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//! Support for long-lived closures in `wasm-bindgen` //! //! This module defines the `Closure` type which is used to pass "owned //! closures" from Rust to JS. Some more details can be found on the `Closure` //! type itself. #[cfg(feature = "nightly")] use std::marker::Unsize; use std::mem::{self, ManuallyDrop}; use std::prelude::v1::*; use JsValue; use convert::*; use describe::*; use throw_str; /// A handle to both a closure in Rust as well as JS closure which will invoke /// the Rust closure. /// /// A `Closure` is the primary way that a `'static` lifetime closure is /// transferred from Rust to JS. `Closure` currently requires that the closures /// it's created with have the `'static` lifetime in Rust for soundness reasons. /// /// This type is a "handle" in the sense that whenever it is dropped it will /// invalidate the JS closure that it refers to. Any usage of the closure in JS /// after the `Closure` has been dropped will raise an exception. It's then up /// to you to arrange for `Closure` to be properly deallocate at an appropriate /// location in your program. /// /// The type parameter on `Closure` is the type of closure that this represents. /// Currently this can only be the `Fn` and `FnMut` traits with up to 7 /// arguments (and an optional return value). The arguments/return value of the /// trait must be numbers like `u32` for now, although this restriction may be /// lifted in the future! /// /// # Example /// /// Sample usage of `Closure` to invoke the `setTimeout` API. /// /// ```rust,no_run /// #[wasm_bindgen] /// extern "C" { /// fn setTimeout(closure: &Closure<FnMut()>, time: u32); /// /// #[wasm_bindgen(js_namespace = console)] /// fn log(s: &str); /// } /// /// #[wasm_bindgen] /// pub struct ClosureHandle(Closure<FnMut()>); /// /// #[wasm_bindgen] /// pub fn run() -> ClosureHandle { /// // First up we use `Closure::wrap` to wrap up a Rust closure and create /// // a JS closure. /// let cb = Closure::wrap(Box::new(move || { /// log("timeout elapsed!"); /// }) as Box<FnMut()>); /// /// // Next we pass this via reference to the `setTimeout` function, and /// // `setTimeout` gets a handle to the corresponding JS closure. /// setTimeout(&cb, 1_000); /// /// // If we were to drop `cb` here it would cause an exception to be raised /// // when the timeout elapses. Instead we *return* our handle back to JS /// // so JS can tell us later when it would like to deallocate this handle. /// ClosureHandle(cb) /// } /// ``` /// /// Sample usage of the same example as above except using `web_sys` instead /// /// ```rust,no_run /// extern crate wasm_bindgen; /// extern crate web_sys; /// /// use wasm_bindgen::JsCast; /// /// #[wasm_bindgen] /// pub struct ClosureHandle(Closure<FnMut()>); /// /// #[wasm_bindgen] /// pub fn run() -> ClosureHandle { /// let cb = Closure::wrap(Box::new(move || { /// web_sys::console::log_1(&"timeout elapsed!".into()); /// }) as Box<FnMut()>); /// /// let window = web_sys::window().unwrap(); /// window.set_timeout_with_callback_and_timeout_and_arguments_0( /// // Note this method call, which uses `as_ref()` to get a `JsValue` /// // from our `Closure` which is then converted to a `&Function` /// // using the `JsCast::unchecked_ref` function. /// cb.as_ref().unchecked_ref(), /// 1_000, /// ); /// /// // same as above /// ClosureHandle(cb) /// } /// ``` pub struct Closure<T: ?Sized> { js: ManuallyDrop<JsValue>, data: ManuallyDrop<Box<T>>, } union FatPtr<T: ?Sized> { ptr: *mut T, fields: (usize, usize), } impl<T> Closure<T> where T: ?Sized + WasmClosure, { /// Creates a new instance of `Closure` from the provided Rust closure. /// /// Note that the closure provided here, `F`, has a few requirements /// associated with it: /// /// * It must implement `Fn` or `FnMut` /// * It must be `'static`, aka no stack references (use the `move` keyword) /// * It can have at most 7 arguments /// * Its arguments and return values are all wasm types like u32/f64. /// /// This is unfortunately pretty restrictive for now but hopefully some of /// these restrictions can be lifted in the future! /// /// *This method requires the `nightly` feature of the `wasm-bindgen` crate /// to be enabled, meaning this is a nightly-only API. Users on stable /// should use `Closure::wrap`.