app: allow app.Main to return on Windows

Fixes golang/go#49499

Change-Id: Ia65b61b9e0707321439ea85d06bac5ed507b58c7
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/369196
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Changkun Ou <mail@changkun.de>
TryBot-Result: Gopher Robot <gobot@golang.org>

app/android.go

@@ -287,8 +287,12 @@ func mainUI(vm, jniEnv, ctx uintptr) error {
 
 	donec := make(chan struct{})
 	go func() {
+		// close the donec channel in a defer statement
+		// so that we could still be able to return even
+		// if mainUserFn panics.
+		defer close(donec)
+
 		mainUserFn(theApp)
-		close(donec)
 	}()
 
 	var pixelsPerPt float32

app/shiny.go

@@ -38,10 +38,23 @@ func main(f func(a App)) {
 			}
 		}()
 
-		go f(theApp)
+		donec := make(chan struct{})
+		go func() {
+			// close the donec channel in a defer statement
+			// so that we could still be able to return even
+			// if f panics.
+			defer close(donec)
+
+			f(theApp)
+		}()
 
 		for {
-			theApp.Send(convertEvent(w.NextEvent()))
+			select {
+			case <-donec:
+				return
+			default:
+				theApp.Send(convertEvent(w.NextEvent()))
+			}
 		}
 	})
 }

app/x11.go

@@ -52,8 +52,12 @@ func main(f func(App)) {
 
 	donec := make(chan struct{})
 	go func() {
+		// close the donec channel in a defer statement
+		// so that we could still be able to return even
+		// if f panics.
+		defer close(donec)
+
 		f(theApp)
-		close(donec)
 	}()
 
 	// TODO: can we get the actual vsync signal?