于FFmpeg中的HLS相关实现，相关代码在libavformat/hls.c中（我所使用的Ffmpeg版本是4.0的），分析hls_demuxer的主要实现逻辑。
本文作为我之前的HLS综述的后续文章，也是ffmpeg框架分析的后续文章。前者介绍了HLS协议相关的理论部分，后者介绍了FFmpeg主要框架分析（本文主要关注demuxer）；要是你对此感兴趣建议了解下。
1 ff_hls_demuxer的主要对外接口
#define OFFSET(x) offsetof(HLSContext, x)
static const AVOption hls_options[] = {
{"live_start_index", "segment index to start live streams at (negative values are from the end)",
OFFSET(live_start_index), AV_OPT_TYPE_INT, {.i64 = -3}, INT_MIN, INT_MAX, FLAGS},
// ... HLS demuxer支持的参数，有删减
{NULL}
};
static const AVClass hls_class = {
.class_name = "hls,applehttp",
.item_name = av_default_item_name,
.option = hls_options,
.version = LIBAVUTIL_VERSION_INT,
};
AVInputFormat ff_hls_demuxer = {
.name = "hls,applehttp",
.long_name = NULL_IF_CONFIG_SMALL("Apple HTTP Live Streaming"),
.priv_class = &hls_class,
.priv_data_size = sizeof(HLSContext),
.flags = AVFMT_NOGENSEARCH,
.read_probe = hls_probe, // 媒体格式探测
.read_header = hls_read_header, // 读取协议头并获取节目信息
.read_packet = hls_read_packet, // 读取音视频包
.read_close = hls_close, // 关闭HLS通信
.read_seek = hls_read_seek, // 实现HLS的seek操作
};
通常分析FFmpeg中的demuxer，我们主要关注其read_probe、read_header、read_packet、read_close、read_seek这五个函数指针所对应的实现代码。FFmpeg在实现demuxer时将其协议解析的部分完整封装了demuxer中。
hls_demuxer中主要的结构是HLSContext，定义如下：
typedef struct HLSContext {
AVClass *class;
AVFormatContext *ctx;
int n_variants;
struct variant **variants;/* master playlist中有多个variants */
int n_playlists;
struct playlist **playlists;/* playlists中包含一个segment的列表 */
int n_renditions;
struct rendition **renditions;
int cur_seq_no;
int live_start_index;
int first_packet;
int64_t first_timestamp;
int64_t cur_timestamp;
AVIOInterruptCB *interrupt_callback;
AVDictionary *avio_opts;
int strict_std_compliance;
char *allowed_extensions;
int max_reload;
int http_persistent;
int http_multiple;
AVIOContext *playlist_pb;
} HLSContext;
接下来我们将逐个函数查看。
2 hls_demuxer核心接口实现
2.1 read_probe -- hls_probe
这个函数主要是HLS格式探测，其实现代码如下：
static int hls_probe(AVProbeData *p)
{
/* HLS协议要求必须以#EXTM3U打头，并至少有下面三个字段之一存在 */
if (strncmp(p->buf, "#EXTM3U", 7))
return 0;
if (strstr(p->buf, "#EXT-X-STREAM-INF:") ||
strstr(p->buf, "#EXT-X-TARGETDURATION:") ||
strstr(p->buf, "#EXT-X-MEDIA-SEQUENCE:"))
return AVPROBE_SCORE_MAX;
return 0;
}
这段代码逻辑比较简单，都是关于字符串匹配的处理，也是按照HLS协议规定的要求进行媒体格式探测的。
2.2 read_header -- hls_read_header
hls_read_header将是比较复杂的处理逻辑，因为这里涉及到master playlist解析、节目信息获取，同时初始化hls解析相关的结构。这个函数实现有300行左右。我们将分为三部分：playlist解析、hls相关初始化、提取节目信息。
2.2.1 playlist解析
hls_read_header第一部分代码如下：
static int hls_read_header(AVFormatContext *s)
{
HLSContext *c = s->priv_data;
int ret = 0, i;
c->ctx = s;
c->interrupt_callback = &s->interrupt_callback;
c->strict_std_compliance = s->strict_std_compliance;
c->first_packet = 1;
c->first_timestamp = AV_NOPTS_VALUE;
c->cur_timestamp = AV_NOPTS_VALUE;
/* 解析m3u8 */
if ((ret = parse_playlist(c, s->url, NULL, s->pb)) < 0)
goto fail;
if (c->n_variants == 0) {
