ndk-glue: Move native activity behind lock and remove it in onDestroy

`NativeActivity` is now behind a lock, and will be removed as soon as
`onDestroy` is called. Due to the async nature of events, make sure to
hold on to the lock received from `native_activity()` _beforehand_ if
you wish to use it during handling of `Event::Destroy`.

ndk-glue/CHANGELOG.md

@@ -1,5 +1,10 @@
 # Unreleased
 
+- **Breaking:** `NativeActivity` is now behind a lock, and will be removed
+  as soon as `onDestroy` is called. Due to the async nature of events, make
+  sure to hold on to the lock received from `native_activity()` _beforehand_
+  if you wish to use it during handling of `Event::Destroy`.
+
 # 0.4.0 (2021-08-02)
 
 - Looper is now created before returning from `ANativeActivity_onCreate`, solving

ndk-glue/src/lib.rs

@@ -47,16 +47,15 @@ pub fn android_log(level: Level, tag: &CStr, msg: &CStr) {
 }
 
 lazy_static! {
+    static ref NATIVE_ACTIVITY: RwLock<Option<NativeActivity>> = Default::default();
     static ref NATIVE_WINDOW: RwLock<Option<NativeWindow>> = Default::default();
     static ref INPUT_QUEUE: RwLock<Option<InputQueue>> = Default::default();
     static ref CONTENT_RECT: RwLock<Rect> = Default::default();
     static ref LOOPER: Mutex<Option<ForeignLooper>> = Default::default();
 }
 
-static mut NATIVE_ACTIVITY: Option<NativeActivity> = None;
-
-pub fn native_activity() -> &'static NativeActivity {
-    unsafe { NATIVE_ACTIVITY.as_ref().unwrap() }
+pub fn native_activity() -> RwLockReadGuard<'static, Option<NativeActivity>> {
+    NATIVE_ACTIVITY.read().unwrap()
 }
 
 pub fn native_window() -> RwLockReadGuard<'static, Option<NativeWindow>> {
@@ -114,6 +113,11 @@ pub enum Event {
     SaveInstanceState,
     Pause,
     Stop,
+    /// The native activity will be stopped and destroyed after this event.
+    /// Due to the async nature of these events the read-lock from
+    /// [`native_activity()`] should be held _before_ this event is called if
+    /// it is still needed. The lock should be released to allow `onDestroy`
+    /// to return.
     Destroy,
     ConfigChanged,
     LowMemory,
@@ -157,7 +161,7 @@ pub unsafe fn init(
     callbacks.onLowMemory = Some(on_low_memory);
 
     let activity = NativeActivity::from_ptr(activity);
-    NATIVE_ACTIVITY = Some(activity);
+    *NATIVE_ACTIVITY.write().unwrap() = Some(activity);
 
     let mut logpipe: [RawFd; 2] = Default::default();
     libc::pipe(logpipe.as_mut_ptr());
@@ -241,6 +245,12 @@ unsafe extern "C" fn on_stop(activity: *mut ANativeActivity) {
 
 unsafe extern "C" fn on_destroy(activity: *mut ANativeActivity) {
     wake(activity, Event::Destroy);
+    let mut native_activity_guard = NATIVE_ACTIVITY.write().unwrap();
+    assert_eq!(
+        native_activity_guard.as_ref().unwrap().ptr().as_ptr(),
+        activity
+    );
+    *native_activity_guard = None;
 }
 
 unsafe extern "C" fn on_configuration_changed(activity: *mut ANativeActivity) {