avformat_find_stream_info 的主要作用就是：解析媒体文件并获取相关的流信息
整体的逻辑如下图所示：

/**
 * Read packets of a media file to get stream information. This
 * is useful for file formats with no headers such as MPEG. This
 * function also computes the real framerate in case of MPEG-2 repeat
 * frame mode.
 * The logical file position is not changed by this function;
 * examined packets may be buffered for later processing.
 *
 * @param ic media file handle
 * @param options  If non-NULL, an ic.nb_streams long array of pointers to
 *                 dictionaries, where i-th member contains options for
 *                 codec corresponding to i-th stream.
 *                 On return each dictionary will be filled with options that were not found.
 * @return >=0 if OK, AVERROR_xxx on error
 *
 * @note this function isn't guaranteed to open all the codecs, so
 *       options being non-empty at return is a perfectly normal behavior.
 *
 * @todo Let the user decide somehow what information is needed so that
 *       we do not waste time getting stuff the user does not need.
 */

/**
 * 从注释可知道，此方法通过读取若干 packet 包来获取流信息，这对于像 MPEG 这种没有 header 的格式比较有用。此函数也计算了像 MPEG-2 这种支持 repeat mode 的真实帧率
 * 这个函数不会修改逻辑文件位置，所读取到的 packet 会缓存起来供后面使用。
 */
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
{
    int i, count = 0, ret = 0, j;
    int64_t read_size;
    AVStream *st;
    AVCodecContext *avctx;
    AVPacket pkt1;
    int64_t old_offset  = avio_tell(ic->pb);
    // new streams might appear, no options for those
    int orig_nb_streams = ic->nb_streams;
    int flush_codecs;
    int64_t max_analyze_duration = ic->max_analyze_duration;
    int64_t max_stream_analyze_duration;
    int64_t max_subtitle_analyze_duration;
    int64_t probesize = ic->probesize;
    int eof_reached = 0;
    int *missing_streams = av_opt_ptr(ic->iformat->priv_class, ic->priv_data, "missing_streams");

    flush_codecs = probesize > 0;

    av_opt_set(ic, "skip_clear", "1", AV_OPT_SEARCH_CHILDREN);

    max_stream_analyze_duration = max_analyze_duration;
    max_subtitle_analyze_duration = max_analyze_duration;
    if (!max_analyze_duration) {
        //默认5*AV_TIME_BASE，针对flv和mpeg默认90*AV_TIME_BASE和7*AV_TIME_BASE
        max_stream_analyze_duration =
        max_analyze_duration        = 5*AV_TIME_BASE;
        max_subtitle_analyze_duration = 30*AV_TIME_BASE;
        if (!strcmp(ic->iformat->name, "flv"))
            max_stream_analyze_duration = 90*AV_TIME_BASE;
        if (!strcmp(ic->iformat->name, "mpeg") || !strcmp(ic->iformat->name, "mpegts"))
            max_stream_analyze_duration = 7*AV_TIME_BASE;
    }

    if (ic->pb)
        av_log(ic, AV_LOG_DEBUG, "Before avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d nb_streams:%d\n",
               avio_tell(ic->pb), ic->pb->bytes_read, ic->pb->seek_count, ic->nb_streams);

    /**
     * -----【第一次循环遍历流】-------
     * 主要就是：初始化 avctx 的 time_base 等参数，初始化解析器，将编解码器的参数信息拷贝到编解码器上下文中，
     * 查找解码器、打开解码器
     */
    for (i = 0; i < ic->nb_streams; i++) {
        const AVCodec *codec;
        AVDictionary *thread_opt = NULL;
        st = ic->streams[i];
        avctx = st->internal->avctx;//获取编解码器上下文

        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO ||
            st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE) {
/*            if (!st->time_base.num)
                st->time_base = */
            if (!avctx->time_base.num)
                avctx->time_base = st->time_base;//设置avctx->time_base
        }