av_log(NULL, AV_LOG_WARNING, "Empty playlist
");
ret = AVERROR_EOF;
goto fail;
}
/* 对于master playlist，逐个解析其中的playlist */
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];
if ((ret = parse_playlist(c, pls->url, pls, NULL)) < 0)
goto fail;
}
}
/* 必须有至少一个variant */
if (c->variants[0]->playlists[0]->n_segments == 0) {
av_log(NULL, AV_LOG_WARNING, "Empty playlist
");
ret = AVERROR_EOF;
goto fail;
}
// ... 部分代码，有删减
从上面代码来看最主要的逻辑就是调用parse_playlist函数来解析m3u8文件。解析部分主要参考HLS协议就可以。其代码如下：
static int parse_playlist(HLSContext *c, const char *url,
struct playlist *pls, AVIOContext *in)
{
int ret = 0, is_segment = 0, is_variant = 0;
int64_t duration = 0;
char line[MAX_URL_SIZE];
const char *ptr;
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;
if (!in) { /* 创建用于HTTP请求的AVIO */
AVDictionary *opts = NULL;
av_dict_copy(&opts, c->avio_opts, 0);
ret = c->ctx->io_open(c->ctx, &in, url, AVIO_FLAG_READ, &opts);
av_dict_free(&opts);
if (ret < 0)
return ret;
}
/* HTTP-URL重定向 */
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")) {/* HLS协议标志起始头 */
ret = AVERROR_INVALIDDATA;
goto fail;
}
/* 释放已经存在的pls及segment */
if (pls) {
free_segment_list(pls);
pls->finished = 0;
pls->type = PLS_TYPE_UNSPECIFIED;
}/* 以下是具体协议的解析 */
while (!avio_feof(in)) {
ff_get_chomp_line(in, line, sizeof(line));
if (av_strstart(line, "#EXT-X-STREAM-INF:", &ptr)) {
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-MEDIA:", &ptr)) {
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)) {
ret = ensure_playlist(c, &pls, url);
if (ret < 0)
goto fail; /* 最大分片时长 */
pls->target_duration = strtoll(ptr, NULL, 10) * AV_TIME_BASE;
} else if (av_strstart(line, "#EXT-X-MEDIA-SEQUENCE:", &ptr)) {
ret = ensure_playlist(c, &pls, url);
if (ret < 0)
goto fail;
pls->start_seq_no = atoi(ptr); /* 起始segment number */
} else if (av_strstart(line, "#EXT-X-PLAYLIST-TYPE:", &ptr)) {
ret = ensure_playlist(c, &pls, url);
if (ret < 0)
goto fail;
if (!strcmp(ptr, "EVENT"))/* HLS类型：VOD/EVENT */
pls->type = PLS_TYPE_EVENT;
else if (!strcmp(ptr, "VOD"))
pls->type = PLS_TYPE_VOD;
} else if (av_strstart(line, "#EXT-X-ENDLIST", &ptr)) {
if (pls) /* playlist结束标志，表示VOD */
pls->finished = 1;
} else if (av_strstart(line, "#EXTINF:", &ptr)) {
is_segment = 1;
duration = atof(ptr) * AV_TIME_BASE;
} else if (av_strstart(line, "#EXT-X-BYTERANGE:", &ptr)) {
seg_size = strtoll(ptr, NULL, 10);
ptr = strchr(ptr, '@');
if (ptr)/* 使用字节划分的m3u8 */
seg_offset = strtoll(ptr+1, NULL, 10);
} else if (av_strstart(line, "#", NULL)) {
continue; /* 忽略无法识别的字段 */
} else if (line[0]) {
if (is_variant) { /* 针对variant的处理，下一行一般是URL */
if (!new_variant(c, &variant_info, line, url)) {
ret = AVERROR(ENOMEM);
goto fail;
}
is_variant = 0;
}
if (is_segment) { /* 针对segment的处理，需要拼接segment的URL */
struct segment *seg;
if (!pls) {
if (!new_variant(c, 0, url, NULL)) {
ret = AVERROR(ENOMEM);
goto fail;
}
pls = c->playlists[c->n_playlists - 1];
}
