app: protect out of touchIDs

From practical observation, out of touchIDs may occur. This conflicts the maximum number of handled touchs in iOS devices. Therefore the only reason that may cause out of touchIDs is that the touchIDs are intrinsically non-concurrent safe. This change adds a read-write lock to the touchIDs array.

Fixes fyne-io/fyne#2407.

app/darwin_ios.go

@@ -125,7 +125,12 @@ func updateConfig(width, height, orientation int32) {
 //
 // It is widely reported that the iPhone can handle up to 5 simultaneous
 // touch events, while the iPad can handle 11.
-var touchIDs [11]uintptr
+var (
+	// touchIDs may arrive concurrently, use a lock for safety.
+	// See https://github.com/fyne-io/fyne/issues/2407.
+	touchMu  sync.RWMutex
+	touchIDs [11]uintptr
+)
 
 var touchEvents struct {
 	sync.Mutex
@@ -135,12 +140,17 @@ var touchEvents struct {
 //export sendTouch
 func sendTouch(cTouch, cTouchType uintptr, x, y float32) {
 	id := -1
+
+	touchMu.RLock()
 	for i, val := range touchIDs {
 		if val == cTouch {
 			id = i
 			break
 		}
 	}
+	touchMu.RUnlock()
+
+	touchMu.Lock()
 	if id == -1 {
 		for i, val := range touchIDs {
 			if val == 0 {
@@ -158,6 +168,7 @@ func sendTouch(cTouch, cTouchType uintptr, x, y float32) {
 	if t == touch.TypeEnd {
 		touchIDs[id] = 0
 	}
+	touchMu.Unlock()
 
 	theApp.eventsIn <- touch.Event{
 		X:        x,