AppHandle is not Send + Sync on Windows

[TheCallSign]

Hello,

My Cargo.toml:

...
[dependencies]
tauri = { version = "1.0.0-beta.5", features = ["api-all"] }

I have a tauri application that compiles fine on Linux but when cross compiling to Windows I get the following error:

error[E0277]: `std::sync::mpsc::Sender<tauri_runtime_wry::Message>` cannot be shared between threads safely
  --> src-tauri/src/commands/device.rs:52:12
   |
52 |     state: State<'_, App>,
   |            ^^^^^^^^^^^^^^^^^^^^^^ `std::sync::mpsc::Sender<tauri_runtime_wry::Message>` cannot be shared between threads safely
   |
  ::: /home/thecallsign/.cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/tauri-1.0.0-beta.5/src/state.rs:13:32
   |
13 | pub struct State<'r, T: Send + Sync + 'static>(&'r T);
   |                                ---- required by this bound in `tauri::State`
   |
   = help: within `AppHandle`, the trait `std::marker::Sync` is not implemented for `std::sync::mpsc::Sender<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tao::platform_impl::platform::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tao::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tauri_runtime_wry::DispatcherContext`
   = note: required because it appears within the type `tauri_runtime_wry::WryHandle`
   = note: required because it appears within the type `AppHandle`
   = note: required because of the requirements on the impl of `std::marker::Sync` for `once_cell::imp::OnceCell<AppHandle>`
   = note: required because it appears within the type `once_cell::sync::OnceCell<AppHandle>`

and

error[E0277]: `*mut winapi::shared::windef::HWND__` cannot be shared between threads safely
  --> src-tauri/src/commands/serial.rs:6:41
   |
6  | pub(crate) async fn rescan_ports(state: State<'_, App>) -> Result<(), AppError> {
   |                                         ^^^^^^^^^^^^^^^^^^^^^^ `*mut winapi::shared::windef::HWND__` cannot be shared between threads safely
   |
  ::: /home/thecallsign/.cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/tauri-1.0.0-beta.5/src/state.rs:13:32
   |
13 | pub struct State<'r, T: Send + Sync + 'static>(&'r T);
   |                                ---- required by this bound in `tauri::State`
   |
   = help: within `AppHandle`, the trait `std::marker::Sync` is not implemented for `*mut winapi::shared::windef::HWND__`
   = note: required because it appears within the type `tao::platform_impl::platform::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tao::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tauri_runtime_wry::DispatcherContext`
   = note: required because it appears within the type `tauri_runtime_wry::WryHandle`
   = note: required because it appears within the type `AppHandle`
   = note: required because of the requirements on the impl of `std::marker::Sync` for `once_cell::imp::OnceCell<AppHandle>`
   = note: required because it appears within the type `once_cell::sync::OnceCell<AppHandle>`

I am holding on to a tauri AppHandle in my application state. My application state is managed with a tauri Application via manage().

Also, as far as I can tell tao::event_loop::EventLoopProxy is Send + Sync on Linux but only Send on Windows.

Windows:

https://github.com/tauri-apps/tao/blob/0d6fcde47a1dd587a7ac2c209d330169496d4c46/src/platform_impl/windows/event_loop.rs#L491

Linux:

https://github.com/tauri-apps/tao/blob/0d6fcde47a1dd587a7ac2c209d330169496d4c46/src/platform_impl/linux/event_loop.rs#L795

I'm not too phased about whether the AppHandle can be kept within tauri managed state but the bahaviour should be consistent across platform.

[TheCallSign]

I've since refactored my code to work around this issue but the inconsistent behaviour remains...

[TheCallSign]

Nevermind, cross compilation still fails. I cannot send the AppHandle to long running background tokio tasks.

Here are some more compiler errors:

Note that ConnectionManager contains an AppHandle, but it is instantiated and sent to a newly spawned async task.

The error wrt to tracing is because I am forwarding tracing log messages to the developer console in the tauri window. For that to work I really need the AppHandle to be Send + Sync.

error: future cannot be sent between threads safely
   --> src-tauri/src/app.rs:78:21
    |
78  |                     tokio::task::spawn(async move {
    |                     ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
    |
   ::: /home/thecallsign/.cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/tokio-1.9.0/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ---- required by this bound in `tokio::spawn`
    |
    = help: within `ConnectionManager`, the trait `std::marker::Sync` is not implemented for `*mut winapi::shared::windef::HWND__`
note: captured value is not `Send`
   --> src-tauri/src/app.rs:84:33
    |
84  | ...                   manager.close(&name);
    |                       ^^^^^^^ has type `Arc<ConnectionManager>` which is not `Send`

error: future cannot be sent between threads safely
   --> src-tauri/src/app.rs:78:21
    |
78  |                     tokio::task::spawn(async move {
    |                     ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
    |
   ::: /home/thecallsign/.cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/tokio-1.9.0/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ---- required by this bound in `tokio::spawn`
    |
    = help: within `ConnectionManager`, the trait `std::marker::Sync` is not implemented for `std::sync::mpsc::Sender<tauri_runtime_wry::Message>`
note: captured value is not `Send`
   --> src-tauri/src/app.rs:84:33
    |
84  | ...                   manager.close(&name);
    |                       ^^^^^^^ has type `Arc<ConnectionManager>` which is not `Send`

