Add flake.nix

[SignalWalker]

Adds a flake.nix (and its attendant flake.lock), containing outputs for all of the packages and devshells output by default.nix and shell.nix, an overlay output, and an apps output (to make it possible to run emuhawk with just nix run github:TASEmulators/BizHawk).

Notes:

• The flake description is just copied straight from the github repo short description
• I set the nixpkgs input to match the default already present in default.nix
• I also set a formatter output (which tells nix fmt what formatter to use), which may or may not be desired
• The default package and app are emuhawk-latest-bin, and the default devshell is emuhawk-latest, because that made sense to me, but it could also make sense to have them all be emuhawk-latest
• The apps output may not be completely correct -- it works for the default, but I haven't checked that the generated program path is accurate for all packages
• nix flake show acts weird if you don't set --option allow-import-from-derivation true

Check if completed:

• I have run any relevant test suites
• I, the commit author, have read the licensing terms for contributors (last updated 2024-06-22) and am compliant

Resolved

[YoshiRulz]

Is https://github.com/numtide/flake-utils considered good practice or nah?

[YoshiRulz]

Wait does this work in Flakes?

[SignalWalker]

Oh, when evaluating as part of a flake, that ./. gets interpreted as a nix store path, so BIZHAWKBUILD_HOME is set to something like /nix/store/1igqk0g3q3bj6zhcfmrgzirzjsqbgpx9-source instead of something like /home/tasbot/bizhawk. That's fine for reading data, but the store is (usually) read-only, so $BIZHAWK_HOME doesn't work as-is (I assume).

I'm not really sure what the best way to solve this would be (especially in the case of someone doing something like nix develop github:tasemulators/bizhawk, since that would mean that the source dir only exists in the store).

nix develop sets environment variables for each of the derivation outputs (ex. $out), which might be useful. They're relative to the directory in which you invoke nix develop, though (ex. if you invoke nix develop ./.#emuhawk-latest in BizHawk/Dist, $out is set to BizHawk/Dist/outputs/out rather than BizHawk/outputs/out).

Do builds output to $BIZHAWKBUILD_HOME/output, or do they output to $BIZHAWK_HOME? If it's the latter, does anything expect $BIZHAWKBUILD_HOME to be writable? If things only expect to be able to write to $BIZHAWK_HOME, we could add an isFlake parameter to the shellHook function and export $BIZHAWK_HOME=$out, if we don't mind that $out isn't guaranteed to be relative to the project root.

[YoshiRulz]

IMO it's fine to assume nix develop will only be called from the repo root, already checked-out.

BIZHAWKBUILD_HOME is used at build-time to locate the repo root (not for the main solution though). BIZHAWK_HOME is used at run-time, and it was mainly added for NixHawk so I don't think anything expects it to be writable, BUT it can't point to ${BizHawk-source}/output since that doesn't exist.

[SignalWalker]

Some scripts seem to expect to be able to find (and write to) $BIZHAWKBUILD_HOME/output, ex.:

BizHawk/waterbox/waterboxhost/build-debug.sh

Line 8 in caeb811

cp target/debug/libwaterboxhost.so "$BIZHAWKBUILD_HOME/output/dll"

For $BIZHAWK_HOME, could we just mkdir -p it in the launch script, since that and PathExtensions.cs seem to be the only places it gets used?

[YoshiRulz]

You can do that, but again, it needs to exist and contain the assemblies and other assets. The launch script is essentially cd $BIZHAWK_HOME; mono ./EmuHawk.exe.

No-one is going to be using EmuHawk's dev shell to build WaterboxHost. It can get its own shell when I eventually package it.

[SignalWalker]

I mean, if the only thing we care about that uses it is the launch script, and we're assuming that nix develop is only being called from the repo root, then could we just do something like:

shellHook = isFlake: drv: ''
    export BIZHAWKBUILD_HOME="${builtins.toString ./.}"
    export BIZHAWK_HOME="${if isFlake then "$(pwd -P)" else "$BIZHAWKBUILD_HOME"}/output/"
    # ...
'';

As-is, if you're in a checked-out repo root, running Dist/BuildDebug.sh in nix develop seems to work completely fine and outputs to ${repo_root}/output (as in, not $BIZHAWKBUILD_HOME/output).

(This would mean, though, that the printf later on that tells the user where the build scripts are is misleading if you're in a flake, so I'll need to fix that.)

[SignalWalker]

Is https://github.com/numtide/flake-utils considered good practice or nah?

Honestly, I just don't use it because downloading it adds a bit of time to flake commands and because nix error messages are so obtuse that I'd rather have as little abstraction as possible. As far as I know, though, it's not considered bad practice; that's just my own preference.

[YoshiRulz]

Fair enough, I just don't like Flakes (as you've probably gathered) and would prefer to minimise the amount of code used for it.

[SignalWalker]

Fair. I could rewrite it to use flake-utils, if you'd prefer that.

[YoshiRulz]

Pinging @OmnipotentEntity who was working with Flakes at some point (#3564).