Parallel GH actions workflow for Nixpkgs eval

Motivated by ofborg struggling [1] and its evaluations taking too long,
inspired by Jörg's initial PR [2]
and Adam's previous attempt to parallelise Nixpkgs evaluation [3],
this PR contains initial work to relief ofborg from its evaluation duty
by using GitHub Actions to evaluate Nixpkgs.

For now this doesn't take care of all of what ofborg does, such as
requesting appropriate reviewers or labeling mass rebuilds, but this can
be follow-up work.

[1]: https://discourse.nixos.org/t/infrastructure-announcement-the-future-of-ofborg-your-help-needed/56025?u=infinisil
[2]: NixOS#352808
[3]: NixOS#269403

Co-Authored-By: Jörg Thalheim <joerg@thalheim.io>
Co-Authored-By: Adam Joseph <adam@westernsemico.com>

.github/workflows/eval.yml

@@ -0,0 +1,139 @@
+name: Eval
+
+on: pull_request_target
+
+permissions:
+  contents: read
+
+jobs:
+  attrs:
+    name: Attributes
+    runs-on: ubuntu-latest
+    outputs:
+      mergedSha: ${{ steps.merged.outputs.mergedSha }}
+      systems: ${{ steps.systems.outputs.systems }}
+    steps:
+      # Important: Because of `pull_request_target`, this doesn't check out the PR,
+      # but rather the base branch of the PR, which is needed so we don't run untrusted code
+      - name: Check out the ci directory of the base branch
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          path: base
+          sparse-checkout: ci
+      - name: Check if the PR can be merged and get the test merge commit
+        id: merged
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          if mergedSha=$(base/ci/get-merge-commit.sh ${{ github.repository }} ${{ github.event.number }}); then
+            echo "Checking the merge commit $mergedSha"
+            echo "mergedSha=$mergedSha" >> "$GITHUB_OUTPUT"
+          else
+            # Skipping so that no notifications are sent
+            echo "Skipping the rest..."
+          fi
+          rm -rf base
+      - name: Check out the PR at the test merge commit
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        # Add this to _all_ subsequent steps to skip them
+        if: steps.merged.outputs.mergedSha
+        with:
+          ref: ${{ env.mergedSha }}
+          path: nixpkgs
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+        if: steps.merged.outputs.mergedSha
+
+      - name: Evaluate the list of all attributes and get the systems matrix
+        id: systems
+        if: steps.merged.outputs.mergedSha
+        run: |
+          nix-build nixpkgs/ci -A eval.attrpathsSuperset
+          echo "systems=$(<result/systems.json)" >> "$GITHUB_OUTPUT"
+
+      - name: Upload the list of all attributes
+        uses: actions/upload-artifact@b4b15b8c7c6ac21ea08fcf65892d2ee8f75cf882 # v4.4.3
+        if: steps.merged.outputs.mergedSha
+        with:
+          name: paths
+          path: result/*
+
+  outpaths:
+    name: Outpaths
+    runs-on: ubuntu-latest
+    needs: attrs
+    # Skip this and future steps if the PR can't be merged
+    if: needs.attrs.outputs.mergedSha
+    strategy:
+      matrix:
+        system: ${{ fromJSON(needs.attrs.outputs.systems) }}
+    steps:
+      - name: Download the list of all attributes
+        uses: actions/download-artifact@fa0a91b85d4f404e444e00e005971372dc801d16 # v4.1.8
+        with:
+          name: paths
+          path: paths
+
+      - name: Check out the PR at the test merge commit
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ needs.attrs.outputs.mergedSha }}
+          path: nixpkgs
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+
+      - name: Evaluate the ${{ matrix.system }} output paths for all derivation attributes
+        run: |
+          nix-build nixpkgs/ci -A eval.singleSystem \
+            --argstr evalSystem ${{ matrix.system }} \
+            --arg attrpathFile ./paths/paths.json \
+            --arg chunkSize 10000
+          # If it uses too much memory, slightly decrease chunkSize
+
+      - name: Upload the output paths and eval stats
+        uses: actions/upload-artifact@b4b15b8c7c6ac21ea08fcf65892d2ee8f75cf882 # v4.4.3
+        if: needs.attrs.outputs.mergedSha
+        with:
+          name: intermediate-${{ matrix.system }}
+          path: result/*
+
+  process:
+    name: Process
+    runs-on: ubuntu-latest
+    needs: outpaths
+    steps:
+      - name: Download output paths and eval stats for all systems
+        uses: actions/download-artifact@fa0a91b85d4f404e444e00e005971372dc801d16 # v4.1.8
+        with:
+          pattern: intermediate-*
+          path: intermediate
+
+      - name: Check out the PR at the test merge commit
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ needs.attrs.outputs.mergedSha }}
+          path: nixpkgs
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+
+      - name: Combine all output paths and eval stats
+        run: |
+          nix-build nixpkgs/ci -A eval.combine \
+            --arg resultsDir ./intermediate
+
+      - name: Upload the combined results
+        uses: actions/upload-artifact@b4b15b8c7c6ac21ea08fcf65892d2ee8f75cf882 # v4.4.3
+        with:
+          name: result
+          path: result/*
+
+
+      # TODO: Run this workflow also on `push` (on at least the main development branches)
+      # Then add an extra step here that waits for the base branch (not the merge base, because that could be very different)
+      # to have completed the eval, then use
+      # gh api --method GET /repos/NixOS/nixpkgs/actions/workflows/eval.yml/runs -f head_sha=<BASE>
+      # and follow it to the artifact results, where you can then download the outpaths.json from the base branch
+      # That can then be used to compare the number of changed paths, get evaluation stats and ping appropriate reviewers

ci/default.nix

@@ -26,4 +26,5 @@ in
   inherit pkgs;
   requestReviews = pkgs.callPackage ./request-reviews { };
   codeownersValidator = pkgs.callPackage ./codeowners-validator { };
+  eval = pkgs.callPackage ./eval { };
 }

ci/eval/README.md

@@ -0,0 +1,19 @@
+# Nixpkgs CI evaluation
+
+The code in this directory is used by the [eval.yml](../../.github/workflows/eval.yml) GitHub Actions workflow to evaluate the majority of Nixpkgs for all PRs, effectively making sure that when the development branches are processed by Hydra, no evaluation failures are encountered.
+
+Furthermore it also allows local evaluation using
+```
+nix-build ci -A eval.full \
+  --max-jobs 4
+  --cores 2
+  --arg chunkSize 10000
+```
+
+- `--max-jobs`: The maximum number of derivations to run at the same time. Only each [supported system](../supportedSystems.nix) gets a separate derivation, so it doesn't make sense to set this higher than that number.
+- `--cores`: The number of cores to use for each job. Recommended to set this to the amount of cores on your system divided by `--max-jobs`.
+- `chunkSize`: The number of attributes that are evaluated simultaneously on a single core. Lowering this decreases memory usage at the cost of increased evaluation time. If this is too high, there won't be enough chunks to process them in parallel, and will also increase evaluation time.
+
+A good default is to set `chunkSize` to 10000, which leads to about 3.6GB max memory usage per core, so suitable for fully utilising machines with 4 cores and 16GB memory, 8 cores and 32GB memory or 16 cores and 64GB memory.
+
+Note that 16GB memory is the recommended minimum, while with less than 8GB memory evaluation time suffers greatly.