seg = av_malloc(sizeof(struct segment));
if (!seg) {
ret = AVERROR(ENOMEM);
goto fail;
}
seg->duration = duration;
seg->key_type = KEY_NONE;
seg->key = NULL;
ff_make_absolute_url(tmp_str, sizeof(tmp_str), url, line);
seg->url = av_strdup(tmp_str);
if (!seg->url) {
av_free(seg->key);
av_free(seg);
ret = AVERROR(ENOMEM);
goto fail;
}
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);
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;
}
这样通过对m3u8的parse之后，我们已经创建了好了playlist、segment、variant以及rendition。接下来是第二部分。
2.2.2 hls相关初始化
主要相关代码如下:
/* 对于非直播流，根据segment的时长计算总的节目时长 */
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;
}
/* 将renditions与variants关联起来 */
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);
}
/* 为每个variant创建一个节目 */
for (i = 0; i < c->n_variants; i++) {
struct variant *v = c->variants[i];
AVProgram *program = av_new_program(s, i);
if (!program)
goto fail;
av_dict_set_int(&program->metadata, "variant_bitrate", v->bandwidth, 0);
}
/* 选择起始segment的索引 */
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);
}
我们看一下如何选择起始segment的索引，代码如下：
static int select_cur_seq_no(HLSContext *c, struct playlist *pls)
{
int seq_no;
/* 直播情况下，定期更新m3u8 */
if (!pls->finished && !c->first_packet &&
av_gettime_relative() - pls->last_load_time >= default_reload_interval(pls))
parse_playlist(c, pls->url, pls, NULL);
/* 对于非直播的情况，直接通过时长查找对应的segment索引号（seek时比较常用的逻辑） */
if (pls->finished && c->cur_timestamp != AV_NOPTS_VALUE) {
find_timestamp_in_playlist(c, pls, c->cur_timestamp, &seq_no);
return seq_no;
}
if (!pls->finished) {
if (!c->first_packet && /* 是在播放中选择segment */
c->cur_seq_no >= pls->start_seq_no &&
c->cur_seq_no < pls->start_seq_no + pls->n_segments)
return c->cur_seq_no;
/* 直播情况下，需要参考live_start_index调整下 */
if (c->live_start_index < 0)
return pls->start_seq_no + FFMAX(pls->n_segments + c->live_start_index, 0);
else
return pls->start_seq_no + FFMIN(c->live_start_index, pls->n_segments - 1);
}
/* 其他情况直接返回起始segment索引号*/
return pls->start_seq_no;
}
经过上面的处理，我们已经创建好FFmpeg框架上需要的AVProgram等信息了，接下来就需要填充AVProgram，并获得所有的AVStream信息。
2.2.3 提取节目信息
在开始介绍前，我们先看一下playlist结构体的定义：
/* 每个playlist都有自己的demuxer，如果该playlist是在用的，它还会有AVIOContext和AVPacket */
struct playlist {
char url[MAX_URL_SIZE];
AVIOContext pb;
uint8_t* read_buffer;
AVIOContext *input;
int input_read_done;
AVIOContext *input_next;
int input_next_requested;
AVFormatContext *parent;// 这个将指向公用的AVFormatContext
int index;
AVFormatContext *ctx; // 这个将用于解析当前playlist的所有segment
AVPacket pkt;
int has_noheader_flag;
/* 当前playlist中包含的AVStream信息 */
AVStream **main_streams;
int n_main_streams;
int finished; /* segment读取状态的相关参数 */
enum PlaylistType type;
int64_t target_duration;
int start_seq_no;
int n_segments; /* 当前playlist中的所有segment数组 */
struct segment **segments;
int needed;
int cur_seq_no;
int64_t cur_seg_offset;
int64_t last_load_time;
/* Media Initialization Section */
struct segment *cur_init_section;