        /* check if the caller has overridden the codec id */
#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
        if (st->codec->codec_id != st->internal->orig_codec_id) {
            st->codecpar->codec_id   = st->codec->codec_id;
            st->codecpar->codec_type = st->codec->codec_type;
            st->internal->orig_codec_id = st->codec->codec_id;
        }
FF_ENABLE_DEPRECATION_WARNINGS
#endif
        // only for the split stuff
        if (!st->parser && !(ic->flags & AVFMT_FLAG_NOPARSE) && st->internal->request_probe <= 0) {
            st->parser = av_parser_init(st->codecpar->codec_id);//解析器为空则初始化解析器
            if (st->parser) {
                if (st->need_parsing == AVSTREAM_PARSE_HEADERS) {
                    st->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
                } else if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW) {
                    st->parser->flags |= PARSER_FLAG_USE_CODEC_TS;
                }
            } else if (st->need_parsing) {
                av_log(ic, AV_LOG_VERBOSE, "parser not found for codec "
                       "%s, packets or times may be invalid.\n",
                       avcodec_get_name(st->codecpar->codec_id));
            }
        }

        if (st->codecpar->codec_id != st->internal->orig_codec_id)
            st->internal->orig_codec_id = st->codecpar->codec_id;

        ret = avcodec_parameters_to_context(avctx, st->codecpar);//参数拷贝
        if (ret < 0)
            goto find_stream_info_err;
        if (st->internal->request_probe <= 0)
            st->internal->avctx_inited = 1;

        codec = find_probe_decoder(ic, st, st->codecpar->codec_id);//根据解码器id查找解码器

        /* Force thread count to 1 since the H.264 decoder will not extract
         * SPS and PPS to extradata during multi-threaded decoding. */
        av_dict_set(options ? &options[i] : &thread_opt, "threads", "1", 0);

        if (ic->codec_whitelist)
            av_dict_set(options ? &options[i] : &thread_opt, "codec_whitelist", ic->codec_whitelist, 0);

        /* Ensure that subtitle_header is properly set. */
        if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE
            && codec && !avctx->codec) {
            if (avcodec_open2(avctx, codec, options ? &options[i] : &thread_opt) < 0)
                av_log(ic, AV_LOG_WARNING,
                       "Failed to open codec in %s\n",__FUNCTION__);
        }

        // Try to just open decoders, in case this is enough to get parameters.
        // 在某些场景下，只需打开解码器，就能获取到编码层的参数
        if (!has_codec_parameters(st, NULL) && st->internal->request_probe <= 0) {
            if (codec && !avctx->codec)
                if (avcodec_open2(avctx, codec, options ? &options[i] : &thread_opt) < 0)//打开解码器
                    av_log(ic, AV_LOG_WARNING,
                           "Failed to open codec in %s\n",__FUNCTION__);
        }
        if (!options)
            av_dict_free(&thread_opt);
    }

    for (i = 0; i < ic->nb_streams; i++) {//-----【第二个循环】-------
#if FF_API_R_FRAME_RATE
        ic->streams[i]->internal->info->last_dts = AV_NOPTS_VALUE;
#endif
        ic->streams[i]->internal->info->fps_first_dts = AV_NOPTS_VALUE;
        ic->streams[i]->internal->info->fps_last_dts  = AV_NOPTS_VALUE;
    }

    //  对 stream 中的数据进行 probe，读取一定的数据到内存中
    read_size = 0;
    for (;;) {//-------------------------【第三个循环】------------------
        const AVPacket *pkt;
        int analyzed_all_streams;
        //  ff_check_interrupt 检测是否有用户层进行中断请求，
        if (ff_check_interrupt(&ic->interrupt_callback)) {
            ret = AVERROR_EXIT;
            av_log(ic, AV_LOG_DEBUG, "interrupted\n");
            break;
        }

        /* 再次对所有的 streams 进行一次遍历，检查 stream 中 codec 信息是否完整，
         * 如果还有没解析出的，那么 break 跳出当前 stream 的遍历（此时还没有跳出 for(;;) 的大循环）
         * 在确认了 has_codec 之后，根据各种可能出现的情况设置好 fps_analyze_framecount；之后，还有一系列判断是否应该 break 遍历。
         */
        /* check if one codec still needs to be handled */
        for (i = 0; i < ic->nb_streams; i++) {//-----------【第三个循环的第一个嵌套循环】---------------
            int fps_analyze_framecount = 20;
            int count;

