Should `ReaperSession` be `Send`?

[Boscop]

After updating to the most recent vst-rs (enabled by https://github.com/helgoboss/reaper-rs/issues/14), Plugin must be Send now. So I get this error:

What's the recommended fix? :)

[helgoboss]

Mmh, rather not. ReaperSession is something that should be created in the main thread and stay in the main thread. Why would you send it between threads?

[Boscop]

I just need to keep it alive to prevent my control surface from being dropped.

[helgoboss]

If this *mut c_void in PluginContext is the only reason why it doesn't implement Send, I guess we could make it Send. The void pointers are not like Rc in that they have some single-threaded reference counting or similar. And apart from it, making it Send wouldn't make things more unsafe than they already are anyway. If you use main-thread-only REAPER functions in another thread than in the one in which you created ReaperSession, reaper-rs will panic at runtime.

Do you want to create a PR?

[Boscop]

I would. But it's not just this one pointer:

error: src\lib.rs:124: `*mut std::ffi::c_void` cannot be sent between threads safely
error: src\lib.rs:124: `*mut std::ffi::c_void` cannot be sent between threads safely
help: src\lib.rs:124: within `MyPlugin`, the trait `std::marker::Send` is not implemented for `*mut std::ffi::c_void`
note: src\lib.rs:124: required because it appears within the type `reaper_low::PluginContext`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<reaper_low::PluginContext>`
note: src\lib.rs:124: required because it appears within the type `reaper_low::Reaper`
note: src\lib.rs:124: required because it appears within the type `reaper_medium::Reaper`
note: src\lib.rs:124: required because it appears within the type `ReaperSession`
note: src\lib.rs:124: required because it appears within the type `ReaperData`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<ReaperData>`
note: src\lib.rs:124: required because it appears within the type `MyPlugin`
error: src\lib.rs:1: required by a bound in this
error: C:\Users\me\.cargo\git\checkouts\vst-rs-7c987c17e49fdc9b\fd8e173\src\plugin.rs:473: required by this bound in `vst::plugin::Plugin`
error: src\lib.rs:124: `NonNull<std::ffi::c_void>` cannot be sent between threads safely
error: src\lib.rs:124: `NonNull<std::ffi::c_void>` cannot be sent between threads safely
help: src\lib.rs:124: within `(NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>)`, the trait `std::marker::Send` is not implemented for `NonNull<std::ffi::c_void>`
note: src\lib.rs:124: required because it appears within the type `(NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>)`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `hashbrown::raw::RawTable<(NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>)>`
note: src\lib.rs:124: required because it appears within the type `hashbrown::map::HashMap<NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>, RandomState>`
note: src\lib.rs:124: required because it appears within the type `HashMap<NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>>`
note: src\lib.rs:124: required because it appears within the type `ReaperSession`
note: src\lib.rs:124: required because it appears within the type `ReaperData`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<ReaperData>`
note: src\lib.rs:124: required because it appears within the type `MyPlugin`
error: src\lib.rs:1: required by a bound in this
error: C:\Users\me\.cargo\git\checkouts\vst-rs-7c987c17e49fdc9b\fd8e173\src\plugin.rs:473: required by this bound in `vst::plugin::Plugin`
error: src\lib.rs:124: `NonNull<gaccel_register_t>` cannot be sent between threads safely
error: src\lib.rs:124: `NonNull<gaccel_register_t>` cannot be sent between threads safely
help: src\lib.rs:124: within `(NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>)`, the trait `std::marker::Send` is not implemented for `NonNull<gaccel_register_t>`
note: src\lib.rs:124: required because it appears within the type `(NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>)`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `hashbrown::raw::RawTable<(NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>)>`
note: src\lib.rs:124: required because it appears within the type `hashbrown::map::HashMap<NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>, RandomState>`
note: src\lib.rs:124: required because it appears within the type `HashMap<NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>>`
note: src\lib.rs:124: required because it appears within the type `reaper_medium::infostruct_keeper::InfostructKeeper<OwnedGaccelRegister, gaccel_register_t>`
note: src\lib.rs:124: required because it appears within the type `ReaperSession`
note: src\lib.rs:124: required because it appears within the type `ReaperData`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<ReaperData>`
