macOS: Remove panic wrapper (#4147)

This is unnecessary nowadays, unwinding in CF observer callbacks is safe
(and is safe in Rust after the introduction of `extern "C-unwind"`).

Panicking elsewhere (such as in NSNotificationCenter callbacks or
delegate methods) _may_ still lead to an abort, if AppKit tries to catch
it with libc++, since Rust panics are not compatible with those.
That's "just" a quality-of-implementation detail of current Rust though,
not an inherent limitation, and should really be solved in rustc.

src/platform_impl/apple/appkit/app_state.rs

@@ -1,6 +1,6 @@
 use std::cell::{Cell, OnceCell, RefCell};
 use std::mem;
-use std::rc::{Rc, Weak};
+use std::rc::Rc;
 use std::sync::Arc;
 use std::time::Instant;
 
@@ -15,7 +15,7 @@ use winit_core::window::WindowId;
 
 use super::super::event_handler::EventHandler;
 use super::super::event_loop_proxy::EventLoopProxy;
-use super::event_loop::{notify_windows_of_exit, stop_app_immediately, ActiveEventLoop, PanicInfo};
+use super::event_loop::{notify_windows_of_exit, stop_app_immediately, ActiveEventLoop};
 use super::menu;
 use super::observer::{EventLoopWaker, RunLoop};
 
@@ -309,13 +309,9 @@ impl AppState {
     }
 
     // Called by RunLoopObserver after finishing waiting for new events
-    pub fn wakeup(self: &Rc<Self>, panic_info: Weak<PanicInfo>) {
+    pub fn wakeup(self: &Rc<Self>) {
-        let panic_info = panic_info
-            .upgrade()
-            .expect("The panic info must exist here. This failure indicates a developer error.");
-
         // Return when in event handler due to https://github.com/rust-windowing/winit/issues/1779
-        if panic_info.is_panicking() || !self.event_handler.ready() || !self.is_running() {
+        if !self.event_handler.ready() || !self.is_running() {
             return;
         }
 
@@ -341,15 +337,11 @@ impl AppState {
     }
 
     // Called by RunLoopObserver before waiting for new events
-    pub fn cleared(self: &Rc<Self>, panic_info: Weak<PanicInfo>) {
+    pub fn cleared(self: &Rc<Self>) {
-        let panic_info = panic_info
-            .upgrade()
-            .expect("The panic info must exist here. This failure indicates a developer error.");
-
         // Return when in event handler due to https://github.com/rust-windowing/winit/issues/1779
         // XXX: how does it make sense that `event_handler.ready()` can ever return `false` here if
         // we're about to return to the `CFRunLoop` to poll for new events?
-        if panic_info.is_panicking() || !self.event_handler.ready() || !self.is_running() {
+        if !self.event_handler.ready() || !self.is_running() {
             return;
         }
 

src/platform_impl/apple/appkit/event_loop.rs

@@ -1,8 +1,4 @@
-use std::any::Any;
+use std::rc::Rc;
-use std::cell::Cell;
-use std::fmt;
-use std::panic::{catch_unwind, resume_unwind, RefUnwindSafe, UnwindSafe};
-use std::rc::{Rc, Weak};
 use std::sync::Arc;
 use std::time::{Duration, Instant};
 
@@ -36,42 +32,6 @@ use super::observer::setup_control_flow_observers;
 use crate::platform::macos::ActivationPolicy;
 use crate::platform_impl::Window;
 