            st = ic->streams[i];
            /**
             * has_codec_parameters：此函数检查 codec 信息是否完整
             */
            if (!has_codec_parameters(st, NULL))
                break;
            /* If the timebase is coarse (like the usual millisecond precision
             * of mkv), we need to analyze more frames to reliably arrive at
             * the correct fps. */
             //如果时基比较粗略（例如 mkv 通常的毫秒精度），我们需要分析更多帧才能可靠地获得正确的 fps，fps_analyze_framecount * 2
            if (av_q2d(st->time_base) > 0.0005)
                fps_analyze_framecount *= 2;
            if (!tb_unreliable(st->internal->avctx))//时间基不正常或不是 mpeg4/mpeg2/HEVC/AVC 影片，不需要分析，fps_analyze_framecount = 0
                fps_analyze_framecount = 0;
            if (ic->fps_probe_size >= 0)//设置了 fps_probe_size，则按 fps_probe_size
                fps_analyze_framecount = ic->fps_probe_size;
            if (st->disposition & AV_DISPOSITION_ATTACHED_PIC)//如果是音频文件附带的图片流或者是视频附带的信息流，就不需要估计帧率
                fps_analyze_framecount = 0;
            /* variable fps and no guess at the real fps */
            // 可变 fps 并且无法猜测真实 fps
            count = (ic->iformat->flags & AVFMT_NOTIMESTAMPS) ?
                       st->internal->info->codec_info_duration_fields/2 :
                       st->internal->info->duration_count;
            //未找到帧率的视频帧，并且 count < fps_analyze_framecount，则退出循环去估算视频帧          
            if (!(st->r_frame_rate.num && st->avg_frame_rate.num) &&
                st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
                if (count < fps_analyze_framecount)
                    break;
            }
            // Look at the first 3 frames if there is evidence of frame delay
            // but the decoder delay is not set.
            //查看前 3 帧是否有帧延迟现象但未设置解码器延迟
            if (st->internal->info->frame_delay_evidence && count < 2 && st->internal->avctx->has_b_frames == 0)
                break;
            if (!st->internal->avctx->extradata &&
                (!st->internal->extract_extradata.inited ||
                 st->internal->extract_extradata.bsf) &&
                extract_extradata_check(st))
                break;
            if (st->first_dts == AV_NOPTS_VALUE &&
                !(ic->iformat->flags & AVFMT_NOTIMESTAMPS) &&
                st->codec_info_nb_frames < ((st->disposition & AV_DISPOSITION_ATTACHED_PIC) ? 1 : ic->max_ts_probe) &&
                (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO ||
                 st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO))
                break;
        }
        analyzed_all_streams = 0;
        /* 如果循环能正常结束，说明流信息的探测完毕，此时 i == ic->nb_streams；
         * 如果中间 break 了，说明某个流的信息还没有完全得到，此时 i < ic->nb_streams， 需继续探测
         */
        if (!missing_streams || !*missing_streams)
            if (i == ic->nb_streams) {// i == ic->nb_streams，说明所有的流都没问题了
                analyzed_all_streams = 1;
                /* NOTE: If the format has no header, then we need to read some
                 * packets to get most of the streams, so we cannot stop here. */
                // 如果是像 MPEG 这种没有头的封装格式，需要解析更多的 packets; 否则 break
                if (!(ic->ctx_flags & AVFMTCTX_NOHEADER)) {
                    /* If we found the info for all the codecs, we can stop. */
                    ret = count;
                    av_log(ic, AV_LOG_DEBUG, "All info found\n");
                    flush_codecs = 0;
                    break;
                }
            }
        // 下面属于继续探测的流程   
        /* We did not get all the codec info, but we read too much data. */
        //虽然流信息还没完全探测出来，但是 read_size >= probesize， 已读取到的大小超过了 probesize，则退出
        if (read_size >= probesize) {
            ret = count;
            av_log(ic, AV_LOG_DEBUG,
                   "Probe buffer size limit of %"PRId64" bytes reached\n", probesize);
            for (i = 0; i < ic->nb_streams; i++)
                if (!ic->streams[i]->r_frame_rate.num &&
                    ic->streams[i]->internal->info->duration_count <= 1 &&
                    ic->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
                    strcmp(ic->iformat->name, "image2"))
                    av_log(ic, AV_LOG_WARNING,
                           "Stream #%d: not enough frames to estimate rate; "
                           "consider increasing probesize\n", i);
            break;
        }