uint8_t *init_sec_buf;
unsigned int init_sec_buf_size;
unsigned int init_sec_data_len;
unsigned int init_sec_buf_read_offset;
char key_url[MAX_URL_SIZE]; /* HLS解密密钥对应的URL */
uint8_t key[16];
int64_t seek_timestamp; /* seek相关的参数 */
int seek_flags;
int seek_stream_index; /* into subdemuxer stream array */
/* 和当前playlist相关的Renditions（可选） */
int n_renditions;
struct rendition **renditions;
/* Media Initialization Sections (EXT-X-MAP)（可选） */
int n_init_sections;
struct segment **init_sections;
};
hls_read_header后续逻辑与playlist结构体相关比较大，节目信息提取的代码如下：
/* 对每个playlist打开其demuxer */
for (i = 0; i < c->n_playlists; i++) {
struct playlist *pls = c->playlists[i];
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;
/* 调整直播流的起播索引号，以保证所有playlist是同步的 */
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;
}/* 这里初始化了AVIOContext，留意read_data函数，这将是后续读包的核心 */
ffio_init_context(&pls->pb, pls->read_buffer, INITIAL_BUFFER_SIZE, 0, pls,
read_data, NULL, NULL);
pls->pb.seekable = 0;
ret = av_probe_input_buffer(&pls->pb, &in_fmt, pls->segments[0]->url,
NULL, 0, 0);
if (ret < 0) {
av_log(s, AV_LOG_ERROR, "Error when loading first segment '%s'
", pls->segments[0]->url);
avformat_free_context(pls->ctx);
pls->ctx = NULL;
goto fail;
}
pls->ctx->pb = &pls->pb;
pls->ctx->io_open = nested_io_open;
pls->ctx->flags |= s->flags & ~AVFMT_FLAG_CUSTOM_IO;
/* 接下来就是打开流，获取其中AVStream信息了 */
ret = avformat_open_input(&pls->ctx, pls->segments[0]->url, in_fmt, NULL);
if (ret < 0)
goto fail;
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;
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;
fail:
hls_close(s);
return ret;
}
2.3 read_close -- hls_close
鉴于read_close实现相对简单，我们先了解下该函数的实现代码：
static void free_playlist_list(HLSContext *c)
{
int i; /* 针对每个playlist释放动态分配的内存 */
for (i = 0; i < c->n_playlists; i++) {
struct playlist *pls = c->playlists[i];
free_segment_list(pls);
free_init_section_list(pls);
av_freep(&pls->main_streams);
av_freep(&pls->renditions);
av_freep(&pls->init_sec_buf);
av_packet_unref(&pls->pkt);
av_freep(&pls->pb.buffer);
if (pls->input)
ff_format_io_close(c->ctx, &pls->input);
pls->input_read_done = 0;
if (pls->input_next)
ff_format_io_close(c->ctx, &pls->input_next);
pls->input_next_requested = 0;
if (pls->ctx) {
pls->ctx->pb = NULL;
avformat_close_input(&pls->ctx);
}
av_free(pls);
}
av_freep(&c->playlists);
c->n_playlists = 0;
}
static void free_variant_list(HLSContext *c)
{
int i;
for (i = 0; i < c->n_variants; i++) {
struct variant *var = c->variants[i];
av_freep(&var->playlists);
av_free(var);
}
av_freep(&c->variants);
c->n_variants = 0;
}
static void free_rendition_list(HLSContext *c)
{
int i;
for (i = 0; i < c->n_renditions; i++)
av_freep(&c->renditions[i]);
av_freep(&c->renditions);
c->n_renditions = 0;
}
static int hls_close(AVFormatContext *s)
{
HLSContext *c = s->priv_data;
/* 基本逻辑就是依次释放：playlist, variant, rendition */
free_playlist_list(c); /* 最复杂的释放 */
