ci: reusable Nix format Nixpkgs pin

This is needed such that in the next commit, we can re-use the same
version from a shell.nix, allowing people to have a guaranteed matching
nixfmt version.

.github/workflows/check-nix-format.yml

@@ -19,13 +19,18 @@ jobs:
         with:
           # pull_request_target checks out the base branch by default
           ref: refs/pull/${{ github.event.pull_request.number }}/merge
+      - name: Get Nixpkgs revision for nixfmt
+        run: |
+          # pin to a commit from nixpkgs-unstable to avoid e.g. building nixfmt
+          # from staging
+          # This should not be a URL, because it would allow PRs to run arbitrary code in CI!
+          rev=$(jq -r .rev ci/pinned-nixpkgs.json)
+          echo "url=https://github.com/NixOS/nixpkgs/archive/$rev.tar.gz" >> "$GITHUB_ENV"
       - uses: cachix/install-nix-action@8887e596b4ee1134dae06b98d573bd674693f47c # v26
         with:
           # explicitly enable sandbox
           extra_nix_config: sandbox = true
-          # fix a commit from nixpkgs-unstable to avoid e.g. building nixfmt
-          # from staging
-          nix_path: nixpkgs=https://github.com/NixOS/nixpkgs/archive/4b455dc2048f73a79eb3713f342369ff58f93e0b.tar.gz
+          nix_path: nixpkgs=${{ env.url }}
       - name: Install nixfmt
         run: "nix-env -f '<nixpkgs>' -iAP nixfmt-rfc-style"
       - name: Check that Nix files are formatted according to the RFC style

ci/README.md

@@ -2,3 +2,11 @@
 
 This directory contains files to support CI, such as [GitHub Actions](https://github.com/NixOS/nixpkgs/tree/master/.github/workflows) and [Ofborg](https://github.com/nixos/ofborg).
 This is in contrast with [`maintainers/scripts`](`../maintainers/scripts`) which is for human use instead.
+
+## Pinned Nixpkgs
+
+CI may need certain packages from Nixpkgs.
+In order to ensure that the needed packages are generally available without building,
+[`pinned-nixpkgs.json`](./pinned-nixpkgs.json) contains a pinned Nixpkgs version tested by Hydra.
+
+Run [`update-pinned-nixpkgs.sh`](./update-pinned-nixpkgs.sh) to update it.

ci/pinned-nixpkgs.json

@@ -0,0 +1,4 @@
+{
+  "rev": "cfb89a95f19bea461fc37228dc4d07b22fe617c2",
+  "sha256": "1yhsacvry6j8r02lk70p9dphjpi8lpzgq2qay8hiy4nqlys0mrch"
+}

ci/update-pinned-nixpkgs.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p jq
+
+set -euo pipefail
+
+# https://stackoverflow.com/a/246128
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+
+repo=https://github.com/nixos/nixpkgs
+branch=nixpkgs-unstable
+file=$SCRIPT_DIR/pinned-nixpkgs.json
+
+rev=$(git ls-remote "$repo" refs/heads/"$branch" | cut -f1)
+sha256=$(nix-prefetch-url --unpack "$repo/archive/$rev.tar.gz" --name source)
+
+jq -n --arg rev "$rev" --arg sha256 "$sha256" '$ARGS.named' | tee /dev/stderr > $file