feat(ci): do not build if already cached

Author: jfroche

.github/workflows/nix-build.yml

@@ -14,10 +14,6 @@ permissions:
   contents: write
   packages: write
 
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
-  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
-
 jobs:
   nix-matrix:
     runs-on:
@@ -33,15 +29,12 @@ jobs:
         name: Generate Nix Matrix
         run: |
           set -Eeu
-          echo matrix="$(nix eval --json '.#githubActions.matrix')" >> "$GITHUB_OUTPUT"
+          echo matrix="$(python scripts/github-matrix.py)" >> "$GITHUB_OUTPUT"
 
   build-run-image:
     name: ${{ matrix.name }} (${{ matrix.system }})
     needs: nix-matrix
-    runs-on:
-      group: ${{ contains(matrix.os, 'blacksmith-32vcpu-ubuntu-2404') && '' || 'self-hosted-runners-nix' }}
-      labels:
-        - ${{ matrix.os }}
+    runs-on: ${{ matrix.runs_on.group && matrix.runs_on || matrix.runs_on.labels }}
     strategy:
       fail-fast: false
       matrix: ${{fromJSON(needs.nix-matrix.outputs.matrix)}}
@@ -73,8 +66,14 @@ jobs:
           aws_secret_access_key = ${AWS_SECRET_ACCESS_KEY}
           aws_session_token = ${AWS_SESSION_TOKEN}
           EOF
-      - name: nix-fast-build
-        run: nix build -L
+      - name: nix build
+        run: |
+           if ${{ matrix.already_cached }}; then
+             echo "${{ matrix.attr }} already cached, skipping build"
+             exit 0
+           fi
+           nix build -L .#${{ matrix.attr }}
+
   run-tests:
     needs: build-run-image
     if: ${{ success() }}

scripts/github-matrix.py

@@ -0,0 +1,150 @@
+#!/usr/bin/env python3
+
+import json
+import os
+import subprocess
+import sys
+from typing import (
+    Any,
+    Dict,
+    Generator,
+    List,
+    Literal,
+    NotRequired,
+    Optional,
+    Set,
+    TypedDict,
+)
+
+
+class NixEvalJobsOutput(TypedDict):
+    """Raw output from nix-eval-jobs command."""
+
+    attr: str
+    attrPath: List[str]
+    cacheStatus: Literal["notBuilt", "cached", "local"]
+    drvPath: str
+    isCached: bool
+    name: str
+    system: str
+    neededBuilds: NotRequired[List[Any]]
+    neededSubstitutes: NotRequired[List[Any]]
+    outputs: NotRequired[Dict[str, str]]
+
+
+class RunsOnConfig(TypedDict):
+    """GitHub Actions runs-on configuration."""
+
+    group: NotRequired[str]
+    labels: List[str]
+
+
+class GitHubActionPackage(TypedDict):
+    """Processed package for GitHub Actions matrix."""
+
+    attr: str
+    name: str
+    system: str
+    already_cached: bool
+    runs_on: RunsOnConfig
+
+
+BUILD_RUNNER_MAP: Dict[str, RunsOnConfig] = {
+    "aarch64-linux": {
+        "group": "self-hosted-runners-nix",
+        "labels": ["aarch64-linux"],
+    },
+    "aarch64-darwin": {
+        "group": "self-hosted-runners-nix",
+        "labels": ["aarch64-darwin"],
+    },
+    "x86_64-linux": {
+        "labels": ["blacksmith-32vcpu-ubuntu-2404"],
+    },
+}
+
+
+def get_worker_count() -> int:
+    """Get optimal worker count based on CPU cores."""
+    try:
+        return max(1, int(os.cpu_count()))
+    except (OSError, AttributeError):
+        print(
+            "Warning: Unable to get CPU count, using default max_workers=1",
+            file=sys.stderr,
+        )
+        return 1
+
+
+def build_nix_eval_command(max_workers: int) -> List[str]:
+    """Build the nix-eval-jobs command with appropriate flags."""
+    return [
+        "nix-eval-jobs",
+        "--flake",
+        ".#checks",
+        "--check-cache-status",
+        "--force-recurse",
+        "--quiet",
+        "--workers",
+        str(max_workers),
+    ]
+
+
+def parse_nix_eval_line(
+    line: str, drv_paths: Set[str]
+) -> Optional[GitHubActionPackage]:
+    """Parse a single line of nix-eval-jobs output"""
+    if not line.strip():
+        return None
+
+    try:
+        data: NixEvalJobsOutput = json.loads(line)
+        if data["drvPath"] in drv_paths:
+            return None
+        drv_paths.add(data["drvPath"])
+
+        runs_on_config = BUILD_RUNNER_MAP[data["system"]]
+
+        return {
+            "attr": "checks." + data["attr"],
+            "name": data["name"],
+            "system": data["system"],
+            "already_cached": data.get("cacheStatus") != "notBuilt",
+            "runs_on": runs_on_config,
+        }
+    except json.JSONDecodeError:
+        print(f"Skipping invalid JSON line: {line}", file=sys.stderr)
+        return None
+
+
+def run_nix_eval_jobs(cmd: List[str]) -> Generator[GitHubActionPackage, None, None]:
+    """Run nix-eval-jobs and yield parsed package data."""
+    print(f"Running command: {' '.join(cmd)}", file=sys.stderr)
+
+    with subprocess.Popen(
+        cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True
+    ) as process:
+        drv_paths = set()
+
+        for line in process.stdout:
+            package = parse_nix_eval_line(line, drv_paths)
+            if package:
+                yield package
+
+        if process.returncode and process.returncode != 0:
+            print("Error: Evaluation failed", file=sys.stderr)
+            sys.stderr.write(process.stderr.read())
+            sys.exit(process.returncode)
+
+
+def main() -> None:
+    max_workers = get_worker_count()
+    cmd = build_nix_eval_command(max_workers)
+
+    gh_action_packages = list(run_nix_eval_jobs(cmd))
+    gh_output = {"include": gh_action_packages}
+    print(json.dumps(gh_output))
+
+
+if __name__ == "__main__":
+    main()