finish the leave process if broadcasting leave timeout (#640)

* finish the leave process if broadcasting leave timeout

* Log broadcast timeout as WARN and finish the Leave process.

* increase timeout

serf/coalesce_member_test.go

@@ -134,7 +134,7 @@ func TestMemberEventCoalesce_TagUpdate(t *testing.T) {
 		Members: []Member{Member{Name: "foo", Tags: map[string]string{"role": "foo"}}},
 	}
 
-	time.Sleep(10 * time.Millisecond)
+	time.Sleep(30 * time.Millisecond)
 
 	select {
 	case e := <-outCh:

serf/serf.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"encoding/base64"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io/ioutil"
 	"log"
@@ -685,6 +684,7 @@ func (s *Serf) broadcastJoin(ltime LamportTime) error {
 
 // Leave gracefully exits the cluster. It is safe to call this multiple
 // times.
+// If the Leave broadcast timeout, Leave() will try to finish the sequence as best effort.
 func (s *Serf) Leave() error {
 	// Check the current state
 	s.stateLock.Lock()
@@ -727,14 +727,14 @@ func (s *Serf) Leave() error {
 		select {
 		case <-notifyCh:
 		case <-time.After(s.config.BroadcastTimeout):
-			return errors.New("timeout while waiting for graceful leave")
+			s.logger.Printf("[WARN] serf: timeout while waiting for graceful leave")
 		}
 	}
 
 	// Attempt the memberlist leave
 	err := s.memberlist.Leave(s.config.BroadcastTimeout)
 	if err != nil {
-		return err
+		s.logger.Printf("[WARN] serf: timeout waiting for leave broadcast: %s", err.Error())
 	}
 
 	// Wait for the leave to propagate through the cluster. The broadcast