        /* NOTE: A new stream can be added there if no header in file
         * (AVFMTCTX_NOHEADER). */
        // 读取 packet 
        ret = read_frame_internal(ic, &pkt1);
        if (ret == AVERROR(EAGAIN))
            continue;

        if (ret < 0) {
            /* EOF or error*/
            eof_reached = 1;
            break;
        }

        if (!(ic->flags & AVFMT_FLAG_NOBUFFER)) {
            ret = avpriv_packet_list_put(&ic->internal->packet_buffer,
                                     &ic->internal->packet_buffer_end,
                                     &pkt1, NULL, 0);
            if (ret < 0)
                goto unref_then_goto_end;

            pkt = &ic->internal->packet_buffer_end->pkt;
        } else {
            pkt = &pkt1;
        }

        st = ic->streams[pkt->stream_index];
        //更新 read_size， read_size 用于下一轮判断是否已经超过 probesize
        if (!(st->disposition & AV_DISPOSITION_ATTACHED_PIC))
            read_size += pkt->size;

        avctx = st->internal->avctx;
        if (!st->internal->avctx_inited) {
            ret = avcodec_parameters_to_context(avctx, st->codecpar);
            if (ret < 0)
                goto unref_then_goto_end;
            st->internal->avctx_inited = 1;
        }

        if (pkt->dts != AV_NOPTS_VALUE && st->codec_info_nb_frames > 1) {
            /* check for non-increasing dts */
            if (st->internal->info->fps_last_dts != AV_NOPTS_VALUE &&
                st->internal->info->fps_last_dts >= pkt->dts) {
                av_log(ic, AV_LOG_DEBUG,
                       "Non-increasing DTS in stream %d: packet %d with DTS "
                       "%"PRId64", packet %d with DTS %"PRId64"\n",
                       st->index, st->internal->info->fps_last_dts_idx,
                       st->internal->info->fps_last_dts, st->codec_info_nb_frames,
                       pkt->dts);
                st->internal->info->fps_first_dts =
                st->internal->info->fps_last_dts  = AV_NOPTS_VALUE;
            }
            /* Check for a discontinuity in dts. If the difference in dts
             * is more than 1000 times the average packet duration in the
             * sequence, we treat it as a discontinuity. */
            if (st->internal->info->fps_last_dts != AV_NOPTS_VALUE &&
                st->internal->info->fps_last_dts_idx > st->internal->info->fps_first_dts_idx &&
                (pkt->dts - (uint64_t)st->internal->info->fps_last_dts) / 1000 >
                (st->internal->info->fps_last_dts     - (uint64_t)st->internal->info->fps_first_dts) /
                (st->internal->info->fps_last_dts_idx - st->internal->info->fps_first_dts_idx)) {
                av_log(ic, AV_LOG_WARNING,
                       "DTS discontinuity in stream %d: packet %d with DTS "
                       "%"PRId64", packet %d with DTS %"PRId64"\n",
                       st->index, st->internal->info->fps_last_dts_idx,
                       st->internal->info->fps_last_dts, st->codec_info_nb_frames,
                       pkt->dts);
                st->internal->info->fps_first_dts =
                st->internal->info->fps_last_dts  = AV_NOPTS_VALUE;
            }

            /* update stored dts values */
            if (st->internal->info->fps_first_dts == AV_NOPTS_VALUE) {
                st->internal->info->fps_first_dts     = pkt->dts;
                st->internal->info->fps_first_dts_idx = st->codec_info_nb_frames;
            }
            st->internal->info->fps_last_dts     = pkt->dts;
            st->internal->info->fps_last_dts_idx = st->codec_info_nb_frames;
        }
        if (st->codec_info_nb_frames>1) {
            int64_t t = 0;
            int64_t limit;

            //下面计算已经读取到的时间长度
            //codec_info_duration：已经读取到的 packet 的总时长
            if (st->time_base.den > 0)
                t = av_rescale_q(st->internal->info->codec_info_duration, st->time_base, AV_TIME_BASE_Q);
            //t = 已经读取到的帧数/帧率    
            if (st->avg_frame_rate.num > 0)
                t = FFMAX(t, av_rescale_q(st->codec_info_nb_frames, av_inv_q(st->avg_frame_rate), AV_TIME_BASE_Q));

