Add post about evaluating nixpkgs.

2020-06-02-evaluating-nixpkgs.md

@@ -0,0 +1,95 @@
+------------------------------------------------------
+title: How to Make Sure Nixpkgs Can Evaluate
+summary: Explain a command to make sure nixpkgs is capable of evaluating
+tags: nixos
+draft: false
+------------------------------------------------------
+
+When sending pull requests to Nixpkgs, sometimes the
+[`ofborg`](https://github.com/NixOS/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](https://github.com/NixOS/nixpkgs/pull/88426) where
+`ofborg` reports an evaluation error.
+
+Here is the evaluation
+[error](https://gist.github.com/GrahamcOfBorg/804dff1aaa3f13aed3a6999c8043a42f)
+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 important[^1]. 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](https://github.com/NixOS/nixpkgs/pull/88426) is
+[f043b2c3](https://github.com/NixOS/nixpkgs/commit/f043b2c31bb89fd2024883e348deb994d98ab8a9),
+so we clone nixpkgs and checkout that commit:
+
+```console
+$ git clone [email protected]:NixOS/nixpkgs.git
+$ cd nixpkgs
+$ git checkout f043b2c3
+```
+
+Now we can try evaluating nixpkgs in order to reproduce this error:
+
+```console
+$ 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](https://github.com/LnL7) in
+[this PR](https://github.com/NixOS/nixpkgs/pull/89308).
+
+## 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](https://discourse.nixos.org/t/haskellpackages-stm-containers-fails-to-build/5416/4)
+    for more information.
+
+    Note that this CI error is not directly related to the
+    [PR](https://github.com/NixOS/nixpkgs/pull/88426) in question.  The
+    underlying `haskell-updates` branch has a problem, which is what the
+    `ofborg` CI is saying.