Lets see how well this works.

.forgejo/workflows/build.yaml

@@ -1,12 +1,18 @@
+# yaml-language-server: $schema=https://json.schemastore.org/github-workflow.json
 name: "Build"
 on:
   pull_request:
   push:
-    branches: ["main"]
+    branches:
-    paths: [".forgejo/workflows/build.yaml"]
+      - main
-  # schedule every night
+    paths:
-  schedule:
+      - ".forgejo/workflows/build.yaml"
-    - cron: "0 0 * * *"
+      - "flake.lock"
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   nix-build:
     if: github.event.pull_request.draft == false
@@ -20,6 +26,8 @@ jobs:
             os: native-x86_64
           - system: gandalf
             os: native-x86_64
+          - system: telperion
+            os: native-x86_64
     runs-on: ${{ matrix.os }}
     env:
       PATH: ${{ format('{0}:{1}', '/run/current-system/sw/bin', env.PATH) }}
@@ -28,31 +36,65 @@ jobs:
         uses: https://github.com/actions/checkout@v4
         with:
           fetch-depth: 0
-      - uses: https://github.com/cachix/cachix-action@v15
+      - name: Set up Cachix
+        uses: https://github.com/cachix/cachix-action@v15
         if: ${{ !github.event.pull_request.head.repo.fork }}
         with:
           name: hsndev
-          # If you chose API tokens for write access OR if you have a private cache
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-        # env:
-          # USER: 'root'
 
       - name: Garbage collect build dependencies
         run: nix-collect-garbage
 
+      - name: Build previous ${{ matrix.system }} system
+        shell: bash
+        run: |
+          nix build git+https://git.hsn.dev/jahanson/mochi#top.${{ matrix.system }} \
+            -v --log-format raw --profile ./profile
       - name: Build new ${{ matrix.system }} system
         shell: bash
         run: |
-          set -o pipefail
+          nix build ".#top.${{ matrix.system }}" --profile ./profile --fallback -v \
-          nix build \
+            > >(tee stdout.log) 2> >(tee /tmp/nix-build-err.log >&2)
-            ".#top.${{ matrix.system }}" \
+      - name: Check for build failure
-            --profile ./profile \
+        if: failure()
-            --fallback \
+        run: |
-            -v \
+          drv=$(grep "For full logs, run" /tmp/nix-build-err.log | grep -oE "/nix/store/.*.drv")
-            --log-format raw \
+          if [ -n $drv ]; then
-             > >(tee stdout.log) 2> >(tee /tmp/nix-build-err.log >&2)
+            nix log $drv
-      - name: Push to Cachix
+            echo $drv
-        if: success()
+          fi
-        env:
+          exit 1
-          CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
+      - name: Diff profile
-        run: nix build ".#top.${{ matrix.system }}" --json | jq -r .[].drvPath | cachix push hsndev
+        id: diff
+        run: |
+          nix profile diff-closures --profile ./profile
+          delimiter="$(openssl rand -hex 16)"
+          echo "diff<<${delimiter}" >> "${GITHUB_OUTPUT}"
+          nix profile diff-closures --profile ./profile | perl -pe 's/\e\[[0-9;]*m(?:\e\[K)?//g' >> "${GITHUB_OUTPUT}"
+          echo "${delimiter}" >> "${GITHUB_OUTPUT}"
+      - name: Comment report in pr
+        uses: https://github.com/marocchino/sticky-pull-request-comment@v2
+        with:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          header: ".#top.${{ matrix.system }}"
+          message: |
+            ### Report for `${{ matrix.system }}`
+
+            <summary> Version changes </summary> <br>
+            <pre> ${{ steps.diff.outputs.diff }} </pre>
+      # - name: Push to Cachix
+      #   if: success()
+      #   env:
+      #     CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
+      #   run: nix build ".#top.${{ matrix.system }}" --json | jq -r .[].drvPath | cachix push hsndev
+  nix-build-success:
+    if: ${{ always() }}
+    needs:
+      - nix-build
+    name: Nix Build Successful
+    runs-on: ubuntu-latest
+    steps:
+      - if: ${{ contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled') }}
+        name: Check matrix status
+        run: exit 1