disallowedRequisites: Show forbidden chain(s) to forbidden path

[roberth]

Motivation

When a build fails because of disallowedRequisites, the error message now includes the chain of references that led to the failure. This makes it easier to see in which derivations the chain can be broken, to resolve the problem.

Example:

$ nix build --no-link /nix/store/1fsjsd98awcf3sgny9zfq4wa7i5dci4n-hercules-ci-agent-0.10.3.drv^out
error: output '/nix/store/xg01gga2qsi9v2qm45r2gqrkk46wvrkd-hercules-ci-agent-0.10.3' is not allowed to refer to the following paths.
       Shown below are chains that lead to the forbidden path(s). More may exist.
         { /nix/store/xg01gga2qsi9v2qm45r2gqrkk46wvrkd-hercules-ci-agent-0.10.3 -> /nix/store/mfzkjrwv8ckqqbsbj5yj24ri9dsgs30l-hercules-ci-cnix-expr-0.3.6.3 -> /nix/store/vxlw2igrncif77fh2qg1jg02bd455mbl-ghc-9.6.5 }

Context

• Very useful for e.g. Haskell binaries with excessive closure sizes on aarch64-darwin nixpkgs#318013

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[cole-h]

I wonder if a more readable output would be something similar to why-depends -- the tree-like output is a lot easier to read than a super-long chain of [store path] -> [store path] -> [store path] -> .....

[roberth]

If the chain is short enough, it will look better, and I expect the chains to be short.

I did consider it, but was non-trivial to achieve, because that code can't simply be reused, and when we use the tree syntax, we also need a proper tree data model in order to render it properly. So it's:

• gather all chains up into a tree
• refactor the tree rendering code to take a more abstract tree
  • or maybe only factor out the very surface level rendering bits, insofar that makes sense
• render it with that

I've already had to do a much deeper dive for this unplanned thing, which should have been quick. That's why I had already added the comment:

// Consider working with paths in a more stable order (https://github.com/NixOS/nix/issues/10875)
// and/or get all possible chains and present them in the `nix-store -q --tree` format.

I'd like to save that improvement for later and focus on more urgent things, but PRs are welcome.

[roberth]

Or do something very ad hoc, and not bother deduplicating common paths so that it forms a tree, but it's worth noting that it also requires more test setup to check a multi-line output, fwiw.

[nixos-discourse]

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

https://discourse.nixos.org/t/2024-07-17-nix-team-meeting-minutes-162/49255/1