feat: add runner health admission checks (#1090)

Opt-in local task-admission checks under a `health_check` config section (disabled by default):

- pause new task fetching when free disk space on the workspace volume is below the configured minimum
- optional executable health-check script — a non-zero exit, timeout, or start failure marks the runner unavailable
- checks run only while the runner is idle; the last result is reused while a job is active, and polling resumes automatically on recovery
- `/readyz` reports task-admission readiness (reusing the poll loop's last check); `/healthz` stays a process-liveness endpoint

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1090
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/pkg/metrics/server.go

@@ -13,19 +13,12 @@ import (
 )
 
 // StartServer starts an HTTP server that serves Prometheus metrics on /metrics
-// and a liveness check on /healthz. The server shuts down when ctx is cancelled.
+// and health checks. The server shuts down when ctx is cancelled.
 // Call Init() before StartServer to register metrics with the Registry.
-func StartServer(ctx context.Context, addr string) {
-	mux := http.NewServeMux()
-	mux.Handle("/metrics", promhttp.HandlerFor(Registry, promhttp.HandlerOpts{}))
-	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
-		w.WriteHeader(http.StatusOK)
-		_, _ = w.Write([]byte("ok"))
-	})
-
+func StartServer(ctx context.Context, addr string, readiness func() (bool, string)) {
 	srv := &http.Server{
 		Addr:              addr,
-		Handler:           mux,
+		Handler:           NewHTTPHandler(readiness),
 		ReadHeaderTimeout: 5 * time.Second,
 		ReadTimeout:       10 * time.Second,
 		WriteTimeout:      10 * time.Second,
@@ -48,3 +41,25 @@ func StartServer(ctx context.Context, addr string) {
 		}
 	}()
 }
+
+// NewHTTPHandler returns the metrics and health endpoints used by StartServer.
+func NewHTTPHandler(readiness func() (bool, string)) http.Handler {
+	mux := http.NewServeMux()
+	mux.Handle("/metrics", promhttp.HandlerFor(Registry, promhttp.HandlerOpts{}))
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("ok"))
+	})
+	mux.HandleFunc("/readyz", func(w http.ResponseWriter, _ *http.Request) {
+		ready, reason := true, "ok"
+		if readiness != nil {
+			ready, reason = readiness()
+		}
+		if !ready {
+			w.WriteHeader(http.StatusServiceUnavailable)
+		}
+		_, _ = w.Write([]byte(reason))
+	})
+
+	return mux
+}