-#[derive(Default)]
-pub struct PanicInfo {
-    inner: Cell<Option<Box<dyn Any + Send + 'static>>>,
-}
-
-impl fmt::Debug for PanicInfo {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("PanicInfo").finish_non_exhaustive()
-    }
-}
-
-// WARNING:
-// As long as this struct is used through its `impl`, it is UnwindSafe.
-// (If `get_mut` is called on `inner`, unwind safety may get broken.)
-impl UnwindSafe for PanicInfo {}
-impl RefUnwindSafe for PanicInfo {}
-impl PanicInfo {
-    pub fn is_panicking(&self) -> bool {
-        let inner = self.inner.take();
-        let result = inner.is_some();
-        self.inner.set(inner);
-        result
-    }
-
-    /// Overwrites the current state if the current state is not panicking
-    pub fn set_panic(&self, p: Box<dyn Any + Send + 'static>) {
-        if !self.is_panicking() {
-            self.inner.set(Some(p));
-        }
-    }
-
-    pub fn take(&self) -> Option<Box<dyn Any + Send + 'static>> {
-        self.inner.take()
-    }
-}
-
 #[derive(Debug)]
 pub struct ActiveEventLoop {
     pub(super) app_state: Rc<AppState>,
@@ -183,7 +143,6 @@ pub struct EventLoop {
     app_state: Rc<AppState>,
 
     window_target: ActiveEventLoop,
-    panic_info: Rc<PanicInfo>,
 
     // Since macOS 10.11, we no longer need to remove the observers before they are deallocated;
     // the system instead cleans it up next time it would have posted a notification to it.
@@ -259,14 +218,12 @@ impl EventLoop {
             },
         );
 
-        let panic_info: Rc<PanicInfo> = Default::default();
+        setup_control_flow_observers(mtm);
-        setup_control_flow_observers(mtm, Rc::downgrade(&panic_info));
 
         Ok(EventLoop {
             app,
             app_state: app_state.clone(),
             window_target: ActiveEventLoop { app_state, mtm },
-            panic_info,
             _did_finish_launching_observer,
             _will_terminate_observer,
         })
@@ -306,15 +263,6 @@ impl EventLoop {
                 // NOTE: Make sure to not run the application re-entrantly, as that'd be confusing.
                 self.app.run();
 
-                // While the app is running it's possible that we catch a panic
-                // to avoid unwinding across an objective-c ffi boundary, which
-                // will lead to us stopping the `NSApplication` and saving the
-                // `PanicInfo` so that we can resume the unwind at a controlled,
-                // safe point in time.
-                if let Some(panic) = self.panic_info.take() {
-                    resume_unwind(panic);
-                }
-
                 self.app_state.internal_exit()
             })
         });
@@ -370,15 +318,6 @@ impl EventLoop {
                     self.app.run();
                 }
 
-                // While the app is running it's possible that we catch a panic
-                // to avoid unwinding across an objective-c ffi boundary, which
-                // will lead to us stopping the application and saving the
-                // `PanicInfo` so that we can resume the unwind at a controlled,
-                // safe point in time.
-                if let Some(panic) = self.panic_info.take() {
-                    resume_unwind(panic);
-                }
-
                 if self.app_state.exiting() {
                     self.app_state.internal_exit();
                     PumpStatus::Exit(0)
@@ -423,29 +362,3 @@ pub(super) fn notify_windows_of_exit(app: &NSApplication) {
         window.close();
     }
 }
-
-/// Catches panics that happen inside `f` and when a panic
-/// happens, stops the `sharedApplication`
-#[inline]
-pub fn stop_app_on_panic<F: FnOnce() -> R + UnwindSafe, R>(
-    mtm: MainThreadMarker,
-    panic_info: Weak<PanicInfo>,
-    f: F,
-) -> Option<R> {
-    match catch_unwind(f) {
-        Ok(r) => Some(r),
-        Err(e) => {
-            // It's important that we set the panic before requesting a `stop`
-            // because some callback are still called during the `stop` message
-            // and we need to know in those callbacks if the application is currently
-            // panicking
-            {
-                let panic_info = panic_info.upgrade().unwrap();
-                panic_info.set_panic(e);
-            }
-            let app = NSApplication::sharedApplication(mtm);
-            stop_app_immediately(&app);
-            None
-        },
-    }
-}

src/platform_impl/apple/appkit/observer.rs

@@ -4,9 +4,7 @@
 //! <https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Multithreading/RunLoopManagement/RunLoopManagement.html>
 use std::cell::Cell;
 use std::ffi::c_void;
-use std::panic::{AssertUnwindSafe, UnwindSafe};
 use std::ptr;
-use std::rc::Weak;
 use std::time::Instant;
 
