Support arbitrary fetching of files as Flake inputs

[MatthewCroughan]

Is your feature request related to a problem? Please describe.
I want to make a flake that fetches people's keys from Github. It looks like this, but I cannot because fetching arbitrary files is not supported via flake inputs:

{
  inputs.matthewcroughanKeys = {
    url = "https://github.com/matthewcroughan.keys";
    flake = false;
  };
  outputs = { self, matthewcroughanKeys }: {
    myKey = matthewcroughanKeys.outPath;
  };
}

Describe the solution you'd like
I want flake inputs to support fetching arbitrary files and locking their hash in the flake.lock

Describe alternatives you've considered
Using a sha256 rather than nix flake update to manually fetch keys, maintaining a long list of sha256s for each person. Or aggregating the hashes of many keys in a set of fetchurl calls.

Additional context
https://discourse.nixos.org/t/fetching-ssh-public-keys/12076

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/fetching-ssh-public-keys/12076/5

[tomberek]

Other use cases:

• fetch a patch and locking it automatically in a TOFU way via flake.lock (related: Support flake references to patches #3920)
• fetch a remote resource and purposefully error out if it changes

One can currently point to a local path using path:./somefile so being unable to do this for remote seems to be an accidental restriction. Ugly hack: https://github.com/tomberek/system-arg/blob/master/systemFile/flake.nix#L4 (my intention is not specifically about the system parameter, but it is an example)

[ShamrockLee]

The interface change might include an extra option

{
  inputs.my-file.unpack = false;
}

to keep the fetcher from unpacking the fetched source.

[tomberek]

This seems to be working: https://github.com/tomberek/nix/tree/fetcher_url It should behave the same as the tarball fetcher, but with opposite boolean logic:

In URL form, the schema must be http://, https:// or file:// URLs and the extension must NOT be .zip, .tar, .tar.gz, .tar.xz, .tar.bz2 or .tar.zst.

[Kranzes]

When fetching keys from github, won't the hash change each time a key is added/removed, since it doesn't have a commit/rev?

[tomberek]

When fetching keys from github, won't the hash change each time a key is added/removed, since it doesn't have a commit/rev?

That particular example will have that annoyance, so it may not be the best use-case or motivating example. But generally being able to fetch a file instead of only trees is potentially useful.

[Kranzes]

I agree, I am currently doing this, which I'd rather not do to just fetch the hosts file.

[nmattia]

Another use case (where the file won't change) is fecthing a single file from a big repo (for instance the git-completion.bash file from the git repo).

[tomberek]

Plan:

• wait for Copy local flakes to the store lazily #3121 , specifically the patches argument
• allow files in addition to trees (or support an unpack boolean) (re-implement fetchers: able to fetch a single remote file instead of a tree #6094 in a better way)
• allow referencing patches with local files as well as other inputs
• examine potential syntax for PRs, git patches, etc?