free_variant_list(c);
free_rendition_list(c);
av_dict_free(&c->avio_opts);
ff_format_io_close(c->ctx, &c->playlist_pb);
return 0;
}
2.4 read_packet -- hls_read_packet
该函数主要功能是读取原始的数据，并解析为AVPacket。代码大约有150行，主要逻辑如下：
static int hls_read_packet(AVFormatContext *s, AVPacket *pkt)
{
HLSContext *c = s->priv_data;
int ret, i, minplaylist = -1;
/* 这个函数会根据AVStream.discard标志来判断关闭不需要的HTTP下载 */
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];
/* 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;
reset_packet(&pls->pkt);
break;
} else {
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);
}
/* 以下部分跟seek有关，无seek操作无需后续判断 */
if (pls->seek_timestamp == AV_NOPTS_VALUE)
break;
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);
reset_packet(&pls->pkt);
}
}
/* 从所有AVStream中找到dts最小的一个，记录该索引值 */
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;
}
}
}
/* 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;
/* 判断stream_index有效性 */
if (pls->pkt.stream_index >= pls->n_main_streams) {
av_packet_unref(&pls->pkt);
reset_packet(&pls->pkt);
return AVERROR_BUG;
}
ist = pls->ctx->streams[pls->pkt.stream_index];
st = pls->main_streams[pls->pkt.stream_index];
*pkt = pls->pkt;
pkt->stream_index = st->index;
reset_packet(&c->playlists[minplaylist]->pkt);
/* 更新当前HLS的读取位置 */
if (pkt->dts != AV_NOPTS_VALUE)
c->cur_timestamp = av_rescale_q(pkt->dts,
ist->time_base,
AV_TIME_BASE_Q);
return 0;
}
return AVERROR_EOF;
}
从这个代码上来看很奇怪，这里没有包含任何http请求、playlist更新、segment的处理逻辑，那么真正的逻辑隐藏在哪里呢？不知道大家还记得2.2节中hls_read_header函数中对AVIOContext的初始化逻辑，其实最终的解析主要部分就在其中一个不起眼的函数中——read_data。上面的av_read_frame通过FFmpeg提供的一系列机制，最终调用read_data（你要是对此感兴趣，建议研究下雷神的自定义AVIO的示例）。这个函数长度在150行左右，但其内部是一个带有多个标签的循环逻辑（实际上这部分代码是最复杂的，使用了c语言中不推荐使用的goto语句），详细代码如下：
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;
restart: /* 标签：重启新的下载 */
if (!v->needed)
return AVERROR_EOF;
if (!v->input || (c->http_persistent && v->input_read_done)) {
int64_t reload_interval;
/* 检查playlist是否需要下载，如果不需要了，直接返回EOF */
v->needed = playlist_needed(v);
if (!v->needed) {
return AVERROR_EOF;
}
/* 对于直播流，定期更新playlist */
reload_interval = default_reload_interval(v);
reload: /* 标签：重新下载playlist */
reload_count++;
if (reload_count > c->max_reload)
return AVERROR_EOF;
if (!v->finished &&
av_gettime_relative() - v->last_load_time >= reload_interval) {
if ((ret = parse_playlist(c, v->url, v, NULL)) < 0)
return ret;
/* 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;
}
if (v->cur_seq_no < v->start_seq_no) {
/* 这种情况下客户端因为某些原因下载过慢，直接调整cur_seq_no，跳过一些segment */
v->cur_seq_no = v->start_seq_no;
}
if (v->cur_seq_no >= v->start_seq_no + v->n_segments) {
if (v->finished) /* 点播的情况下，表示读取完成 */
return AVERROR_EOF;
while (av_gettime_relative() - v->last_load_time < reload_interval) {
if (ff_check_interrupt(c->interrupt_callback))
return AVERROR_EXIT;
av_usleep(100*1000);
}
/* 上面的循环用于等待给定的时间，然后重新加载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);
