workflows/check-by-name: Move tool fetching into script

This part of the CI can also be reproduced locally

.github/workflows/check-by-name.yml

@@ -79,40 +79,8 @@ jobs:
 
           echo "headSha=$(git rev-parse HEAD^2)" >> "$GITHUB_ENV"
       - uses: cachix/install-nix-action@7ac1ec25491415c381d9b62f0657c7a028df52a7 # v24
-      - name: Determining channel to use for dependencies
+      - name: Fetching the tool
-        run: |
+        run: pkgs/test/nixpkgs-check-by-name/scripts/fetch-tool.sh "$GITHUB_BASE_REF" result
-          echo "Determining the preferred channel to use for PR base branch $GITHUB_BASE_REF"
-          if [[ "$GITHUB_BASE_REF" =~ ^(release|staging|staging-next)-([0-9][0-9]\.[0-9][0-9])$ ]]; then
-              # Use the release channel for all PRs to release-XX.YY, staging-XX.YY and staging-next-XX.YY
-              channel=nixos-${BASH_REMATCH[2]}
-              echo "PR is for a release branch, preferred channel is $channel"
-          else
-              # Use the nixos-unstable channel for all other PRs
-              channel=nixos-unstable
-              echo "PR is for a non-release branch, preferred channel is $channel"
-          fi
-          # Check that the channel exists. It doesn't exist for fresh release branches
-          if ! curl -fSs "https://channels.nixos.org/$channel"; then
-            # Fall back to nixos-unstable, makes sense for fresh release branches
-            echo "Preferred channel $channel could not be fetched, falling back to nixos-unstable"
-            channel=nixos-unstable
-          fi
-          echo "channel=$channel" >> "$GITHUB_ENV"
-      - name: Fetching latest version of channel
-        run: |
-          echo "Fetching latest version of channel $channel"
-          # This is probably the easiest way to get Nix to output the path to a downloaded channel!
-          nixpkgs=$(nix-instantiate --find-file nixpkgs -I nixpkgs=channel:"$channel")
-          # This file only exists in channels
-          rev=$(<"$nixpkgs"/.git-revision)
-          echo "Channel $channel is at revision $rev"
-          echo "nixpkgs=$nixpkgs" >> "$GITHUB_ENV"
-          echo "rev=$rev" >> "$GITHUB_ENV"
-      - name: Fetching pre-built nixpkgs-check-by-name from the channel
-        run: |
-          echo "Fetching pre-built nixpkgs-check-by-name from channel $channel at revision $rev"
-          # Passing --max-jobs 0 makes sure that we won't build anything
-          nix-build "$nixpkgs" -A tests.nixpkgs-check-by-name --max-jobs 0
       - name: Running nixpkgs-check-by-name
         run: |
           echo "Checking whether the check succeeds on the base branch $GITHUB_BASE_REF"

pkgs/test/nixpkgs-check-by-name/scripts/fetch-tool.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+# Fetches the prebuilt nixpkgs-check-by-name to use from
+# the NixOS channel corresponding to the given base branch
+
+set -euo pipefail
+
+if (( $# < 2 )); then
+    echo >&2 "Usage: $0 BASE_BRANCH OUTPUT_PATH"
+    echo >&2 "BASE_BRANCH: The base branch to use, e.g. master or release-23.11"
+    echo >&2 "OUTPUT_PATH: The output symlink path for the tool"
+    exit 1
+fi
+baseBranch=$1
+output=$2
+
+echo >&2 -n "Determining the channel to use for PR base branch $baseBranch.. "
+if [[ "$baseBranch" =~ ^(release|staging|staging-next)-([0-9][0-9]\.[0-9][0-9])$ ]]; then
+  # Use the release channel for all PRs to release-XX.YY, staging-XX.YY and staging-next-XX.YY
+  preferredChannel=nixos-${BASH_REMATCH[2]}
+else
+  # Use the nixos-unstable channel for all other PRs
+  preferredChannel=nixos-unstable
+fi
+
+# Check that the channel exists. It doesn't exist for fresh release branches
+if curl -fSs "https://channels.nixos.org/$preferredChannel"; then
+    channel=$preferredChannel
+    echo >&2 "$channel"
+else
+    # Fall back to nixos-unstable, makes sense for fresh release branches
+    channel=nixos-unstable
+    echo >&2 -e "\e[33mWarning: Preferred channel $preferredChannel could not be fetched, using fallback: $channel\e[0m"
+fi
+
+echo >&2 -n "Fetching latest version of channel $channel.. "
+# This is probably the easiest way to get Nix to output the path to a downloaded channel!
+nixpkgs=$(nix-instantiate --find-file nixpkgs -I nixpkgs=channel:"$channel")
+echo >&2 "$nixpkgs"
+
+# This file only exists in channels
+echo >&2 -e "Git revision of channel $channel is \e[34m$(<"$nixpkgs/.git-revision")\e[0m"
+
+echo >&2 -n "Fetching the prebuilt version of nixpkgs-check-by-name.. "
+nix-build -o "$output" "$nixpkgs" -A tests.nixpkgs-check-by-name -j 0 >/dev/null
+realpath >&2 "$output"