Don't use read-only mode for nix build --dry-run

[madjar]

In dry run mode, new derivations can't be create, so running the command on anything that has not been evaluated before results in an error message of the form don't know how to build these paths (may be caused by read-only store access).

For comparison, the classical nix-build --dry-run doesn't use read-only mode.

Closes #1795

[dtzWill]

cc 94eb5fa, which does not mention the impact as including this but (untested) looks like maybe it was motivated by this sort of issue? Regardless maybe this can be fixed while visiting that logic, if it hasn't already been addressed? 😇

(if there are known issues in the way, please share as maybe someone can fix them ^_^)

[madjar]

94eb5fa doesn't fix the issue, so I rebased on top of it and updated the patch.

I'm guessing this is ready for review.

[madjar]

@edolstra Would you be interested in merging that? It would enable a few workflows, including https://github.com/Mic92/nix-build-uncached for flakes.

[zimbatm]

Note that the current version works fine if the repo doesn't use IFD.

[madjar]

@zimbatm I'm not sure that's true. I have here a repo without IFD, and I still get the issue (because nix build --dry-run won't create the .drv files).

You can reproduce with nix build --dry-run nixpkgs#hello.

[stale]

I marked this as stale due to inactivity. → More info

[madjar]

Hey folks!

I just rebased this PR on top of master.

Reminder of what this PR does

So, right now, running nix build --dry-run on a derivation that is not already in the store will say "don't know how to build these paths (may be caused by read-only store access):", which is not super helpful.

This PR changes the realisation mode for --dry-run from Nothing to Derivation. As a result, the derivation will get instantiated.

I believe it's good to change the behaviour of --dry-run instead of introducing a new flag, because that is the behaviour of the classical nix-build command, and because I'm not sure that a command that only returns a usable result if some other command has been run before is a use case.

[thufschmitt]

I think you can remove the Realise::Nothing enum member altogether as it's never used anymore (It was used in only two places, #4541 removed one and you remove the other)

[anka-213]

Could we also introduce a new flag that never builds anything for those who want to preserve the current behaviour (if anyone does)?
Or even better, one that says what --dry-run would build, but that is definitely out of scope for this pull request.

[Ma27]

In case anybody needs a version compatible with the latest master: https://gist.github.com/Ma27/4337efe6be1e3858b0209ad1c9cbdcfe

(cc @edolstra would be awesome if this could be merged, though)

[edolstra]

@madjar @Ma27 Thanks, applied!