ci: re-enable cache push step (named-volume bind for relative path)

.gitea/workflows/validate-pr.yml

@@ -68,9 +68,7 @@ jobs:
             done
           done
 
-      # 4. Build smoke — every changed package must build. Cache push
+      # 4. Build smoke — every changed package must build.
-      #    is intentionally absent for now (no shared binary cache);
-      #    add a step here once cache infra is decided.
       - name: build smoke
         if: steps.changed.outputs.packages != ''
         run: |
@@ -79,6 +77,18 @@ jobs:
               .#${pkg} --no-link --print-out-paths
           done
 
+      # 4b. Push the validated outputs to the binary cache. The runner's
+      #     config.yaml bind-mounts /srv/cargoxx-cache and the signing
+      #     key into every job container.
+      - name: push to binary cache
+        if: steps.changed.outputs.packages != ''
+        run: |
+          for pkg in ${{ steps.changed.outputs.packages }}; do
+            nix copy --extra-experimental-features 'nix-command flakes' \
+              --to "file:///srv/cargoxx-cache/store?secret-key=/srv/cargoxx-cache/keys/cache.sec" \
+              .#${pkg}
+          done
+
       # 5. Maintainer check — PR must come from someone listed in
       #    recipes/<pkg>/maintainers.txt (auto-pass for new packages,
       #    since the PR introduces the file in the same commit).

runner/.gitignore

@@ -1,3 +1,8 @@
 .env
 data/
 result
+
+# Binary cache state + signing keys. The cache.sec must never be
+# committed; the public key is regenerated per deployment too
+# (`nix-store --generate-binary-cache-key`).
+cache/

runner/README.md

@@ -46,7 +46,39 @@ Self-hosted Gitea Actions runner that validates package PRs.
    GITEA_RUNNER_LABELS=self-hosted
    ```
 
-4. **Start the runner**:
+4. **Generate the binary-cache signing key** + cache directory. The
+   workflow's "push to binary cache" step writes here; nginx (or
+   anything you point at it) serves it back over HTTPS to consumers.
+
+   ```sh
+   mkdir -p cache/store
+   nix-store --generate-binary-cache-key \
+       cache.cargoxx.<your-domain> \
+       cache/cache.sec cache/cache.pub
+   chmod 600 cache/cache.sec
+   ```
+
+   The `cache/` directory is gitignored. Both keys live alongside
+   `compose.yml`; the named volume binds use `${PWD}/cache/...`.
+
+5. **(optional) Front the store with nginx** so substituters can read it:
+
+   ```nginx
+   # /etc/nginx/sites-available/cargoxx-cache
+   server {
+       listen 443 ssl;
+       server_name cache.cargoxx.<your-domain>;
+       root /path/to/cargoxx-pkgs/runner/cache/store;
+       autoindex off;
+       location / { try_files $uri =404; }
+   }
+   ```
+
+   Consumers later need `substituters = https://cache.cargoxx.<your-domain>`
+   and `trusted-public-keys = <contents of cache.pub>` in their nix
+   config (bake this into the cargoxx wrapper once ready).
+
+6. **Start the runner**:
 
    ```sh
    docker compose up -d

runner/compose.yml

@@ -18,3 +18,24 @@ services:
       - ./config.yaml:/config.yaml:ro
       - ./data:/data
       - /var/run/docker.sock:/var/run/docker.sock
+      # Binary cache — `validate-pr.yml`'s push step writes `$out` NAR
+      # archives here. Named volumes (defined below) make the same
+      # storage reachable from both this runner container AND every
+      # job container act_runner spawns. nginx (on the host) serves
+      # ./cache/store over HTTPS for consumers' substituter config.
+      - cargoxx-cache-store:/srv/cargoxx-cache/store
+      - cargoxx-cache-keys:/srv/cargoxx-cache/keys:ro
+
+volumes:
+  cargoxx-cache-store:
+    driver: local
+    driver_opts:
+      type: none
+      o: bind
+      device: "${PWD}/cache/store"
+  cargoxx-cache-keys:
+    driver: local
+    driver_opts:
+      type: none
+      o: bind
+      device: "${PWD}/cache/keys"

runner/config.yaml

@@ -18,7 +18,12 @@ cache:
 container:
   network: bridge
   privileged: false
-  options: ""
+  # Bind the binary cache into every job container by referencing the
+  # named volumes defined in compose.yml — those, in turn, are bound
+  # to ./cache/{store,cache.sec} via `${PWD}` so the path is
+  # deployment-relative, not absolute.
+  options: "-v cargoxx-cache-store:/srv/cargoxx-cache/store
+            -v cargoxx-cache-keys:/srv/cargoxx-cache/keys:ro"
   workdir_parent: /workspace
   valid_volumes: []
   docker_host: "unix:///var/run/docker.sock"