            //根据 fps_last_dts - fps_first_dts 来计算
            if (   t == 0
                && st->codec_info_nb_frames>30
                && st->internal->info->fps_first_dts != AV_NOPTS_VALUE
                && st->internal->info->fps_last_dts  != AV_NOPTS_VALUE)
                t = FFMAX(t, av_rescale_q(st->internal->info->fps_last_dts - st->internal->info->fps_first_dts, st->time_base, AV_TIME_BASE_Q));

            // 如果所有流都探测完
            if (analyzed_all_streams)                                limit = max_analyze_duration;
            else if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE) limit = max_subtitle_analyze_duration;
            else                                                     limit = max_stream_analyze_duration;

            // 当前读取到的 packet 总时长 >= limit，则 break
            if (t >= limit) {
                av_log(ic, AV_LOG_VERBOSE, "max_analyze_duration %"PRId64" reached at %"PRId64" microseconds st:%d\n",
                       limit,
                       t, pkt->stream_index);
                if (ic->flags & AVFMT_FLAG_NOBUFFER)
                    av_packet_unref(&pkt1);
                break;
            }
            // 更新已经读取到的 packet 的总时长
            if (pkt->duration) {
                if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && st->start_time != AV_NOPTS_VALUE && pkt->pts >= st->start_time) {
                    st->internal->info->codec_info_duration = FFMIN(pkt->pts - st->start_time, st->internal->info->codec_info_duration + pkt->duration);
                } else
                    st->internal->info->codec_info_duration += pkt->duration;
                st->internal->info->codec_info_duration_fields += st->parser && st->need_parsing && avctx->ticks_per_frame ==2 ? st->parser->repeat_pict + 1 : 2;
            }
        }
        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
#if FF_API_R_FRAME_RATE
            ff_rfps_add_frame(ic, st, pkt->dts);
#endif
            if (pkt->dts != pkt->pts && pkt->dts != AV_NOPTS_VALUE && pkt->pts != AV_NOPTS_VALUE)
                st->internal->info->frame_delay_evidence = 1;
        }
        if (!st->internal->avctx->extradata) {
            ret = extract_extradata(st, pkt);
            if (ret < 0)
                goto unref_then_goto_end;
        }

        /* If still no information, we try to open the codec and to
         * decompress the frame. We try to avoid that in most cases as
         * it takes longer and uses more memory. For MPEG-4, we need to
         * decompress for QuickTime.
         *
         * If AV_CODEC_CAP_CHANNEL_CONF is set this will force decoding of at
         * least one frame of codec data, this makes sure the codec initializes
         * the channel configuration and does not only trust the values from
         * the container. */
        // 在某些场景下，打开解码器还获取不到编码层参数，就需要尝试解码，通过解码来获取到流的解码器信息
        try_decode_frame(ic, st, pkt,
                         (options && i < orig_nb_streams) ? &options[i] : NULL);

        if (ic->flags & AVFMT_FLAG_NOBUFFER)
            av_packet_unref(&pkt1);

        //已经探测的帧数 + 1，count 总数 + 1
        st->codec_info_nb_frames++;
        count++;
    }

    // 是否已经到达文件尾部
    if (eof_reached) {
        int stream_index;
        // 再遍历一次流，检查编码器参数，如果参数完整会再次调用 avcodec_open2
        for (stream_index = 0; stream_index < ic->nb_streams; stream_index++) {
            st = ic->streams[stream_index];
            avctx = st->internal->avctx;
            if (!has_codec_parameters(st, NULL)) {
                const AVCodec *codec = find_probe_decoder(ic, st, st->codecpar->codec_id);
                if (codec && !avctx->codec) {
                    AVDictionary *opts = NULL;
                    if (ic->codec_whitelist)
                        av_dict_set(&opts, "codec_whitelist", ic->codec_whitelist, 0);
                    if (avcodec_open2(avctx, codec, (options && stream_index < orig_nb_streams) ? &options[stream_index] : &opts) < 0)
                        av_log(ic, AV_LOG_WARNING,
                               "Failed to open codec in %s\n",__FUNCTION__);
                    av_dict_free(&opts);
                }
            }

