Cleanup/deregister a mailbox if we fail to register (#172)

* Cleanup/deregister a mailbox if we fail to register

* Fix the defers and mutex locking in mailboxes

mailbox.go

@@ -121,40 +121,18 @@ func newMailbox(s *Server, name, nsName string, size int) (*Mailbox, error) {
 	s.mu.Unlock()
 
 	s.mumb.Lock()
-	defer s.mumb.Unlock()
-
 	_, ok := s.mailboxes[nsName]
 	if ok {
 		return nil, fmt.Errorf("%w: nsName=%s", ErrAlreadyRegistered, nsName)
 	}
+	s.mumb.Unlock()
 
-	timeout, cancel := context.WithTimeout(context.Background(), s.cfg.Timeout)
-	err := s.registry.Register(timeout, nsName)
-	cancel()
-	// Check if the error is a particular fatal error
-	// from etcd. Some errors have no recovery. See
-	// the list of all possible errors here:
-	//
-	// https://github.com/etcd-io/etcd/blob/master/etcdserver/api/v3rpc/rpctypes/error.go
-	//
-	// They are unfortunately not classidied into
-	// recoverable or non-recoverable.
-	if err != nil && strings.Contains(err.Error(), "etcdserver: requested lease not found") {
-		s.reportFatalError(err)
-		return nil, err
-	}
-	if err != nil {
-		return nil, err
-	}
-
-	boxC := make(chan Request, size)
 	cleanup := func() {
 		s.mumb.Lock()
-		defer s.mumb.Unlock()
-
 		// Immediately delete the subscription so that no one
 		// can send to it, at least from this host.
 		delete(s.mailboxes, nsName)
+		s.mumb.Unlock()
 
 		// Deregister the name.
 		var err error
@@ -165,13 +143,37 @@ func newMailbox(s *Server, name, nsName string, size int) (*Mailbox, error) {
 				cancel()
 				return err != nil
 			})
-		// Ingnore ErrNotOwner because most likely the previous owner panic'ed or exited badly. 
+		// Ingnore ErrNotOwner because most likely the previous owner panic'ed or exited badly.
 		// So we'll ignore the error and let the mailbox creator retry later.  We don't want to panic
-		// in that case because it will take down more mailboxes and make it worse. 
+		// in that case because it will take down more mailboxes and make it worse.
 		if err != nil && err != registry.ErrNotOwner {
 			panic(fmt.Errorf("%w: unable to deregister mailbox: %v, error: %v", errDeregisteredFailed, nsName, err))
 		}
 	}
+
+	timeout, cancel := context.WithTimeout(context.Background(), s.cfg.Timeout)
+	err := s.registry.Register(timeout, nsName)
+	cancel()
+	// Check if the error is a particular fatal error
+	// from etcd. Some errors have no recovery. See
+	// the list of all possible errors here:
+	//
+	// https://github.com/etcd-io/etcd/blob/master/etcdserver/api/v3rpc/rpctypes/error.go
+	//
+	// They are unfortunately not classidied into
+	// recoverable or non-recoverable.
+	if err != nil && strings.Contains(err.Error(), "etcdserver: requested lease not found") {
+		s.reportFatalError(err)
+		return nil, err
+	}
+	if err != nil {
+		cleanup()
+		return nil, err
+	}
+
+	s.mumb.Lock()
+	defer s.mumb.Unlock()
+	boxC := make(chan Request, size)
 	box := &Mailbox{
 		name:    name,
 		nsName:  nsName,