android: Hold NativeWindow lock until after notifying the user with…

… `Event::Suspended`

This applies rust-mobile/ndk#117
on the `winit` side: Android destroys its window/surface as soon as the
user returns from [`onNativeWindowDestroyed`], and we "fixed" this on
the `ndk-glue` side by sending the `WindowDestroyed` event before
locking the window and removing it: this lock has to wait for any user
of `ndk-glue` - ie. `winit` - to give up its readlock on the window,
which is what we utilize here to give users of `winit` "time" to destroy
any resource created on top of a `RawWindowHandle`.

since we can't pass the user a `RawWindowHandle` through the
`HasRawWindowHandle` trait we have to document this case explicitly and
keep the lock alive on the `winit` side instead.

[`onNativeWindowDestroyed`]: https://developer.android.com/ndk/reference/struct/a-native-activity-callbacks#onnativewindowdestroyed

Cargo.toml

@@ -39,8 +39,10 @@ simple_logger = "2.1.0"
 
 [target.'cfg(target_os = "android")'.dependencies]
 # Coordinate the next winit release with android-ndk-rs: https://github.com/rust-windowing/winit/issues/1995
-ndk = { git = "https://github.com/rust-windowing/android-ndk-rs", rev = "7e33384" }
-ndk-glue = { git = "https://github.com/rust-windowing/android-ndk-rs", rev = "7e33384" }
+# https://github.com/rust-windowing/android-ndk-rs/pull/282
+# https://github.com/rust-windowing/android-ndk-rs/pull/288
+ndk = { git = "https://github.com/MarijnS95/android-ndk-rs", rev = "130d013" }
+ndk-glue = { git = "https://github.com/MarijnS95/android-ndk-rs", rev = "130d013" }
 
 [target.'cfg(any(target_os = "ios", target_os = "macos"))'.dependencies]
 objc = "0.2.7"

src/platform_impl/android/mod.rs

@@ -10,8 +10,9 @@ use ndk::{
     configuration::Configuration,
     event::{InputEvent, KeyAction, Keycode, MotionAction},
     looper::{ForeignLooper, Poll, ThreadLooper},
+    native_window::NativeWindow,
 };
-use ndk_glue::{Event, Rect};
+use ndk_glue::{Event, LockReadGuard, Rect};
 use once_cell::sync::Lazy;
 use raw_window_handle::{HasRawWindowHandle, RawWindowHandle};
 
@@ -238,6 +239,7 @@ pub struct EventLoop<T: 'static> {
     start_cause: event::StartCause,
     looper: ThreadLooper,
     running: bool,
+    window_lock: Option<LockReadGuard<NativeWindow>>,
 }
 
 #[derive(Default, Debug, Copy, Clone, PartialEq, Eq, Hash)]
@@ -267,6 +269,7 @@ impl<T: 'static> EventLoop<T> {
             start_cause: event::StartCause::Init,
             looper: ThreadLooper::for_thread().unwrap(),
             running: false,
+            window_lock: None,
         }
     }
 
@@ -299,6 +302,17 @@ impl<T: 'static> EventLoop<T> {
             match self.first_event.take() {
                 Some(EventSource::Callback) => match ndk_glue::poll_events().unwrap() {
                     Event::WindowCreated => {
+                        // Acquire a lock on the window to prevent Android from destroying
+                        // it until we've notified and waited for the user in Event::Suspended.
+                        // XXX: Don't send this event if Android already removed the window
+                        // before we managed to handle this event?
+                        let next_window_lock = ndk_glue::native_window().expect(
+                            "`WindowCreated` while ndk_glue::native_window() provides no window",
+                        );
+                        assert!(
+                            self.window_lock.replace(next_window_lock).is_none(),
+                            "`WindowCreated` called while we were already holding a lock"
+                        );
                         call_event_handler!(
                             event_handler,
                             self.window_target(),
@@ -315,6 +329,10 @@ impl<T: 'static> EventLoop<T> {
                             control_flow,
                             event::Event::Suspended
                         );
+                        // Release the lock, allowing Android to clean up this surface
+                        self.window_lock.take().expect(
+                            "`WindowDestroyed` called while we were not holding a window lock",
+                        );
                     }
                     Event::Pause => self.running = false,
                     Event::Resume => self.running = true,
@@ -786,7 +804,7 @@ impl Window {
     }
 
     pub fn raw_window_handle(&self) -> RawWindowHandle {
-        if let Some(native_window) = ndk_glue::native_window().as_ref() {
+        if let Some(native_window) = ndk_glue::native_window() {
             native_window.raw_window_handle()
         } else {
             panic!("Cannot get the native window, it's null and will always be null before Event::Resumed and after Event::Suspended. Make sure you only call this function between those events.");

src/window.rs

@@ -1024,12 +1024,21 @@ impl Window {
 }
 
 unsafe impl raw_window_handle::HasRawWindowHandle for Window {
-    /// Returns a `raw_window_handle::RawWindowHandle` for the Window
+    /// Returns a [`raw_window_handle::RawWindowHandle`] for the Window
     ///
     /// ## Platform-specific
     ///
-    /// - **Android:** Only available after receiving the Resumed event and before Suspended. *If you*
-    /// *try to get the handle outside of that period, this function will panic*!
+    /// ### Android
+    ///
+    /// Only available after receiving [`Event::Resumed`] and before [`Event::Suspended`]. *If you
+    /// try to get the handle outside of that period, this function will panic*!
+    ///
+    /// Make sure to release or destroy any resources created from this `RawWindowHandle` (ie. Vulkan
+    /// or OpenGL surfaces) before returning from [`Event::Suspended`], at which point Android will
+    /// release the underlying window/surface: any subsequent interaction is undefined behavior.
+    ///
+    /// [`Event::Resumed`]: crate::event::Event::Resumed
+    /// [`Event::Suspended`]: crate::event::Event::Suspended
     fn raw_window_handle(&self) -> raw_window_handle::RawWindowHandle {
         self.window.raw_window_handle()
     }