            // EOF already reached while reading the stream above.
            // So continue with reoordering DTS with whatever delay we have.
            if (ic->internal->packet_buffer && !has_decode_delay_been_guessed(st)) {
                update_dts_from_pts(ic, stream_index, ic->internal->packet_buffer);
            }
        }
    }

    /* 
     * 刷新解码器
     * 有一些帧可能在缓存区，需要 flush
     */
    if (flush_codecs) {
        AVPacket empty_pkt = { 0 };
        int err = 0;
        av_init_packet(&empty_pkt);

        for (i = 0; i < ic->nb_streams; i++) {//------【第四个循环】-------

            st = ic->streams[i];

            /* flush the decoders */
            if (st->internal->info->found_decoder == 1) {
                do {
                    err = try_decode_frame(ic, st, &empty_pkt,
                                            (options && i < orig_nb_streams)
                                            ? &options[i] : NULL);
                } while (err > 0 && !has_codec_parameters(st, NULL));

                if (err < 0) {
                    av_log(ic, AV_LOG_INFO,
                        "decoding for stream %d failed\n", st->index);
                }
            }
        }
    }

    ff_rfps_calculate(ic);

    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];
        avctx = st->internal->avctx;
        if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
            if (avctx->codec_id == AV_CODEC_ID_RAWVIDEO && !avctx->codec_tag && !avctx->bits_per_coded_sample) {
                uint32_t tag= avcodec_pix_fmt_to_codec_tag(avctx->pix_fmt);
                if (avpriv_find_pix_fmt(avpriv_get_raw_pix_fmt_tags(), tag) == avctx->pix_fmt)
                    avctx->codec_tag= tag;
            }

            /* estimate average framerate if not set by demuxer */
            if (st->internal->info->codec_info_duration_fields &&
                !st->avg_frame_rate.num &&
                st->internal->info->codec_info_duration) {
                int best_fps      = 0;
                double best_error = 0.01;
                AVRational codec_frame_rate = avctx->framerate;

                if (st->internal->info->codec_info_duration        >= INT64_MAX / st->time_base.num / 2||
                    st->internal->info->codec_info_duration_fields >= INT64_MAX / st->time_base.den ||
                    st->internal->info->codec_info_duration        < 0)
                    continue;
                av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                          st->internal->info->codec_info_duration_fields * (int64_t) st->time_base.den,
                          st->internal->info->codec_info_duration * 2 * (int64_t) st->time_base.num, 60000);

                /* Round guessed framerate to a "standard" framerate if it's
                 * within 1% of the original estimate. */
                for (j = 0; j < MAX_STD_TIMEBASES; j++) {
                    AVRational std_fps = { get_std_framerate(j), 12 * 1001 };
                    double error       = fabs(av_q2d(st->avg_frame_rate) /
                                              av_q2d(std_fps) - 1);

                    if (error < best_error) {
                        best_error = error;
                        best_fps   = std_fps.num;
                    }

                    if (ic->internal->prefer_codec_framerate && codec_frame_rate.num > 0 && codec_frame_rate.den > 0) {
                        error       = fabs(av_q2d(codec_frame_rate) /
                                           av_q2d(std_fps) - 1);
                        if (error < best_error) {
                            best_error = error;
                            best_fps   = std_fps.num;
                        }
                    }
                }
                if (best_fps)
                    av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                              best_fps, 12 * 1001, INT_MAX);
            }