/* 如果存在，加载并更新Media Initialization Section */
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->input_next_requested = 0;
ret = 0;
} else { /* 这里发起了新的HTTP请求 */
ret = open_input(c, v, seg, &v->input);
}
if (ret < 0) { /* 请求失败的情况下的处理，默认直接跳过当前segment */
if (ff_check_interrupt(c->interrupt_callback))
return AVERROR_EXIT;
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) == 0;
av_freep(&http_version_opt);
}
}
seg = next_segment(v); /* 这是使用单HTTP发起请求的处理 */
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 %d of playlist %d
",
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) {
/* 在任何实际数据返回之前，首先返回init section（解码器可能会依赖这些数据做初始化） */
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，这个数据是交织后的数据，从server上获取的 */
ret = read_from_url(v, seg, buf, buf_size);
if (ret > 0)
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);
}
/* 更新当前读取的seg_no，同时更新全局的，保证流切换之后可以同步 */
v->cur_seq_no++;
c->cur_seq_no = v->cur_seq_no;
goto restart; /* 回到上面继续循环了 */
}
这是需要强调下，read_data就是完成一个I/O该做的事情，从server上去数据，放到缓冲区中，真正的数据解析是由FFmpeg的通用框架实现的。换句话来说，HLS demuxer仅仅做了HLS协议相关的解析，关于mpeg-ts/mp4的解析实际上有它们各自的demuxer完成的。
2.5 read_seek -- hls_read_seek
seek的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。实际代码如下：
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, seq_no;
int j;
int stream_subdemuxer_index;
int64_t first_timestamp, seek_timestamp, duration;
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;
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);
/* 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;
}
}
}
/* 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);
/* 所有参数都有效了，这就可以设置目标位置了 */
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++) {
struct playlist *pls = c->playlists[i];
if (pls->input)
ff_format_io_close(pls->parent, &pls->input);
pls->input_read_done = 0;
if (pls->input_next)
ff_format_io_close(pls->parent, &pls->input_next);
pls->input_next_requested = 0;
av_packet_unref(&pls->pkt);
reset_packet(&pls->pkt);
pls->pb.eof_reached = 0;
/* 清空所有缓存的数据 */
pls->pb.buf_end = pls->pb.buf_ptr = pls->pb.buffer;
/* 重置读取位置，以确保demuxer知道需要seek */
pls->pb.pos = 0;
/* 清空subdemuxer缓存的所有AVPacket队列 */
ff_read_frame_flush(pls->ctx);
pls->seek_timestamp = seek_timestamp;
pls->seek_flags = flags;
if (pls != seek_pls) {
/* 对于不是seek的playlist，将其读取位置设置到seek目标点的位置 */
find_timestamp_in_playlist(c, pls, seek_timestamp, &pls->cur_seq_no);
pls->seek_stream_index = -1; /* 这两个标志将在read_packet中使用 */
pls->seek_flags |= AVSEEK_FLAG_ANY;
}
}
/* 最后记录seek目标位置 */
c->cur_timestamp = seek_timestamp;
return 0;
}
读完seek的代码，基本上是处理了下正在读取的流，并没有处理新收到的数据。对新收到数据的过滤处理明显是在hls_read_packet中完成（通过seek_stream_index、seek_flags、seek_timestamp等参数）。需要说明的是seek操作和read_packet必须位于同一个线程，否则从上面的实现来看，明显存在多线程逻辑上的问题。
3 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。
HLS协议中还涉及一些比较细节的部分，比如subtitle、rendetion、group、init_section、fragment mp4，对这些感兴趣的建议参考HLS官方标准。