[Windows] multiwindow example panics when opening additional windows

[filnet]

Running the master multiwindow example and pressing space to open more windows yields:

# cargo run --example multiwindow
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
     Running `target\debug\examples\multiwindow.exe`
thread 'main' panicked at 'already mutably borrowed: BorrowError', src\libcore\result.rs:1165:5
stack backtrace:
   0: core::fmt::write
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libcore\fmt\mod.rs:1028
   1: std::io::Write::write_fmt<std::sys::windows::stdio::Stderr>
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libstd\io\mod.rs:1412
   2: std::panicking::default_hook::{{closure}}
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libstd\panicking.rs:188
   3: std::panicking::default_hook
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libstd\panicking.rs:205
   4: std::panicking::rust_panic_with_hook
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libstd\panicking.rs:464
   5: std::panicking::continue_panic_fmt
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libstd\panicking.rs:373
   6: std::panicking::rust_begin_panic
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libstd\panicking.rs:302
   7: core::panicking::panic_fmt
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libcore\panicking.rs:139
   8: core::result::unwrap_failed
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libcore\result.rs:1165
   9: core::result::Result<core::cell::Ref<core::option::Option<winit::platform_impl::platform::event_loop::runner::EventLoopRunner<()>>>, core::cell::BorrowError>::expect<core::cell::Ref<core::option::Option<winit::platform_impl::platform::event_loop::runner::
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\src\libcore\result.rs:960
  10: core::cell::RefCell<core::option::Option<winit::platform_impl::platform::event_loop::runner::EventLoopRunner<()>>>::borrow<core::option::Option<winit::platform_impl::platform::event_loop::runner::EventLoopRunner<()>>>
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\src\libcore\cell.rs:788
  11: gapfnScSendMessage
  12: winit::platform_impl::platform::event_loop::SubclassInput<()>::send_event<()>
             at .\src\platform_impl\windows\event_loop.rs:104
  13: winit::platform_impl::platform::event_loop::public_window_callback<()>
             at .\src\platform_impl\windows\event_loop.rs:1367
  14: DefSubclassProc
  15: DPA_Clone
  16: gapfnScSendMessage
  17: GetThreadDesktop
  18: GetThreadDesktop
  19: GetThreadDesktop
  20: KiUserCallbackDispatcher
  21: gapfnScSendMessage
  22: gapfnScSendMessage
  23: gapfnScSendMessage
  24: GetThreadDesktop
  25: GetThreadDesktop
  26: GetThreadDesktop
  27: KiUserCallbackDispatcher
  28: winit::platform_impl::platform::window::{{impl}}::set_visible::{{closure}}
             at .\src\platform_impl\windows\window.rs:134
  29: winit::platform_impl::platform::event_loop::EventLoopThreadExecutor::execute_in_thread<closure-0>
             at .\src\platform_impl\windows\event_loop.rs:420
  30: winit::platform_impl::platform::window::Window::set_visible
             at .\src\platform_impl\windows\window.rs:135
  31: winit::event::WindowEvent::to_static
             at .\src\event.rs:402
  32: winit::platform_impl::platform::window::Window::new<()>
             at .\src\platform_impl\windows\window.rs:72
  33: winit::window::WindowBuilder::build<()>
             at .\src\window.rs:333
  34: winit::window::Window::new<()>
             at .\src\window.rs:360
  35: multiwindow::main::{{closure}}
             at .\examples\multiwindow.rs:42
  36: winit::platform_impl::platform::event_loop::{{impl}}::run_return::{{closure}}<(),closure-0>
             at .\src\platform_impl\windows\event_loop.rs:208
  37: alloc::boxed::{{impl}}::call_mut<(winit::event::Event<()>, mut winit::event_loop::ControlFlow*),FnMut<(winit::event::Event<()>, mut winit::event_loop::ControlFlow*)>>
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\src\liballoc\boxed.rs:950
  38: winit::platform_impl::platform::event_loop::runner::{{impl}}::call_event_handler::{{closure}}<()>
             at .\src\platform_impl\windows\event_loop\runner.rs:565
  39: std::panic::{{impl}}::call_once<(),closure-0>
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\src\libstd\panic.rs:318
  40: std::panicking::try::do_call<std::panic::AssertUnwindSafe<closure-0>,()>
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\src\libstd\panicking.rs:289
  41: panic_unwind::__rust_maybe_catch_panic
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\/src\libpanic_unwind\lib.rs:78
  42: std::panicking::try<(),std::panic::AssertUnwindSafe<closure-0>>
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\src\libstd\panicking.rs:265
  43: std::panic::catch_unwind<std::panic::AssertUnwindSafe<closure-0>,()>
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\src\libstd\panic.rs:396
  44: winit::platform_impl::platform::event_loop::runner::EventLoopRunner<()>::call_event_handler<()>
             at .\src\platform_impl\windows\event_loop\runner.rs:563
  45: winit::platform_impl::platform::event_loop::runner::EventLoopRunner<()>::process_event<()>
             at .\src\platform_impl\windows\event_loop\runner.rs:439
  46: winit::platform_impl::platform::event_loop::runner::ELRShared<()>::send_event_unbuffered<()>
             at .\src\platform_impl\windows\event_loop\runner.rs:161
  47: winit::platform_impl::platform::event_loop::runner::ELRShared<()>::send_event<()>
             at .\src\platform_impl\windows\event_loop\runner.rs:150
  48: winit::platform_impl::platform::event_loop::SubclassInput<()>::send_event<()>
             at .\src\platform_impl\windows\event_loop.rs:104
  49: winit::platform_impl::platform::event_loop::public_window_callback<()>
             at .\src\platform_impl\windows\event_loop.rs:940
  50: DefSubclassProc
  51: DPA_Clone
  52: gapfnScSendMessage
  53: GetThreadDesktop
  54: CharPrevW
  55: DispatchMessageW
  56: winit::platform_impl::platform::event_loop::EventLoop<()>::run_return<(),closure-0>
             at .\src\platform_impl\windows\event_loop.rs:235
  57: winit::platform_impl::platform::event_loop::EventLoop<()>::run<(),closure-0>
             at .\src\platform_impl\windows\event_loop.rs:192
  58: winit::event_loop::EventLoop<()>::run<(),closure-0>
             at .\src\event_loop.rs:148
  59: multiwindow::main
             at .\examples\multiwindow.rs:18
  60: std::rt::lang_start::{{closure}}<()>
             at /rustc/73528e339aae0f17a15ffa49a8ac608f50c6cf14\src\libstd\rt.rs:61

