How to Make Sure Nixpkgs Can Evaluate

2020-06-02

When sending pull requests to Nixpkgs, sometimes the ofborg CI will display evaluation errors. This blog post explains how to reproduce these errors locally for debugging.

Example Error

Here is an example PR where ofborg reports an evaluation error.

Here is the evaluation error that ofborg is reporting:

nix-env failed:
warning: ignoring the user-specified setting 'restrict-eval', because it is a restricted setting and you are not a trusted user
error: while evaluating anonymous function at .gc-of-borg-outpaths.nix:44:12, called from undefined position:
while evaluating anonymous function at pkgs/top-level/release-lib.nix:121:6, called from lib/attrsets.nix:292:43:
while evaluating 'hydraJob' at lib/customisation.nix:171:14, called from pkgs/top-level/release-lib.nix:121:14:
while evaluating the attribute 'drvPath' at lib/customisation.nix:188:13:
while evaluating the attribute 'drvPath' at lib/customisation.nix:155:13:
while evaluating the attribute 'buildInputs' of the derivation 'blake2-0.3.0' at pkgs/development/haskell-modules/generic-builder.nix:289:3:
while evaluating the attribute 'propagatedBuildInputs' of the derivation 'hlint-3.1.1' at pkgs/development/haskell-modules/generic-builder.nix:289:3:
while evaluating 'getOutput' at lib/attrsets.nix:464:23, called from undefined position:
while evaluating anonymous function at pkgs/stdenv/generic/make-derivation.nix:157:17, called from undefined position:
attribute 'ghc-lib-parser_8_10_1_20200412' missing, at pkgs/development/haskell-modules/configuration-common.nix:1510:22

This exact error is not important1. In the next section I will explain how to replicate this CI check locally.

Replicating This Error Locally

In order to replicate this CI check in your local environment, first you'll have to checkout the commit in nixpkgs where the error is occurring.

The base of the above PR is f043b2c3, so we clone nixpkgs and checkout that commit:

$ git clone git@github.com:NixOS/nixpkgs.git
$ cd nixpkgs
$ git checkout f043b2c3

Now we can try evaluating nixpkgs in order to reproduce this error:

$ nix-env --query --available --out-path --file ./. --show-trace
error: while querying the derivation named 'hlint-3.1.1':
while evaluating the attribute 'buildInputs' of the derivation 'hlint-3.1.1' at pkgs/development/haskell-modules/generic-builder.nix:289:3:
while evaluating 'getOutput' at lib/attrsets.nix:464:23, called from undefined position:
while evaluating anonymous function at pkgs/stdenv/generic/make-derivation.nix:143:17, called from undefined position:
attribute 'ghc-lib-parser_8_10_1_20200412' missing, at pkgs/development/haskell-modules/configuration-common.nix:1510:22

This nix-env command is the take away from this blog post:

nix-env --query --available --out-path --file ./. --show-trace

I'm not sure why the --out-path flag is needed, but if you don't add it then you don't get this error.

Conclusion

The above nix-env command is a convenient way to make sure there are no evaluation errors in nixpkgs, or to figure out where an evaluation error is coming from.

I picked up this trick from @LnL7 in this PR.

Footnotes

  1. This error is saying that the ghc-lib-parser_8_10_1_20200412 derivation is used, but it is never defined anywhere.

    This is caused because of how Haskell packages are updated automatically in the haskell-updates branch. See this for more information.

    Note that this CI error is not directly related to the PR in question. The underlying haskell-updates branch has a problem, which is what the ofborg CI is saying.↩︎

tags: nixos