Let the route job run off GitHub-hosted minutes

.github/workflows/route.yml

@@ -42,6 +42,10 @@ on:
         description: 'GitHub App id. Default is the ROUTACT_APP_ID variable.'
         type: string
         required: false
+      runs-on:
+        description: 'Runner for the route job itself, as JSON (a label or a label array). Default is the ROUTACT_ROUTER_RUNS_ON variable, else "ubuntu-slim".'
+        type: string
+        required: false
     secrets:
       ROUTACT_APP_PRIVATE_KEY:
         required: false
@@ -66,9 +70,12 @@ on:
 
 jobs:
   route:
-    # The smallest, fastest-starting GitHub-hosted runner. Never self-hosted:
-    # the router must not depend on the pool it is checking.
-    runs-on: ubuntu-slim
+    # The smallest, fastest-starting GitHub-hosted runner by default. Set the
+    # runs-on input or the ROUTACT_ROUTER_RUNS_ON variable to route from a
+    # runner that does not use GitHub's minutes, such as Blacksmith. Never use
+    # a pool the router checks: it could not route away from that pool when it
+    # is full.
+    runs-on: ${{ fromJSON(inputs.runs-on || vars.ROUTACT_ROUTER_RUNS_ON || '"ubuntu-slim"') }}
     timeout-minutes: 3
     env:
       HAS_APP_KEY: ${{ secrets.ROUTACT_APP_PRIVATE_KEY != '' }}

README.md

@@ -161,6 +161,16 @@ The check reads the organization's [billing usage summary](https://docs.github.c
 - GitHub updates billing usage with a delay, so a few jobs can still start on paid minutes right after the included minutes run out.
 - The check counts the whole organization. Public repositories use GitHub-hosted runners for free, so do not route them through a pool that ends on a self-hosted runner.
 
+### Where the route job runs
+
+The reusable workflow runs the route job on GitHub's `ubuntu-slim`. When a private repository's GitHub-hosted minutes are used up and the spending limit is reached, GitHub does not start that job, so nothing can be routed. To route from a runner that does not use GitHub's minutes, set the org variable `ROUTACT_ROUTER_RUNS_ON` (or the `runs-on` input) to a JSON label or label array:
+
+```sh
+gh variable set ROUTACT_ROUTER_RUNS_ON --org your-org --body '"blacksmith-2vcpu-ubuntu-2404"'
+```
+
+Do not point it at a self-hosted pool that the router checks. When that pool is full, the route job would wait behind the jobs it is meant to route away from.
+
 ### Fork pull requests
 
 Fork PRs skip the primary and every fallback step labelled `self-hosted`. Other checks still run, so a fork PR can use Blacksmith when it is enabled.