futures can react to shutdown (hashicorp#390)

Author: hanshasselberg

future.go

@@ -84,9 +84,10 @@ func (e errorFuture) Index() uint64 {
 // deferError can be embedded to allow a future
 // to provide an error in the future.
 type deferError struct {
-	err       error
-	errCh     chan error
-	responded bool
+	err        error
+	errCh      chan error
+	responded  bool
+	ShutdownCh chan struct{}
 }
 
 func (d *deferError) init() {
@@ -103,7 +104,11 @@ func (d *deferError) Error() error {
 	if d.errCh == nil {
 		panic("waiting for response on nil channel")
 	}
-	d.err = <-d.errCh
+	select {
+	case d.err = <-d.errCh:
+	case <-d.ShutdownCh:
+		d.err = ErrRaftShutdown
+	}
 	return d.err
 }
 

snapshot.go

@@ -146,6 +146,7 @@ func (r *Raft) takeSnapshot() (string, error) {
 	// We have to use the future here to safely get this information since
 	// it is owned by the main thread.
 	configReq := &configurationsFuture{}
+	configReq.ShutdownCh = r.shutdownCh
 	configReq.init()
 	select {
 	case r.configurationsCh <- configReq: