command: UIOutput is functinal

command/ui_output.go

@@ -1,6 +1,8 @@
 package command
 
 import (
+	"sync"
+
 	"github.com/mitchellh/cli"
 	"github.com/mitchellh/colorstring"
 )
@@ -9,7 +11,18 @@ import (
 type UIOutput struct {
 	Colorize *colorstring.Colorize
 	Ui       cli.Ui
+
+	once sync.Once
+	ui        cli.Ui
 }
 
 func (u *UIOutput) Output(v string) {
+	u.once.Do(u.init)
+	u.ui.Output(v)
+}
+
+func (u *UIOutput) init() {
+	// Wrap the ui so that it is safe for concurrency regardless of the
+	// underlying reader/writer that is in place.
+	u.ui = &cli.ConcurrentUi{Ui: u.Ui}
 }