client: avoid registering node twice right away

I noticed that `watchNodeUpdates()` almost immediately after
`registerAndHeartbeat()` calls `retryRegisterNode()`, well after 5
seconds.

This call is unnecessary and made debugging a bit harder.  So here, we
ensure that we only re-register node for new node events, not for
initial registration.

client/client.go

@@ -1468,13 +1468,7 @@ func (c *Client) watchNodeEvents() {
 	// batchEvents stores events that have yet to be published
 	var batchEvents []*structs.NodeEvent
 
-	// Create and drain the timer
-	timer := time.NewTimer(0)
-	timer.Stop()
-	select {
-	case <-timer.C:
-	default:
-	}
+	timer := stoppedTimer()
 	defer timer.Stop()
 
 	for {
@@ -1930,7 +1924,8 @@ func (c *Client) updateNodeLocked() {
 // it will update the client node copy and re-register the node.
 func (c *Client) watchNodeUpdates() {
 	var hasChanged bool
-	timer := time.NewTimer(c.retryIntv(nodeUpdateRetryIntv))
+
+	timer := stoppedTimer()
 	defer timer.Stop()
 
 	for {

client/util.go

@@ -3,6 +3,7 @@ package client
 import (
 	"fmt"
 	"math/rand"
+	"time"
 
 	"github.com/hashicorp/nomad/nomad/structs"
 )
@@ -63,3 +64,13 @@ func shuffleStrings(list []string) {
 		list[i], list[j] = list[j], list[i]
 	}
 }
+
+// stoppedTimer returns a timer that's stopped and wouldn't fire until
+// it's reset
+func stoppedTimer() *time.Timer {
+	timer := time.NewTimer(0)
+	if !timer.Stop() {
+		<-timer.C
+	}
+	return timer
+}