ndk-glue: Switch to parking_lot for mappable and Sendable lock guards (#282)

It has come up previously that the `native_window()` function is easier
to use if we could transpose a `LockGuard<Option<>>` into an
`Option<LockGuard<>>`.  After all, as soon as you have the lock you only
need to check the `Option<>` value once and are thereafter guaranteed
that its value won't change, until the lock is given up.

At the same time `parking_lot` has a `send_guard` feature which allows
moving a lock guard to another thread (as long as `deadlock_detection`
is disabled); a use case for this recently came up [in glutin] where the
`NativeWindow` handle lock should be stored with a GL context so that
our `fn on_window_destroyed()` callback doesn't return until after the
`Surface` created on it is removed from the context and destroyed.
Glutin forces this context to be `Send`, and a lock guard already allows
`Sync` if `NativeWindow` is `Sync` (which it is).

[in glutin]: rust-windowing/glutin#1411 (comment)

Author: MarijnS95

ndk-glue/CHANGELOG.md

@@ -1,6 +1,7 @@
 # Unreleased
 
 - **Breaking:** Provide a `LockReadGuard` newtype around `NativeWindow`/`InputQueue` to hide the underlying lock implementation. (#288)
+- **Breaking:** Transpose `LockReadGuard<Option<T>>` into `Option<LockReadGuard<T>>` to only necessitate an `Option` unpack/`unwrap()` once. (#282)
 
 # 0.6.2 (2022-04-19)
 

ndk-glue/Cargo.toml

@@ -12,14 +12,15 @@ homepage = "https://github.com/rust-windowing/android-ndk-rs"
 repository = "https://github.com/rust-windowing/android-ndk-rs"
 
 [dependencies]
+android_logger = { version = "0.11", optional = true }
+libc = "0.2.84"
+log = "0.4.14"
 ndk = { path = "../ndk", version = "0.6.0" }
 ndk-context = { path = "../ndk-context", version = "0.1.1" }
 ndk-macro = { path = "../ndk-macro", version = "0.3.0" }
 ndk-sys = { path = "../ndk-sys", version = "0.3.0" }
 once_cell = "1"
-libc = "0.2.84"
-log = "0.4.14"
-android_logger = { version = "0.11", optional = true }
+parking_lot = "0.12"
 
 [features]
 default = []

ndk-glue/src/lib.rs

@@ -7,6 +7,7 @@ use ndk::native_activity::NativeActivity;
 use ndk::native_window::NativeWindow;
 use ndk_sys::{AInputQueue, ANativeActivity, ANativeWindow, ARect};
 use once_cell::sync::Lazy;
+use parking_lot::{MappedRwLockReadGuard, RwLock, RwLockReadGuard};
 use std::ffi::{CStr, CString};
 use std::fmt;
 use std::fs::File;
@@ -15,7 +16,7 @@ use std::ops::Deref;
 use std::os::raw;
 use std::os::unix::prelude::*;
 use std::ptr::NonNull;
-use std::sync::{Arc, Condvar, Mutex, RwLock, RwLockReadGuard};
+use std::sync::{Arc, Condvar, Mutex};
 use std::thread;
 
 #[cfg(feature = "logger")]
@@ -67,7 +68,20 @@ pub fn native_activity() -> &'static NativeActivity {
     unsafe { NATIVE_ACTIVITY.as_ref().unwrap() }
 }
 
-pub struct LockReadGuard<T: ?Sized + 'static>(RwLockReadGuard<'static, T>);
+pub struct LockReadGuard<T: ?Sized + 'static>(MappedRwLockReadGuard<'static, T>);
+
+impl<T> LockReadGuard<T> {
+    /// Transpose an [`Option`] wrapped inside a [`LockReadGuard`]
+    ///
+    /// This is a _read_ lock for which the contents can't change; hence allowing the user to only
+    /// check for [`None`] once and hold a lock containing `T` directly thereafter, without
+    /// subsequent infallible [`Option::unwrap()`]s.
+    fn from_wrapped_option(wrapped: RwLockReadGuard<'static, Option<T>>) -> Option<Self> {
+        RwLockReadGuard::try_map(wrapped, Option::as_ref)
+            .ok()
+            .map(Self)
+    }
+}
 
