Make middleware survive a restart (#142)

Make middleware that sets up a (http) handler survive a graceful
restart. We calls the middleware's Shutdown function(s). If restart
fails the Start function is called again.

* middleware/health: OK
* middleware/pprof: OK
* middleware/metrics: OK

All restart OK.

middleware/health/health.go

@@ -3,6 +3,7 @@ package health
 import (
 	"io"
 	"log"
+	"net"
 	"net/http"
 	"sync"
 )
@@ -11,28 +12,45 @@ var once sync.Once
 
 type Health struct {
 	Addr string
+	ln   net.Listener
+	mux  *http.ServeMux
 }
 
 func health(w http.ResponseWriter, r *http.Request) {
 	io.WriteString(w, ok)
 }
 
-func (h Health) ListenAndServe() error {
+func (h *Health) Start() error {
 	if h.Addr == "" {
 		h.Addr = defAddr
 	}
+
 	once.Do(func() {
-		http.HandleFunc("/health", health)
+		if ln, err := net.Listen("tcp", h.Addr); err != nil {
+			log.Printf("[ERROR] Failed to start health handler: %s", err)
+			return
+		} else {
+			h.ln = ln
+		}
+		h.mux = http.NewServeMux()
+
+		h.mux.HandleFunc(path, health)
 		go func() {
-			if err := http.ListenAndServe(h.Addr, nil); err != nil {
-				log.Printf("[ERROR] Failed to start health handler: %s", err)
-			}
+			http.Serve(h.ln, h.mux)
 		}()
 	})
 	return nil
 }
 
+func (h *Health) Shutdown() error {
+	if h.ln != nil {
+		return h.ln.Close()
+	}
+	return nil
+}
+
 const (
 	ok      = "OK"
 	defAddr = ":8080"
+	path    = "/health"
 )