I downloaded this movie with youtube-dl, zipped it, built mpv with nix build '(with import (builtins.fetchTarball { url = "https://github.com/bb010g/nixpkgs/archive/mpv-libarchive-locale.tar.gz"; }) {}; pkgs.mpv.override { archiveSupport = true; })' and successfully played the movie through the zip file. Does that mean this is working properly?

If it fails for you without this patch, then yes. (Although I suppose that just testing this doesn't mean it's working improperly, either. :) )

Building mpv using nix build '(with import <nixpkgs> {}; pkgs.mpv.override { archiveSupport = true; })' gives me an mpv that fails with "Failed to recognize file format."

Looks good?

Upstream currently relies on the system having a "C.UTF-8" locale for libarchive, which NixOS does not have.

Hmm, aren't there patches in staging and 19.03 staging that would fix this?
Pretty sure we have that now.

Yes, that's #57974 / #58009.

So yeah, didn't see that. (Based this off NixOS/nixpkgs-channels:nixos-unstable, and been testing on a system following nixos-18.09.)

I'm not sure if this is still worth merging, as locales look to be strange and non-obvious. I could see this patch possibly helping with Nix-built binaries ran on other platforms, but I'm not sure?

https://github.com/mpv-player/mpv/blob/d6d6da4711ca1ad20e14386c4b29a955eb32322d/stream/stream_libarchive.c#L245

 mpa->locale = newlocale(LC_ALL_MASK, "C.UTF-8", (locale_t)0);

This looks like a runtime dep on locale presence to me, which would still cause issues on other distributions. Would that use case be valuable enough to check this issue on updates until something happens upstream?

Closing due to a patch downstream (mpv-player/mpv#6683) solving this differently, and 19.03 being the main release.