m3u: introduce EXT-MPV to allow specifying per-file parameters

[vvuk]

m3u files are handy, but it would be nice to be able to specify per-file parameters. This PR introduces a #EXT-MPV:param=value directive that applies to the next file in the playlist. The existing per-file params mechanism is used.

Sample usage:

#EXT-MPV:image-display-duration=5
image.jpg
#EXT-MPV:start=120
#EXT-MPV:length=20
foo.mp4
#EXT-MPV:image-display-duration=60
image2.jpg

Much of this can already be done via .edl files, but edl files (seem to?) require edl to parse all of the contents, which can be prohibitive in case of lots of entries.

[kasper93]

If you introduce custom #EXT-MPV it has to be documented

[vvuk]

If you introduce custom #EXT-MPV it has to be documented

Yep, happy to! I should have mentioned -- would like to get some feedback first and see if this approach/format is fine, and then I'll add some docs (+ changelog etc, whatever's required)

[kasper93]

It should be #EXT-X-MPV, X denotes that extension is not standard

[sfan5]

There is a problem:
Arbitrary mpv options cannot be safely set from untrusted data (try ctrl+f on the manual for "untrusted", "insecure" or "unsafe")
Do I have a better suggestion? No, but whatever use case you have is probably better left up to a script than added to mpv itself.

[kasper93]

would like to get some feedback first and see if this approach/format is fine

There are two issues from this approach

1. hooking into m3u will possibly pollute playlist in the wild with pointless mpv extension
2. executing any option from the playlist is security issue

So either we define some subset of safe options to be able to specify like #EXT-MPV-LENGTH:20 and possibly few others. It is question to you what would be useful. But arbitrary option parsing is imho no-go in main player.

EDIT:

I see sfan5 had similar comment. The idea of external script is probably better, this way it would be opt-in by users that are aware of this functionality.

[vvuk]

Ah, hmm. I wasn't thinking of security issues since I was just thinking of local usage. Perhaps adding a command line option to allow m3u options, so it's opt-in? For m3u extensions though.. VLC and others already do it, so I think that ship has sailed. But requiring a command line option to enable it should resolve this too, since there just won't be any point to putting the extension flags in the wild.

Doing things via a script is interesting though; I keep forgetting that mpv is scriptable. I'll look into that if this doesn't seem workable.

[kasper93]

I don't oppose that such future may be useful, but it needs to be thought through. That's why I asked for docs initially, because it is easier to discuss if we have full specification of a idea.

As for the option to enable parsing of this extension, it could be a list of allowed properties/options to be set from inside m3u. Which by default, could be empty or have few safe options. Extending to more would be opt-in on user discretion.