plugin/health: make reload work (#1585)

* plugin/health: make reload work

Remove the once.Do from the startup, so we can re-bind the HTTP
listener. Also clarify the usage of health in multiple server blocks
(this is not the best approach - but there isn't a generic solution at
this point).

Manual tested as we lack testing infra, i.e kill -SIGUSR1 and some
CURLing of the health endpoint.

* Readme test fix

* update

* dont need this

plugin/health/health.go

@@ -10,8 +10,6 @@ import (
 	"time"
 )
 
-var once sync.Once
-
 // Health implements healthchecks by polling plugins.
 type health struct {
 	Addr     string
@@ -39,33 +37,26 @@ func (h *health) OnStartup() error {
 		h.Addr = defAddr
 	}
 
-	once.Do(func() {
-		ln, err := net.Listen("tcp", h.Addr)
-		if err != nil {
-			log.Printf("[ERROR] Failed to start health handler: %s", err)
+	ln, err := net.Listen("tcp", h.Addr)
+	if err != nil {
+		return err
+	}
+
+	h.ln = ln
+	h.mux = http.NewServeMux()
+
+	h.mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
+		if h.Ok() {
+			w.WriteHeader(http.StatusOK)
+			io.WriteString(w, ok)
 			return
 		}
-
-		h.ln = ln
-
-		h.mux = http.NewServeMux()
-
-		h.mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
-			if h.Ok() {
-				w.WriteHeader(http.StatusOK)
-				io.WriteString(w, ok)
-				return
-			}
-			w.WriteHeader(http.StatusServiceUnavailable)
-		})
-
-		go func() {
-			http.Serve(h.ln, h.mux)
-		}()
-		go func() {
-			h.overloaded()
-		}()
+		w.WriteHeader(http.StatusServiceUnavailable)
 	})
+
+	go func() { http.Serve(h.ln, h.mux) }()
+	go func() { h.overloaded() }()
+
 	return nil
 }