* #[cfg(feature = "nightly")] pub fn new<F>(t: F) -> Closure<T> where F: Unsize<T> + 'static { Closure::wrap(Box::new(t) as Box<T>) } /// A mostly internal function to wrap a boxed closure inside a `Closure` /// type. /// /// This is the function where the JS closure is manufactured. pub fn wrap(mut data: Box<T>) -> Closure<T> { assert_eq!(mem::size_of::<*const T>(), mem::size_of::<FatPtr<T>>()); let (a, b) = unsafe { FatPtr { ptr: &mut *data as *mut T }.fields }; // Here we need to create a `JsValue` with the data and `T::invoke()` // function pointer. To do that we... take a few unconventional turns. // In essence what happens here is this: // // 1. First up, below we call a function, `breaks_if_inlined`. This // function, as the name implies, does not work if it's inlined. // More on that in a moment. // 2. This function internally calls a special import recognized by the // `wasm-bindgen` CLI tool, `__wbindgen_describe_closure`. This // imported symbol is similar to `__wbindgen_describe` in that it's // not intended to show up in the final binary but it's an // intermediate state for a `wasm-bindgen` binary. // 3. The `__wbindgen_describe_closure` import is namely passed a // descriptor function, monomorphized for each invocation. // // Most of this doesn't actually make sense to happen at runtime! The // real magic happens when `wasm-bindgen` comes along and updates our // generated code. When `wasm-bindgen` runs it performs a few tasks: // // * First, it finds all functions that call // `__wbindgen_describe_closure`. These are all `breaks_if_inlined` // defined below as the symbol isn't called anywhere else. // * Next, `wasm-bindgen` executes the `breaks_if_inlined` // monomorphized functions, passing it dummy arguments. This will // execute the function just enough to invoke the special import, // namely telling us about the function pointer that is the describe // shim. // * This knowledge is then used to actually find the descriptor in the // function table which is then executed to figure out the signature // of the closure. // * Finally, and probably most heinously, the call to // `breaks_if_inlined` is rewritten to call an otherwise globally // imported function. This globally imported function will generate // the `JsValue` for this closure specialized for the signature in // question. // // Later on `wasm-gc` will clean up all the dead code and ensure that // we don't actually call `__wbindgen_describe_closure` at runtime. This // means we will end up not actually calling `breaks_if_inlined` in the // final binary, all calls to that function should be pruned. // // See crates/cli-support/src/js/closures.rs for a more information // about what's going on here. extern "C" fn describe<T: WasmClosure + ?Sized>() { inform(CLOSURE); T::describe() } #[inline(never)] unsafe fn breaks_if_inlined<T: WasmClosure + ?Sized>( a: usize, b: usize, ) -> u32 { super::__wbindgen_describe_closure( a as u32, b as u32, describe::<T> as u32, ) } let idx = unsafe { breaks_if_inlined::<T>(a, b) }; Closure { js: ManuallyDrop::new(JsValue::_new(idx)), data: ManuallyDrop::new(data), } } /// Leaks this `Closure` to ensure it remains valid for the duration of the /// entire program. /// /// > **Note**: this function will leak memory. It should be used sparingly /// > to ensure the memory leak doesn't affect the program too much. /// /// When a `Closure` is dropped it will invalidate the associated JS /// closure, but this isn't always desired. Some callbacks are alive for /// the entire duration of the program, so this can be used to conveniently /// leak this instance of `Closure` while performing as much internal /// cleanup as it can. pub fn forget(self) { unsafe { super::__wbindgen_cb_forget(self.js.idx); mem::forget(self); } } } impl<T: ?Sized> AsRef<JsValue> for Closure<T> { fn as_ref(&self) -> &JsValue { &self.js } } impl<T> WasmDescribe for Closure<T> where T: WasmClosure + ?Sized, { fn