 use objc2::MainThreadMarker;
@@ -18,47 +16,18 @@ use objc2_core_foundation::{
 use tracing::error;
 
 use super::app_state::AppState;
-use super::event_loop::{stop_app_on_panic, PanicInfo};
-
-unsafe fn control_flow_handler<F>(panic_info: *mut c_void, f: F)
-where
-    F: FnOnce(Weak<PanicInfo>) + UnwindSafe,
-{
-    let info_from_raw = unsafe { Weak::from_raw(panic_info as *mut PanicInfo) };
-    // Asserting unwind safety on this type should be fine because `PanicInfo` is
-    // `RefUnwindSafe` and `Rc<T>` is `UnwindSafe` if `T` is `RefUnwindSafe`.
-    let panic_info = AssertUnwindSafe(Weak::clone(&info_from_raw));
-    // `from_raw` takes ownership of the data behind the pointer.
-    // But if this scope takes ownership of the weak pointer, then
-    // the weak pointer will get free'd at the end of the scope.
-    // However we want to keep that weak reference around after the function.
-    std::mem::forget(info_from_raw);
-
-    let mtm = MainThreadMarker::new().unwrap();
-    stop_app_on_panic(mtm, Weak::clone(&panic_info), move || {
-        let _ = &panic_info;
-        f(panic_info.0)
-    });
-}
 
 // begin is queued with the highest priority to ensure it is processed before other observers
 extern "C-unwind" fn control_flow_begin_handler(
     _: *mut CFRunLoopObserver,
     activity: CFRunLoopActivity,
-    panic_info: *mut c_void,
+    _info: *mut c_void,
 ) {
-    unsafe {
+    match activity {
-        control_flow_handler(panic_info, |panic_info| {
+        CFRunLoopActivity::AfterWaiting => {
-            #[allow(non_upper_case_globals)]
+            AppState::get(MainThreadMarker::new().unwrap()).wakeup();
-            match activity {
+        },
-                CFRunLoopActivity::AfterWaiting => {
+        _ => unreachable!(),
-                    // trace!("Triggered `CFRunLoopAfterWaiting`");
-                    AppState::get(MainThreadMarker::new().unwrap()).wakeup(panic_info);
-                    // trace!("Completed `CFRunLoopAfterWaiting`");
-                },
-                _ => unreachable!(),
-            }
-        });
     }
 }
 
@@ -67,21 +36,14 @@ extern "C-unwind" fn control_flow_begin_handler(
 extern "C-unwind" fn control_flow_end_handler(
     _: *mut CFRunLoopObserver,
     activity: CFRunLoopActivity,
-    panic_info: *mut c_void,
+    _info: *mut c_void,
 ) {
-    unsafe {
+    match activity {
-        control_flow_handler(panic_info, |panic_info| {
+        CFRunLoopActivity::BeforeWaiting => {
-            #[allow(non_upper_case_globals)]
+            AppState::get(MainThreadMarker::new().unwrap()).cleared();
-            match activity {
+        },
-                CFRunLoopActivity::BeforeWaiting => {
+        CFRunLoopActivity::Exit => (), // unimplemented!(), // not expected to ever happen
-                    // trace!("Triggered `CFRunLoopBeforeWaiting`");
+        _ => unreachable!(),
-                    AppState::get(MainThreadMarker::new().unwrap()).cleared(panic_info);
-                    // trace!("Completed `CFRunLoopBeforeWaiting`");
-                },
-                CFRunLoopActivity::Exit => (), /* unimplemented!(), // not expected to ever happen */
-                _ => unreachable!(),
-            }
-        });
     }
 }
 
@@ -180,11 +142,11 @@ impl RunLoop {
     }
 }
 
-pub fn setup_control_flow_observers(mtm: MainThreadMarker, panic_info: Weak<PanicInfo>) {
+pub fn setup_control_flow_observers(mtm: MainThreadMarker) {
     let run_loop = RunLoop::main(mtm);
     unsafe {
         let mut context = CFRunLoopObserverContext {
-            info: Weak::into_raw(panic_info) as *mut _,
+            info: ptr::null_mut(),
             version: 0,
             retain: None,
             release: None,