            if (!st->r_frame_rate.num) {
                if (    avctx->time_base.den * (int64_t) st->time_base.num
                    <= avctx->time_base.num * avctx->ticks_per_frame * (uint64_t) st->time_base.den) {
                    av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den,
                              avctx->time_base.den, (int64_t)avctx->time_base.num * avctx->ticks_per_frame, INT_MAX);
                } else {
                    st->r_frame_rate.num = st->time_base.den;
                    st->r_frame_rate.den = st->time_base.num;
                }
            }
            if (st->internal->display_aspect_ratio.num && st->internal->display_aspect_ratio.den) {
                AVRational hw_ratio = { avctx->height, avctx->width };
                st->sample_aspect_ratio = av_mul_q(st->internal->display_aspect_ratio,
                                                   hw_ratio);
            }
        } else if (avctx->codec_type == AVMEDIA_TYPE_AUDIO) {
            // 对于音频流，基于音频流服务类型初始化 disposition
            if (!avctx->bits_per_coded_sample)
                avctx->bits_per_coded_sample =
                    av_get_bits_per_sample(avctx->codec_id);
            // set stream disposition based on audio service type
            switch (avctx->audio_service_type) {
            case AV_AUDIO_SERVICE_TYPE_EFFECTS:
                st->disposition = AV_DISPOSITION_CLEAN_EFFECTS;
                break;
            case AV_AUDIO_SERVICE_TYPE_VISUALLY_IMPAIRED:
                st->disposition = AV_DISPOSITION_VISUAL_IMPAIRED;
                break;
            case AV_AUDIO_SERVICE_TYPE_HEARING_IMPAIRED:
                st->disposition = AV_DISPOSITION_HEARING_IMPAIRED;
                break;
            case AV_AUDIO_SERVICE_TYPE_COMMENTARY:
                st->disposition = AV_DISPOSITION_COMMENT;
                break;
            case AV_AUDIO_SERVICE_TYPE_KARAOKE:
                st->disposition = AV_DISPOSITION_KARAOKE;
                break;
            }
        }
    }

    // 计算时间相关参数
    if (probesize)
        estimate_timings(ic, old_offset);

    av_opt_set(ic, "skip_clear", "0", AV_OPT_SEARCH_CHILDREN);

    if (ret >= 0 && ic->nb_streams)
        /* We could not have all the codec parameters before EOF. */
        ret = -1;
    for (i = 0; i < ic->nb_streams; i++) {
        const char *errmsg;
        st = ic->streams[i];

        /* if no packet was ever seen, update context now for has_codec_parameters */
        if (!st->internal->avctx_inited) {
            if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&
                st->codecpar->format == AV_SAMPLE_FMT_NONE)
                st->codecpar->format = st->internal->avctx->sample_fmt;
            ret = avcodec_parameters_to_context(st->internal->avctx, st->codecpar);
            if (ret < 0)
                goto find_stream_info_err;
        }
        if (!has_codec_parameters(st, &errmsg)) {
            char buf[256];
            avcodec_string(buf, sizeof(buf), st->internal->avctx, 0);
            av_log(ic, AV_LOG_WARNING,
                   "Could not find codec parameters for stream %d (%s): %s\n"
                   "Consider increasing the value for the 'analyzeduration' (%"PRId64") and 'probesize' (%"PRId64") options\n",
                   i, buf, errmsg, ic->max_analyze_duration, ic->probesize);
        } else {
            ret = 0;
        }
    }

    ret = compute_chapters_end(ic);
    if (ret < 0)
        goto find_stream_info_err;

    // 更新 stream 各个结构的数据
    /* update the stream parameters from the internal codec contexts */
    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];

        if (st->internal->avctx_inited) {
            int orig_w = st->codecpar->width;
            int orig_h = st->codecpar->height;
            ret = avcodec_parameters_from_context(st->codecpar, st->internal->avctx);
            if (ret < 0)
                goto find_stream_info_err;
            ret = add_coded_side_data(st, st->internal->avctx);
            if (ret < 0)
                goto find_stream_info_err;
#if FF_API_LOWRES
            // The decoder might reduce the video size by the lowres factor.
            if (st->internal->avctx->lowres && orig_w) {
                st->codecpar->width = orig_w;
                st->codecpar->height = orig_h;
            }
#endif
        }

#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
        ret = avcodec_parameters_to_context(st->codec, st->codecpar);
        if (ret < 0)
            goto find_stream_info_err;