note: src\lib.rs:124: required because it appears within the type `MyPlugin`
error: src\lib.rs:1: required by a bound in this
error: C:\Users\me\.cargo\git\checkouts\vst-rs-7c987c17e49fdc9b\fd8e173\src\plugin.rs:473: required by this bound in `vst::plugin::Plugin`
error: src\lib.rs:124: `NonNull<audio_hook_register_t>` cannot be sent between threads safely
error: src\lib.rs:124: `NonNull<audio_hook_register_t>` cannot be sent between threads safely
help: src\lib.rs:124: within `(NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>)`, the trait `std::marker::Send` is not implemented for `NonNull<audio_hook_register_t>`
note: src\lib.rs:124: required because it appears within the type `(NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>)`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `hashbrown::raw::RawTable<(NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>)>`
note: src\lib.rs:124: required because it appears within the type `hashbrown::map::HashMap<NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>, RandomState>`
note: src\lib.rs:124: required because it appears within the type `HashMap<NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>>`
note: src\lib.rs:124: required because it appears within the type `reaper_medium::infostruct_keeper::InfostructKeeper<reaper_medium::audio_hook_register::OwnedAudioHookRegister, audio_hook_register_t>`
note: src\lib.rs:124: required because it appears within the type `ReaperSession`
note: src\lib.rs:124: required because it appears within the type `ReaperData`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<ReaperData>`
note: src\lib.rs:124: required because it appears within the type `MyPlugin`
error: src\lib.rs:1: required by a bound in this
error: C:\Users\me\.cargo\git\checkouts\vst-rs-7c987c17e49fdc9b\fd8e173\src\plugin.rs:473: required by this bound in `vst::plugin::Plugin`
error: src\lib.rs:124: `*mut HWND__` cannot be sent between threads safely
error: src\lib.rs:124: `*mut HWND__` cannot be sent between threads safely
help: src\lib.rs:124: within `MyPlugin`, the trait `std::marker::Send` is not implemented for `*mut HWND__`
note: src\lib.rs:124: required because it appears within the type `reaper_low::ExtensionPluginContext`
note: src\lib.rs:124: required because it appears within the type `reaper_low::TypeSpecificPluginContext`
note: src\lib.rs:124: required because it appears within the type `reaper_low::PluginContext`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<reaper_low::PluginContext>`
note: src\lib.rs:124: required because it appears within the type `reaper_low::Reaper`
note: src\lib.rs:124: required because it appears within the type `reaper_medium::Reaper`
note: src\lib.rs:124: required because it appears within the type `ReaperSession`
note: src\lib.rs:124: required because it appears within the type `ReaperData`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<ReaperData>`
note: src\lib.rs:124: required because it appears within the type `MyPlugin`
error: src\lib.rs:1: required by a bound in this
error: C:\Users\me\.cargo\git\checkouts\vst-rs-7c987c17e49fdc9b\fd8e173\src\plugin.rs:473: required by this bound in `vst::plugin::Plugin`
error: src\lib.rs:124: `(dyn reaper_low::IReaperControlSurface + 'static)` cannot be sent between threads safely
error: src\lib.rs:124: `(dyn reaper_low::IReaperControlSurface + 'static)` cannot be sent between threads safely
help: src\lib.rs:124: the trait `std::marker::Send` is not implemented for `(dyn reaper_low::IReaperControlSurface + 'static)`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<(dyn reaper_low::IReaperControlSurface + 'static)>`
note: src\lib.rs:124: required because it appears within the type `Box<(dyn reaper_low::IReaperControlSurface + 'static)>`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>`
note: src\lib.rs:124: required because it appears within the type `Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>`
note: src\lib.rs:124: required because it appears within the type `(NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>)`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `hashbrown::raw::RawTable<(NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>)>`
note: src\lib.rs:124: required because it appears within the type `hashbrown::map::HashMap<NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>, RandomState>`
note: src\lib.rs:124: required because it appears within the type `HashMap<NonNull<std::ffi::c_void>, Box<Box<(dyn reaper_low::IReaperControlSurface + 'static)>>>`
note: src\lib.rs:124: required because it appears within the type `ReaperSession`
note: src\lib.rs:124: required because it appears within the type `ReaperData`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<ReaperData>`
note: src\lib.rs:124: required because it appears within the type `MyPlugin`
error: src\lib.rs:1: required by a bound in this