 impl<T: ?Sized> Deref for LockReadGuard<T> {
     type Target = T;
@@ -102,8 +116,8 @@ impl<T: ?Sized + fmt::Display> fmt::Display for LockReadGuard<T> {
 /// # Warning
 /// This function accesses a `static` variable internally and must only be used if you are sure
 /// there is exactly one version of `ndk_glue` in your dependency tree.
-pub fn native_window() -> LockReadGuard<Option<NativeWindow>> {
-    LockReadGuard(NATIVE_WINDOW.read().unwrap())
+pub fn native_window() -> Option<LockReadGuard<NativeWindow>> {
+    LockReadGuard::from_wrapped_option(NATIVE_WINDOW.read())
 }
 
 /// Returns an [`InputQueue`] held inside a lock, preventing Android from freeing it immediately
@@ -117,14 +131,14 @@ pub fn native_window() -> LockReadGuard<Option<NativeWindow>> {
 /// # Warning
 /// This function accesses a `static` variable internally and must only be used if you are sure
 /// there is exactly one version of `ndk_glue` in your dependency tree.
-pub fn input_queue() -> LockReadGuard<Option<InputQueue>> {
-    LockReadGuard(INPUT_QUEUE.read().unwrap())
+pub fn input_queue() -> Option<LockReadGuard<InputQueue>> {
+    LockReadGuard::from_wrapped_option(INPUT_QUEUE.read())
 }
 
 /// This function accesses a `static` variable internally and must only be used if you are sure
 /// there is exactly one version of `ndk_glue` in your dependency tree.
 pub fn content_rect() -> Rect {
-    CONTENT_RECT.read().unwrap().clone()
+    CONTENT_RECT.read().clone()
 }
 
 static PIPE: Lazy<[RawFd; 2]> = Lazy::new(|| {
@@ -345,7 +359,6 @@ unsafe extern "C" fn on_window_focus_changed(
 unsafe extern "C" fn on_window_created(activity: *mut ANativeActivity, window: *mut ANativeWindow) {
     NATIVE_WINDOW
         .write()
-        .unwrap()
         .replace(NativeWindow::clone_from_ptr(NonNull::new(window).unwrap()));
     wake(activity, Event::WindowCreated);
 }
@@ -369,7 +382,7 @@ unsafe extern "C" fn on_window_destroyed(
     window: *mut ANativeWindow,
 ) {
     wake(activity, Event::WindowDestroyed);
-    let mut native_window_guard = NATIVE_WINDOW.write().unwrap();
+    let mut native_window_guard = NATIVE_WINDOW.write();
     assert_eq!(native_window_guard.as_ref().unwrap().ptr().as_ptr(), window);
     native_window_guard.take();
 }
@@ -384,7 +397,7 @@ unsafe extern "C" fn on_input_queue_created(
     // future code cleans it up and sets it back to `None` again.
     let looper = locked_looper.as_ref().expect("Looper does not exist");
     input_queue.attach_looper(looper, NDK_GLUE_LOOPER_INPUT_QUEUE_IDENT);
-    INPUT_QUEUE.write().unwrap().replace(input_queue);
+    INPUT_QUEUE.write().replace(input_queue);
     wake(activity, Event::InputQueueCreated);
 }
 
@@ -393,7 +406,7 @@ unsafe extern "C" fn on_input_queue_destroyed(
     queue: *mut AInputQueue,
 ) {
     wake(activity, Event::InputQueueDestroyed);
-    let mut input_queue_guard = INPUT_QUEUE.write().unwrap();
+    let mut input_queue_guard = INPUT_QUEUE.write();
     assert_eq!(input_queue_guard.as_ref().unwrap().ptr().as_ptr(), queue);
     let input_queue = InputQueue::from_ptr(NonNull::new(queue).unwrap());
     input_queue.detach_looper();
@@ -407,6 +420,6 @@ unsafe extern "C" fn on_content_rect_changed(activity: *mut ANativeActivity, rec
         right: (*rect).right as _,
         bottom: (*rect).bottom as _,
     };
-    *CONTENT_RECT.write().unwrap() = rect;
+    *CONTENT_RECT.write() = rect;
     wake(activity, Event::ContentRectChanged);
 }