Automatic GitHub action updates

.github/workflows/main.yml

@@ -5,7 +5,7 @@ on:
       - main
 
 jobs:
-  check:
+  build:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -14,3 +14,23 @@ jobs:
 
       - name: build
         run: nix-build -A ci
+
+  test-update:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/install-nix-action@v26
+
+      - name: test update script
+        run: |
+          nix-build -A autoPrUpdate
+          {
+            result/bin/auto-pr-update .
+            echo ""
+            echo '```diff'
+            git diff
+            echo '```'
+          } > $GITHUB_STEP_SUMMARY
+        env:
+          GH_TOKEN: ${{ github.token }}

.github/workflows/update.yml

@@ -18,6 +18,8 @@ jobs:
         run: |
           nix-build repo -A autoPrUpdate
           result/bin/auto-pr-update repo > body
+        env:
+          GH_TOKEN: ${{ github.token }}
 
       - name: Create Pull Request
         uses: peter-evans/create-pull-request@v6

default.nix

@@ -11,6 +11,7 @@ let
     config = {};
     overlays = [];
   };
+  inherit (pkgs) lib;
 
   runtimeExprPath = ./src/eval.nix;
   testNixpkgsPath = ./tests/mock-nixpkgs.nix;
@@ -32,8 +33,12 @@ let
 
 
   results = {
-    build = pkgs.callPackage ./package.nix {
-      inherit nixpkgsLibPath initNix runtimeExprPath testNixpkgsPath;
+    # We're using this value as the root result. By default, derivations expose all of their
+    # internal attributes, which is very messy. We prevent this using lib.lazyDerivation
+    build = lib.lazyDerivation {
+      derivation = pkgs.callPackage ./package.nix {
+        inherit nixpkgsLibPath initNix runtimeExprPath testNixpkgsPath;
+      };
     };
 
     shell = pkgs.mkShell {
@@ -48,31 +53,61 @@ let
     };
 
     # Run regularly by CI and turned into a PR
-    autoPrUpdate = pkgs.writeShellApplication {
-      name = "auto-pr-update";
-      runtimeInputs = with pkgs; [
-        npins
-        cargo
-      ];
-      text =
-        let
-          commands = {
-            "npins changes" = ''
-              npins update --directory "$REPO_ROOT/npins"'';
-            "cargo changes" = ''
-              cargo update --manifest-path "$REPO_ROOT/Cargo.toml"'';
+    autoPrUpdate =
+      let
+        updateScripts = {
+          npins = pkgs.writeShellApplication {
+            name = "update-npins";
+            runtimeInputs = with pkgs; [
+              npins
+            ];
+            text = ''
+              echo "<details><summary>npins changes</summary>"
+              # Needed because GitHub's rendering of the first body line breaks down otherwise
+              echo ""
+              echo '```'
+              npins update --directory "$1/npins" 2>&1
+              echo  '```'
+              echo "</details>"
+            '';
           };
-        in
-        ''
-          REPO_ROOT=$1
-          echo "Run automated updates"
-        ''
-        + pkgs.lib.concatStrings (pkgs.lib.mapAttrsToList (title: command: ''
-          echo -e '<details><summary>${title}</summary>\n\n```'
-          ${command} 2>&1
-          echo -e '```\n</details>'
-        '') commands);
-    };
+          cargo = pkgs.writeShellApplication {
+            name = "update-cargo";
+            runtimeInputs = with pkgs; [
+              cargo
+            ];
+            text = ''
+              echo "<details><summary>cargo changes</summary>"
+              # Needed because GitHub's rendering of the first body line breaks down otherwise
+              echo ""
+              echo '```'
+              cargo update --manifest-path "$1/Cargo.toml" 2>&1
+              echo  '```'
+              echo "</details>"
+            '';
+          };
+          githubActions = pkgs.writeShellApplication {
+            name = "update-github-actions";
+            runtimeInputs = with pkgs; [
+              dependabot-cli
+              jq
+              github-cli
+            ];
+            text = builtins.readFile ./scripts/update-github-actions.sh;
+          };
+        };
+      in
+      pkgs.writeShellApplication {
+        name = "auto-pr-update";
+        text = ''
+          # Prevent impurities
+          unset PATH
+          ${lib.concatMapStringsSep "\n" (script: ''
+            echo >&2 "Running ${script}"
+            ${lib.getExe script} "$1"
+          '') (lib.attrValues updateScripts)}
+        '';
+      };
 
     # Tests the tool on the pinned Nixpkgs tree, this is a good sanity check
     nixpkgsCheck = pkgs.runCommand "test-nixpkgs-check-by-name" {

scripts/update-github-actions.sh

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+
+# This script calls the dependabot CLI (https://github.com/dependabot/cli)
+# to determine updates to GitHub Action dependencies in the local repository.
+# It then also applies the updates and outputs the results to standard output.
+
+set -euo pipefail
+
+REPO_ROOT=$1
+
+echo -e "<details><summary>GitHub Action updates</summary>\n\n"
+
+# CI sets the GH_TOKEN env var, which `gh auth token` defaults to if set
+githubToken=$(gh auth token)
+
+# Each dependabot update call tries to update all dependencies,
+# but it outputs individual results for each dependency,
+# with the intention of creating a PR for each.
+#
+# We want to have all changes together though,
+# so we'd need to merge updates of the same files together,
+# which could cause merge conflicts, no good.
+#
+# Instead, we run dependabot repeatedly,
+# each time only taking the first dependency update and updating the files with it,
+# such that the next iteration takes into account the previous updates.
+# We do this until there's no more dependencies to be updated,
+# at which point --exit-status will make jq return with a non-zero exit code.
+#
+# This does mean that dependabot internally needs to perform O(n^2) updates,
+# but this isn't a problem in practice, since we run these updates regularly,
+# so n is low.
+while
+  # Unused argument would be the remote GitHub repo, which is not used if we pass --local
+  create_pull_request=$(LOCAL_GITHUB_ACCESS_TOKEN="$githubToken" \
+    dependabot update github_actions this-argument-is-unused --local "$REPO_ROOT" \
+    | jq --exit-status --compact-output --slurp 'map(select(.type == "create_pull_request")) | .[0].data')
+do
+  title=$(jq --exit-status --raw-output '."pr-title"' <<< "$create_pull_request")
+  echo "<details><summary>$title</summary>"
+
+  # Needed because GitHub's rendering of the first body line breaks down otherwise
+  echo ""
+
+  jq --exit-status --raw-output '."pr-body"' <<< "$create_pull_request"
+  echo '</details>'
+
+  jq --compact-output '."updated-dependency-files"[]' <<< "$create_pull_request" \
+    | while read -r fileUpdate; do
+      file=$(jq --exit-status --raw-output '.name' <<< "$fileUpdate")
+      # --join-output makes sure to not output a trailing newline
+      jq --exit-status --raw-output --join-output '.content' <<< "$fileUpdate" > "$REPO_ROOT/$file"
+    done
+done
+
+echo -e "</details>"