error: C:\Users\me\.cargo\git\checkouts\vst-rs-7c987c17e49fdc9b\fd8e173\src\plugin.rs:473: required by this bound in `vst::plugin::Plugin`
error: src\lib.rs:124: `*const i8` cannot be sent between threads safely
error: src\lib.rs:124: `*const i8` cannot be sent between threads safely
help: src\lib.rs:124: within `OwnedGaccelRegister`, the trait `std::marker::Send` is not implemented for `*const i8`
note: src\lib.rs:124: required because it appears within the type `gaccel_register_t`
note: src\lib.rs:124: required because it appears within the type `OwnedGaccelRegister`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<OwnedGaccelRegister>`
note: src\lib.rs:124: required because it appears within the type `Box<OwnedGaccelRegister>`
note: src\lib.rs:124: required because it appears within the type `(NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>)`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `hashbrown::raw::RawTable<(NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>)>`
note: src\lib.rs:124: required because it appears within the type `hashbrown::map::HashMap<NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>, RandomState>`
note: src\lib.rs:124: required because it appears within the type `HashMap<NonNull<gaccel_register_t>, Box<OwnedGaccelRegister>>`
note: src\lib.rs:124: required because it appears within the type `reaper_medium::infostruct_keeper::InfostructKeeper<OwnedGaccelRegister, gaccel_register_t>`
note: src\lib.rs:124: required because it appears within the type `ReaperSession`
note: src\lib.rs:124: required because it appears within the type `ReaperData`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<ReaperData>`
note: src\lib.rs:124: required because it appears within the type `MyPlugin`
error: src\lib.rs:1: required by a bound in this
error: C:\Users\me\.cargo\git\checkouts\vst-rs-7c987c17e49fdc9b\fd8e173\src\plugin.rs:473: required by this bound in `vst::plugin::Plugin`
error: src\lib.rs:124: `(dyn OnAudioBuffer + 'static)` cannot be sent between threads safely
error: src\lib.rs:124: `(dyn OnAudioBuffer + 'static)` cannot be sent between threads safely
help: src\lib.rs:124: the trait `std::marker::Send` is not implemented for `(dyn OnAudioBuffer + 'static)`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<(dyn OnAudioBuffer + 'static)>`
note: src\lib.rs:124: required because it appears within the type `Box<(dyn OnAudioBuffer + 'static)>`
note: src\lib.rs:124: required because it appears within the type `reaper_medium::audio_hook_register::OwnedAudioHookRegister`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<reaper_medium::audio_hook_register::OwnedAudioHookRegister>`
note: src\lib.rs:124: required because it appears within the type `Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>`
note: src\lib.rs:124: required because it appears within the type `(NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>)`
note: src\lib.rs:124: required because of the requirements on the impl of `std::marker::Send` for `hashbrown::raw::RawTable<(NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>)>`
note: src\lib.rs:124: required because it appears within the type `hashbrown::map::HashMap<NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>, RandomState>`
note: src\lib.rs:124: required because it appears within the type `HashMap<NonNull<audio_hook_register_t>, Box<reaper_medium::audio_hook_register::OwnedAudioHookRegister>>`
note: src\lib.rs:124: required because it appears within the type `reaper_medium::infostruct_keeper::InfostructKeeper<reaper_medium::audio_hook_register::OwnedAudioHookRegister, audio_hook_register_t>`
note: src\lib.rs:124: required because it appears within the type `ReaperSession`
note: src\lib.rs:124: required because it appears within the type `ReaperData`
note: src\lib.rs:124: required because it appears within the type `std::option::Option<ReaperData>`
note: src\lib.rs:124: required because it appears within the type `MyPlugin`
error: src\lib.rs:1: required by a bound in this
error: C:\Users\me\.cargo\git\checkouts\vst-rs-7c987c17e49fdc9b\fd8e173\src\plugin.rs:473: required by this bound in `vst::plugin::Plugin`

Is it safe (i.e. not more unsafe than it already is) to make all necessary types Send?

[helgoboss]

I think yes. This all boils down to the same thing: Raw pointers. And that OnAudioBuffer trait object - which I think should really have a Send bound anyway because it needs to be sent from the main thread to the audio thread to make sense.

But I can't tell for sure. This would need some thinking time to check thoroughly.

For the moment you could implement Send unsafely for your plug-in and see how that goes.

[helgoboss]

I had to make ReaperSession and a bunch of other things Send now anyway because of the requirements of the new vst crate version. Having looked into it a bit closer, I'm pretty sure those raw pointers don't do any harm when being sent to another thread.