#if FF_API_LOWRES
        // The old API (AVStream.codec) "requires" the resolution to be adjusted
        // by the lowres factor.
        if (st->internal->avctx->lowres && st->internal->avctx->width) {
            st->codec->lowres = st->internal->avctx->lowres;
            st->codec->width = st->internal->avctx->width;
            st->codec->height = st->internal->avctx->height;
        }
#endif

        if (st->codec->codec_tag != MKTAG('t','m','c','d')) {
            st->codec->time_base = st->internal->avctx->time_base;
            st->codec->ticks_per_frame = st->internal->avctx->ticks_per_frame;
        }
        st->codec->framerate = st->avg_frame_rate;

        if (st->internal->avctx->subtitle_header) {
            st->codec->subtitle_header = av_malloc(st->internal->avctx->subtitle_header_size);
            if (!st->codec->subtitle_header)
                goto find_stream_info_err;
            st->codec->subtitle_header_size = st->internal->avctx->subtitle_header_size;
            memcpy(st->codec->subtitle_header, st->internal->avctx->subtitle_header,
                   st->codec->subtitle_header_size);
        }

        // Fields unavailable in AVCodecParameters
        st->codec->coded_width = st->internal->avctx->coded_width;
        st->codec->coded_height = st->internal->avctx->coded_height;
        st->codec->properties = st->internal->avctx->properties;
FF_ENABLE_DEPRECATION_WARNINGS
#endif

        st->internal->avctx_inited = 0;
    }

find_stream_info_err:
    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];
        if (st->internal->info)
            av_freep(&st->internal->info->duration_error);
        avcodec_close(ic->streams[i]->internal->avctx);
        av_freep(&ic->streams[i]->internal->info);
        av_bsf_free(&ic->streams[i]->internal->extract_extradata.bsf);
        av_packet_free(&ic->streams[i]->internal->extract_extradata.pkt);
    }
    if (ic->pb)
        av_log(ic, AV_LOG_DEBUG, "After avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d frames:%d\n",
               avio_tell(ic->pb), ic->pb->bytes_read, ic->pb->seek_count, count);
    return ret;

unref_then_goto_end:
    av_packet_unref(&pkt1);
    goto find_stream_info_err;
}

// 检查当前的音视频流信息是否完整
static int has_codec_parameters(AVStream *st, const char **errmsg_ptr)
{
    AVCodecContext *avctx = st->internal->avctx;

#define FAIL(errmsg) do {                                         \
        if (errmsg_ptr)                                           \
            *errmsg_ptr = errmsg;                                 \
        return 0;                                                 \
    } while (0)

    if (   avctx->codec_id == AV_CODEC_ID_NONE
        && avctx->codec_type != AVMEDIA_TYPE_DATA)
        FAIL("unknown codec");
    switch (avctx->codec_type) {
    case AVMEDIA_TYPE_AUDIO://音频的需要检测frame_size、sample_fmt、sample_rate、channels等
        if (!avctx->frame_size && determinable_frame_size(avctx))
            FAIL("unspecified frame size");
        if (st->internal->info->found_decoder >= 0 &&
            avctx->sample_fmt == AV_SAMPLE_FMT_NONE)
            FAIL("unspecified sample format");
        if (!avctx->sample_rate)
            FAIL("unspecified sample rate");
        if (!avctx->channels)
            FAIL("unspecified number of channels");
        if (st->internal->info->found_decoder >= 0 && !st->internal->nb_decoded_frames && avctx->codec_id == AV_CODEC_ID_DTS)
            FAIL("no decodable DTS frames");
        break;
    case AVMEDIA_TYPE_VIDEO://视频的需要检测width、pix_fmt、sample_aspect_ratio等
        if (!avctx->width)
            FAIL("unspecified size");
        if (st->internal->info->found_decoder >= 0 && avctx->pix_fmt == AV_PIX_FMT_NONE)
            FAIL("unspecified pixel format");
        if (st->codecpar->codec_id == AV_CODEC_ID_RV30 || st->codecpar->codec_id == AV_CODEC_ID_RV40)
            if (!st->sample_aspect_ratio.num && !st->codecpar->sample_aspect_ratio.num && !st->codec_info_nb_frames)
                FAIL("no frame in rv30/40 and no sar");
        break;
    case AVMEDIA_TYPE_SUBTITLE:
        if (avctx->codec_id == AV_CODEC_ID_HDMV_PGS_SUBTITLE && !avctx->width)
            FAIL("unspecified size");
        break;
    case AVMEDIA_TYPE_DATA:
        if (avctx->codec_id == AV_CODEC_ID_NONE) return 1;
    }

    return 1;
}