[filnet]

fullscreen example seems to panic in the same way

[filnet]

This is what I believe happens:

Pressing any key in one of the windows will call runner::call_event_handler which mutably borrows the runner.

    pub(crate) unsafe fn call_event_handler(&self, event: Event<'static, T>) {
        if let Ok(mut runner_ref) = self.runner.try_borrow_mut() {
            if let Some(ref mut runner) = *runner_ref {
                runner.call_event_handler(event);
                return;
            }
        }
    }

On key press, the multiwindow example event handler will create a new window and make it visible.
This will steal focus from the current window and that message gets dispatched and handled.
We end up in runner::send_event() where the same runner is borrowed again (see the 1st instruction that checks if we are handling redraws). This borrow fails.

    pub(crate) unsafe fn send_event(&self, event: Event<'_, T>) {
        let handling_redraw = self
            .runner
            .borrow()
            .as_ref()
            .map(|r| RunnerState::HandlingRedraw == r.runner_state)
            .unwrap_or(false);
        let mut send = None;
        if handling_redraw {
            if let Event::RedrawRequested(_) = event {
                send = Some(event);
            } else {
                self.buffer_event(event);
            }
        } else {
            send = Some(event);
        }
        if let Some(event) = send {
            if let Err(event) = self.send_event_unbuffered(event) {
                // If the runner is already borrowed, we're in the middle of an event loop invocation. Add
                // the event to a buffer to be processed later.
                self.buffer_event(event);
            }
        }
    }

A fix would be to try_borrow() instead of borrow(). But what to do if the try_borrow() fails ?

Note that the two other places, where the runner is borrowed, use try_borrow() because the runner can already be borrowed. But runner::send_event() does not bother with that.

[filnet]

The borrow that fails was introduced recently (after 0.20.0).
See 6a330a2#diff-e76532bd258be7f13f34d877ce8718b4R132

[filnet]

Seems to duplicate #1400