Nix: Remove the build hook mechanism

[edolstra]

Like the substituter mechanism, we can use the Store abstraction instead. I.e. we call remoteStore->buildPaths(...) directly rather than calling an external process that does the same thing.

[expipiplus1]

Would this just be an internal change to Nix? If not, how would this affect those of us who are using NIX_BUILD_HOOK with things other than build-remote.pl?

[edolstra]

It would break your build hook. What are you using NIX_BUILD_HOOK for?

[expipiplus1]

I'm using NIX_BUILD_HOOK with nix-instantiate and nix-store --realise in order to easily get machine readable output of the build in progress. By using a build hook with these tools I can have precise control over what's being built when, and the output of the builder.

It's also been useful in getting some better --dry-run functionality as described here too.

[veprbl]

@expipiplus1
Which entity executes NIX_BUILD_HOOK in your scenario? I'm asking because both nix-build and nix-store -r will try to lock the output path, so you can't have one just call another.

EDIT: seems like NIX_HELD_LOCKS is the mechanism for this, but it is ignored by nix-store -r.

[LisannaAtHome]

Would doing this prevent implementation of use cases like https://github.com/NixOS/nix/issues/1914?

[edolstra]

@ledettwy No, it shouldn't affect that.

[edolstra]

@volth You can force a local build by passing --builders ''.

[edolstra]

You're right, it requires a trusted user. We could require the untrusted user's builders to be a subset of the daemons builders though.

[arianvp]

I'm using build hooks to incrementally push things to the cache (as opposed to only after a full derivation built). Is there a better mechanism for doing this?

[arianvp]

Nevermind. I'm using post-build hooks. sorry