Fix reloading in plugin/pprof. (#3454)

* Fix reloading in plugin/pprof.

Reloading the server without changing the listen address results in an
error because Startup is called for newly set up plugins before Shutdown
is called for the old ones.
Signed-off-by: Gonzalo Paniagua Javier <gonzalo.mono@gmail.com>

* Use pkg/reuseport when listening.

Use coredns' newly added reuseport.
Signed-off-by: Gonzalo Paniagua Javier <gonzalo.mono@gmail.com>

* Revert go.{mod,sum} changes.
Signed-off-by: Gonzalo Paniagua Javier <gonzalo.mono@gmail.com>

plugin/pprof/pprof.go

@@ -7,6 +7,8 @@ import (
 	"net/http"
 	pp "net/http/pprof"
 	"runtime"
+
+	"github.com/coredns/coredns/plugin/pkg/reuseport"
 )
 
 type handler struct {
@@ -17,7 +19,10 @@ type handler struct {
 }
 
 func (h *handler) Startup() error {
-	ln, err := net.Listen("tcp", h.addr)
+	// Reloading the plugin without changing the listening address results
+	// in an error unless we reuse the port because Startup is called for
+	// new handlers before Shutdown is called for the old ones.
+	ln, err := reuseport.Listen("tcp", h.addr)
 	if err != nil {
 		log.Errorf("Failed to start pprof handler: %s", err)
 		return err

plugin/pprof/setup.go

@@ -3,7 +3,6 @@ package pprof
 import (
 	"net"
 	"strconv"
-	"sync"
 
 	"github.com/coredns/coredns/plugin"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
@@ -62,12 +61,7 @@ func setup(c *caddy.Controller) error {
 
 	}
 
-	pprofOnce.Do(func() {
-		c.OnStartup(h.Startup)
-		c.OnShutdown(h.Shutdown)
-	})
-
+	c.OnStartup(h.Startup)
+	c.OnShutdown(h.Shutdown)
 	return nil
 }
-
-var pprofOnce sync.Once