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"
 )

middleware/metrics/metrics.go

@@ -2,6 +2,7 @@ package metrics
 
 import (
 	"log"
+	"net"
 	"net/http"
 	"sync"
 
@@ -17,12 +18,12 @@ var (
 	responseRcode   *prometheus.CounterVec
 )
 
-const path = "/metrics"
-
 // Metrics holds the prometheus configuration. The metrics' path is fixed to be /metrics
 type Metrics struct {
 	Next      middleware.Handler
-	Addr      string // where to we listen
+	Addr      string
+	ln        net.Listener
+	mux       *http.ServeMux
 	Once      sync.Once
 	ZoneNames []string
 }
@@ -31,21 +32,35 @@ func (m *Metrics) Start() error {
 	m.Once.Do(func() {
 		define()
 
+		if ln, err := net.Listen("tcp", m.Addr); err != nil {
+			log.Printf("[ERROR] Failed to start metrics handler: %s", err)
+			return
+		} else {
+			m.ln = ln
+		}
+		m.mux = http.NewServeMux()
+
 		prometheus.MustRegister(requestCount)
 		prometheus.MustRegister(requestDuration)
 		prometheus.MustRegister(responseSize)
 		prometheus.MustRegister(responseRcode)
 
-		http.Handle(path, prometheus.Handler())
+		m.mux.Handle(path, prometheus.Handler())
+
 		go func() {
-			if err := http.ListenAndServe(m.Addr, nil); err != nil {
-				log.Printf("[ERROR] Failed to start prometheus handler: %s", err)
-			}
+			http.Serve(m.ln, m.mux)
 		}()
 	})
 	return nil
 }
 
+func (m *Metrics) Shutdown() error {
+	if m.ln != nil {
+		return m.ln.Close()
+	}
+	return nil
+}
+
 func define() {
 	requestCount = prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: middleware.Namespace,
@@ -80,7 +95,7 @@ func define() {
 
 const (
 	// Dropped indicates we dropped the query before any handling. It has no closing dot, so it can not be a valid zone.
-	Dropped = "dropped"
-
+	Dropped   = "dropped"
 	subsystem = "dns"
+	path      = "/metrics"
 )

middleware/pprof/README.md

@@ -1,7 +1,7 @@
 # pprof
 
 pprof publishes runtime profiling data at endpoints under /debug/pprof. You can visit /debug/pprof
-on your site for an index of the available endpoints. By default it will listen on localhost:8053.
+on your site for an index of the available endpoints. By default it will listen on localhost:6053.
 
 > This is a debugging tool. Certain requests (such as collecting execution traces) can be slow. If
 > you use pprof on a live site, consider restricting access or enabling it only temporarily.

middleware/pprof/pprof.go

@@ -2,8 +2,9 @@ package pprof
 
 import (
 	"log"
+	"net"
 	"net/http"
-	_ "net/http/pprof"
+	pp "net/http/pprof"
 
 	"github.com/miekg/coredns/middleware"
 
@@ -11,10 +12,10 @@ import (
 	"golang.org/x/net/context"
 )
 
-const addr = "localhost:8053"
-
 type Handler struct {
 	Next middleware.Handler
+	ln   net.Listener
+	mux  *http.ServeMux
 }
 
 // ServeDNS passes all other requests up the chain.
@@ -23,10 +24,34 @@ func (h *Handler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 }
 
 func (h *Handler) Start() error {
+	if ln, err := net.Listen("tcp", addr); err != nil {
+		log.Printf("[ERROR] Failed to start pprof handler: %s", err)
+		return err
+	} else {
+		h.ln = ln
+	}
+
+	h.mux = http.NewServeMux()
+	h.mux.HandleFunc(path+"/", pp.Index)
+	h.mux.HandleFunc(path+"/cmdline", pp.Cmdline)
+	h.mux.HandleFunc(path+"/profile", pp.Profile)
+	h.mux.HandleFunc(path+"/symbol", pp.Symbol)
+	h.mux.HandleFunc(path+"/trace", pp.Trace)
+
 	go func() {
-		if err := http.ListenAndServe(addr, nil); err != nil {
-			log.Printf("[ERROR] Failed to start pprof handler: %s", err)
-		}
+		http.Serve(h.ln, h.mux)
 	}()
 	return nil
 }
+
+func (h *Handler) Shutdown() error {
+	if h.ln != nil {
+		return h.ln.Close()
+	}
+	return nil
+}
+
+const (
+	addr = "localhost:6053"
+	path = "/debug/pprof"
+)