describe() { inform(ANYREF); } } // `Closure` can only be passed by reference to imports. impl<'a, T> IntoWasmAbi for &'a Closure<T> where T: WasmClosure + ?Sized, { type Abi = u32; fn into_abi(self, extra: &mut Stack) -> u32 { (&*self.js).into_abi(extra) } } fn _check() { fn _assert<T: IntoWasmAbi>() {} _assert::<&Closure<Fn()>>(); _assert::<&Closure<Fn(String)>>(); _assert::<&Closure<Fn() -> String>>(); _assert::<&Closure<FnMut()>>(); _assert::<&Closure<FnMut(String)>>(); _assert::<&Closure<FnMut() -> String>>(); } impl<T> Drop for Closure<T> where T: ?Sized, { fn drop(&mut self) { unsafe { // this will implicitly drop our strong reference in addition to // invalidating all future invocations of the closure if super::__wbindgen_cb_drop(self.js.idx) != 0 { ManuallyDrop::drop(&mut self.data); } } } } /// An internal trait for the `Closure` type. /// /// This trait is not stable and it's not recommended to use this in bounds or /// implement yourself. #[doc(hidden)] pub unsafe trait WasmClosure: 'static { fn describe(); } // The memory safety here in these implementations below is a bit tricky. We // want to be able to drop the `Closure` object from within the invocation of a // `Closure` for cases like promises. That means that while it's running we // might drop the `Closure`, but that shouldn't invalidate the environment yet. // // Instead what we do is to wrap closures in `Rc` variables. The main `Closure` // has a strong reference count which keeps the trait object alive. Each // invocation of a closure then *also* clones this and gets a new reference // count. When the closure returns it will release the reference count. // // This means that if the main `Closure` is dropped while it's being invoked // then destruction is deferred until execution returns. Otherwise it'll // deallocate data immediately. macro_rules! doit { ($( ($($var:ident)*) )*) => ($( unsafe impl<$($var,)* R> WasmClosure for Fn($($var),*) -> R where $($var: FromWasmAbi + 'static,)* R: ReturnWasmAbi + 'static, { fn describe() { #[allow(non_snake_case)] unsafe extern "C" fn invoke<$($var: FromWasmAbi,)* R: ReturnWasmAbi>( a: usize, b: usize, $($var: <$var as FromWasmAbi>::Abi),* ) -> <R as ReturnWasmAbi>::Abi { if a == 0 { throw_str("closure invoked recursively or destroyed already"); } // Make sure all stack variables are converted before we // convert `ret` as it may throw (for `Result`, for // example) let ret = { let f: *const Fn($($var),*) -> R = FatPtr { fields: (a, b) }.ptr; let mut _stack = GlobalStack::new(); $( let $var = <$var as FromWasmAbi>::from_abi($var, &mut _stack); )* (*f)($($var),*) }; ret.return_abi(&mut GlobalStack::new()) } inform(invoke::<$($var,)* R> as u32); unsafe extern fn destroy<$($var: FromWasmAbi,)* R: ReturnWasmAbi>( a: usize, b: usize, ) { debug_assert!(a != 0); drop(Box::from_raw(FatPtr::<Fn($($var,)*) -> R> { fields: (a, b) }.ptr)); } inform(destroy::<$($var,)* R> as u32); <&Self>::describe(); } } unsafe impl<$($var,)* R> WasmClosure for FnMut($($var),*) -> R where $($var: FromWasmAbi + 'static,)* R: ReturnWasmAbi + 'static, { fn describe() { #[allow(non_snake_case)] unsafe extern "C" fn invoke<$($var: FromWasmAbi,)* R: ReturnWasmAbi>( a: usize, b: usize, $($var: <$var as FromWasmAbi>::Abi),* ) -> <R as ReturnWasmAbi>::Abi { if a == 0 { throw_str("closure invoked recursively or destroyed already"); } // Make sure all stack variables are converted before we // convert `ret` as it may throw (for `Result`, for // example) let ret = { let f: *const FnMut($($var),*) -> R = FatPtr { fields: (a, b) }.ptr; let f = f as *mut FnMut($($var),*) -> R; let mut _stack = GlobalStack::new(); $( let $var = <$var as FromWasmAbi>::from_abi($var, &mut _stack); )* (*f)($($var),*) }; ret.return_abi(&mut GlobalStack::new()) } inform(invoke::<$($var,)* R> as u32); unsafe extern fn destroy<$($var: FromWasmAbi,)* R: ReturnWasmAbi>( a: usize, b: usize, ) { debug_assert!(a != 0); drop(Box::from_raw(FatPtr::<FnMut($($var,)*) -> R> { fields: (a, b) }.ptr)); } inform(destroy::<$($var,)* R> as u32); <&mut Self>::describe(); } } )*) } doit! { () (A) (A B) (A B C) (A B C D) (A B C D E) (A B C D E F) (A B C D E F G) }