By default the option "flv_metadata" (internally using the field
name "trust_metadata") is set to 0, meaning that we don't allocate
streams based on information in the metadata, only based on
actual streams we encounter. However the "datastream" metadata field
still would allocate a subtitle stream.

When muxing, the "datastream" field is added if either a data stream
or subtitle stream is present - but the same metadata field is used
to preemtively create a subtitle stream only. Thus, if the field
was added due to a data stream, not a subtitle stream, the demuxer
would create a stream which won't get any actual packets.

If there was such an extra, empty subtitle stream, running
avformat_find_stream_info still used to terminate within reasonable
time before 3749eede. After that
commit, it no longer would terminate until it reaches the max
analyze duration, which is 90 seconds for flv streams (see
e6a08464,
24fdf733 and
f58e011a).

Before that commit (which removed the deprecated AVStream.codec), the
"st->codecpar->codec_id = AV_CODEC_ID_TEXT", set within the demuxer,
would get propagated into st->codec->codec_id by numerous
avcodec_parameters_to_context(st->codec, st->codecpar), then further
into st->internal->avctx->codec_id by update_stream_avctx within
read_frame_internal in libavformat/utils.c (demux.c these days).
Signed-off-by: Martin Storsjö <martin@martin.st>