add message to CI that shows what version of nixpkgs is being used to build

[cdepillabout]

I wanted to try to figure out what version (commit) of nixpkgs was used to build https://buildkite.com/nh2/static-haskell-nix/builds/203.

However, it appears that the version is just whatever is the latest version on the channel:

https://github.com/nh2/static-haskell-nix/blob/7699f2f7a465bfadadbfdd3e86ee8b1b0132ed76/nixpkgs.nix#L5

It would be great if this version was output in the buildkite log somehow, so that I could try to reproduce this build locally.

I'm not sure what would be the best way to output the commit. Maybe a builtins.trace message in the nixpkgs.nix file? That should show up in the buildkite log.

[nh2]

I'm not sure actually, I wondered how to get that info as well!

[nh2]

Equally relevant: https://buildkite.com/nh2/static-haskell-nix/builds/204

Where nixpkgs master broke the build, and it's not clear which master commit it is.

[nh2]

Where nixpkgs master broke the build

Correction: It was actually nixos-unstable that we use, and that build break is fixed by https://github.com/NixOS/nixpkgs/pull/76073.

However, master is also broken, due to https://github.com/NixOS/nixpkgs/issues/74904, with my fix https://github.com/NixOS/nixpkgs/issues/75676 still having open questions.

[cdepillabout]

@nh2 I thought about this a little more. Maybe a simple patch like this would suffice?

diff --git a/nixpkgs.nix b/nixpkgs.nix
index cfafb35..9f2a9fe 100644
--- a/nixpkgs.nix
+++ b/nixpkgs.nix
@@ -2,7 +2,12 @@
 # We use that for scheduled builds tracking nixpkgs unstable on CI.
 # Of course that is NOT reproducible.
 if builtins.getEnv "STATIC_HASKELL_NIX_CI_NIXPKGS_UNSTABLE_BUILD" == "1"
-  then import (fetchTarball https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz) {}
+  then
+    let
+      nixpkgs =
+        import (fetchTarball https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz) {};
+      msg = "Using version ${nixpkgs.lib.version} of nixpkgs-unstable channel.";
+    in builtins.trace msg nixpkgs
   else
     # If a `./nixpkgs` submodule exists, use that.
     # Note that this will take precedence over setting NIX_PATH!

[nh2]

I'll be on holiday until the 7th, can look at this again after that time!

[nh2]

PR: #77