use realChannel to overlay

[GTrunSec]

fixes: #464, #469

[montchr]

This is awesome, thanks!

Though I am not sure at the moment what would happen if one of the packages in a function overlay is only supported on Darwin systems. Since the real channel arg uses the default NixOS system, would that result in an error for Darwin-only packages?

I don’t think there are any such packages in the examples, so while the checks pass now, I’d want to make sure we have a Darwin-only package to include in one of the examples before merging.

[montchr]

To my surprise, it seemed like darwin-specific packages were working in c55f079 and 24cb8eb, so I'm not sure that 327b569 is actually necessary at this point.

[GTrunSec]

so I'm not sure that 327b569 is actually necessary at this point.

but, user must set the nixos.hostDefaults = { channelName = "xxx", system ="x86_64-linux"}

[montchr]

To clarify my initial concern about cross-host-type packages, I had thought that the system would cause problems, not the channel itself. But upon inspection, it turns out that darwin-only packages like skhd and mas are still present under unsupported system attrs (on a NixOS machine fwiw):

nix-repl> flake.inputs.nixos-stable.legacyPackages.x86_64-linux.skhd.name
"skhd-0.3.5"

nix-repl> flake.inputs.nixos-stable.legacyPackages.x86_64-linux.mas.name  
"mas-1.8.6"

But, as expected, the derivations can't be evaluated if called. Which is fine as long as there's no IFD happening.

nix-repl> flake.inputs.nixos-stable.legacyPackages.x86_64-linux.mas     
error: Package ‘mas-1.8.6’ in /nix/store/p02y3rdbshbsrdl47vnrf10bq197yf9f-source/pkgs/os-specific/darwin/mas/default.nix:35 is not supported on ‘x86_64-linux’, refusing to evaluate.

       a) To temporarily allow packages that are unsupported for this system, you can use an environment variable
          for a single invocation of the nix tools.

            $ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1

        Note: For `nix shell`, `nix build`, `nix develop` or any other Nix 2.4+
        (Flake) command, `--impure` must be passed in order to read this
        environment variable.

       b) For `nixos-rebuild` you can set
         { nixpkgs.config.allowUnsupportedSystem = true; }
       in configuration.nix to override this.

       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
         { allowUnsupportedSystem = true; }
       to ~/.config/nixpkgs/config.nix.
«derivation 

[GTrunSec]

This pr only solves the problem of overlay detection.

[montchr]

@GTrunSec thanks again for this, it's a huge help 😄 -- i've updated my personal flake (uses emacs-overlay) and was able to rebuild successfully without the weird workarounds.