error[E0277]: `*mut winapi::shared::windef::HWND__` cannot be shared between threads safely
  --> src-tauri/src/logging.rs:83:5
   |
83 |     tracing::subscriber::set_global_default(subscriber).expect("Set global default log subscriber");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `*mut winapi::shared::windef::HWND__` cannot be shared between threads safely
   |
  ::: /home/thecallsign/.cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/tracing-0.1.26/src/subscriber.rs:41:28
   |
41 |     S: Subscriber + Send + Sync + 'static,
   |                            ---- required by this bound in `tracing::subscriber::set_global_default`
   |
   = help: within `Layered<BunyanFormattingLayer<FrontendLogger>, Layered<BunyanFormattingLayer<NonBlocking>, Layered<tracing_subscriber::fmt::Layer<Registry, DefaultFields, Format, NonBlocking>, Registry>>>`, the trait `std::marker::Sync` is not implemented for `*mut winapi::shared::windef::HWND__`
   = note: required because it appears within the type `tao::platform_impl::platform::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tao::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tauri_runtime_wry::DispatcherContext`
   = note: required because it appears within the type `tauri_runtime_wry::WryHandle`
   = note: required because it appears within the type `AppHandle`
   

[TheCallSign]

With regards to this error:

error[E0277]: `*mut winapi::shared::windef::HWND__` cannot be shared between threads safely
  --> src-tauri/src/logging.rs:83:5
   |
83 |     tracing::subscriber::set_global_default(subscriber).expect("Set global default log subscriber");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `*mut winapi::shared::windef::HWND__` cannot be shared between threads safely
   |
  ::: /home/thecallsign/.cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/tracing-0.1.26/src/subscriber.rs:41:28
   |
41 |     S: Subscriber + Send + Sync + 'static,
   |                            ---- required by this bound in `tracing::subscriber::set_global_default`
   |
   = help: within `Layered<BunyanFormattingLayer<FrontendLogger>, Layered<BunyanFormattingLayer<NonBlocking>, Layered<tracing_subscriber::fmt::Layer<Registry, DefaultFields, Format, NonBlocking>, Registry>>>`, the trait `std::marker::Sync` is not implemented for `*mut winapi::shared::windef::HWND__`
   = note: required because it appears within the type `tao::platform_impl::platform::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tao::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tauri_runtime_wry::DispatcherContext`
   = note: required because it appears within the type `tauri_runtime_wry::WryHandle`
   = note: required because it appears within the type `AppHandle`

Wrapping the AppHandle with an Arc<RwLock<T>> does not work (tauri_runtime_wry::Message does not impl Send):

error[E0277]: `std::sync::mpsc::Sender<tauri_runtime_wry::Message>` cannot be shared between threads safely
  --> src-tauri/src/logging.rs:87:5
   |
87 |     tracing::subscriber::set_global_default(subscriber).expect("Set global default log subscriber");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `std::sync::mpsc::Sender<tauri_runtime_wry::Message>` cannot be shared between threads safely
   |
  ::: /home/thecallsign/.cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/tracing-0.1.26/src/subscriber.rs:41:21
   |
41 |     S: Subscriber + Send + Sync + 'static,
   |                     ---- required by this bound in `tracing::subscriber::set_global_default`
   |
   = help: within `AppHandle`, the trait `std::marker::Sync` is not implemented for `std::sync::mpsc::Sender<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tao::platform_impl::platform::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tao::event_loop::EventLoopProxy<tauri_runtime_wry::Message>`
   = note: required because it appears within the type `tauri_runtime_wry::DispatcherContext`
   = note: required because it appears within the type `tauri_runtime_wry::WryHandle`
   = note: required because it appears within the type `AppHandle`
   = note: required because of the requirements on the impl of `std::marker::Sync` for `parking_lot::lock_api::RwLock<parking_lot::RawRwLock, AppHandle>`
   = note: required because of the requirements on the impl of `std::marker::Send` for `Arc<parking_lot::lock_api::RwLock<parking_lot::RawRwLock, AppHandle>>`

[wusyong]

I believe tao::event_loop::EventLoopProxy should be Send only.
It turns out glib::Sender is actually Send + Sync.
Thanks for notifying this!
@lucasfernog Should AppHandle be Send + Sync? Or it just needs Send to send the messages?

[lucasfernog]

I think AppHandle only needs to be Send since it implements Clone so it's easy to manage it. But it needs to be consistent across platforms.

[wusyong]

For EventLoopProxy it should be Send only imho.
So if that's the only thing that affect AppHandle, I think it should be Send only too.
I'll check it across platform later.

[lucasfernog]

I'll take a look on Windows.

[lucasfernog]

I just checked https://docs.rs/tao/0.4.0/tao/event_loop/struct.EventLoopProxy.html and https://docs.rs/tao/0.4.0/x86_64-pc-windows-msvc/tao/event_loop/struct.EventLoopProxy.html
the proxy is Sync on Linux, and !Sync on Windows. Maybe we should force them to be !Sync on all platforms (the proxy and also AppHandle).

[wusyong]

I'm thinking about maybe we could use channel from crossbeam since the performance from std is quite slow anyway.
It's sender is also Send + Sync that could ease our pain a lot

[lucasfernog]

@wusyong can you patch it in tao? Yesterday I saw that tao 0.5 has a breaking change on macOS because it's no longer Sync (it used to work, now the same code only works on Linux and I had to make some changes).

[lucasfernog]

AppHandle will be Sync on Windows on the next release, @wusyong fixed it here: tauri-apps/tao#181