Make terminal UI shutdown idempotent

fix.txt

@@ -231,7 +231,7 @@ Priority 3: configuration, UI, and binding hardening
     never closes the FIFO descriptor. Exit the reader on terminal errors,
     close the descriptor, and make shutdown cancellable.
 
-30. Empty UI tree can panic; UI startup errors are swallowed
+[x] 30. Empty UI tree can panic; UI startup errors are swallowed
     Files: uiterm/tree.go, uiterm/ui.go
     Tree.uiKeyEvent indexes lines[activeLine] for a non-arrow key even when
     no lines exist. Guard empty trees. Ui.Run returns nil when termbox.Init

uiterm/ui.go

@@ -3,6 +3,7 @@ package uiterm
 import (
 	"errors"
 	"strings"
+	"sync"
 	"sync/atomic"
 
 	"github.com/nsf/termbox-go"
@@ -20,8 +21,9 @@ type UiManager interface {
 type Ui struct {
 	Fg, Bg Attribute
 
-	close   chan bool
-	manager UiManager
+	close     chan struct{}
+	closeOnce sync.Once
+	manager   UiManager
 
 	drawCount     int32
 	elements      map[string]*uiElement
@@ -39,7 +41,7 @@ type uiElement struct {
 
 func New(manager UiManager) *Ui {
 	ui := &Ui{
-		close:            make(chan bool, 10),
+		close:            make(chan struct{}),
 		elements:         make(map[string]*uiElement),
 		manager:          manager,
 		keyListeners:     make(map[Key][]KeyListener),
@@ -48,10 +50,9 @@ func New(manager UiManager) *Ui {
 	return ui
 }
 
+// Close is safe to call repeatedly and never blocks a caller.
 func (ui *Ui) Close() {
-	if termbox.IsInit {
-		ui.close <- true
-	}
+	ui.closeOnce.Do(func() { close(ui.close) })
 }
 
 func (ui *Ui) Refresh() {
@@ -97,7 +98,7 @@ func (ui *Ui) Run(cmds chan string) error {
 		return nil
 	}
 	if err := termbox.Init(); err != nil {
-		return nil
+		return err
 	}
 	defer termbox.Close()
 	termbox.SetInputMode(termbox.InputAlt)
@@ -119,7 +120,11 @@ func (ui *Ui) Run(cmds chan string) error {
 		select {
 		case <-ui.close:
 			return nil
-		case cmd := <-cmds:
+		case cmd, ok := <-cmds:
+			if !ok {
+				cmds = nil
+				continue
+			}
 			ui.onCommandEvent(cmd)
 		case event := <-events:
 			switch event.Type {

uiterm/ui_regression_test.go

@@ -0,0 +1,16 @@
+package uiterm
+
+import "testing"
+
+// Regression: Close sent to a bounded channel and could block or enqueue
+// duplicate shutdowns when called more than once.
+func TestCloseIsNonblockingAndIdempotent(t *testing.T) {
+	ui := New(nil)
+	ui.Close()
+	ui.Close()
+	select {
+	case <-ui.close:
+	default:
+		t.Fatal("Close did not signal shutdown")
+	}
+}