properly unlock mutex

Author: masci

Diff for: arduino/discovery/discovery.go

@@ -90,7 +90,11 @@ func (d *Discovery) Start() error {
 
 // List retrieve the port list from this discovery
 func (d *Discovery) List() ([]*BoardPort, error) {
+	// ensure the connection to the discoverer is unique to avoid messing up
+	// the messages exchanged
 	d.Lock()
+	defer d.Unlock()
+
 	if _, err := d.in.Write([]byte("LIST\n")); err != nil {
 		return nil, fmt.Errorf("sending LIST command to discovery: %s", err)
 	}
@@ -112,7 +116,6 @@ func (d *Discovery) List() ([]*BoardPort, error) {
 		return nil, fmt.Errorf("decoding LIST command: %s", err)
 	}
 	done <- true
-	d.Unlock()
 	return event.Ports, nil
 }