mirror of
https://github.com/fluencelabs/wasm-bindgen
synced 2025-05-03 09:32:16 +00:00
42053ddd4e
Currently closure shims are communicated to JS at runtime, although at runtime the same constant value is always passed to JS! More pressing, however, work in #1002 requires knowledge of closure descriptor indices at `wasm-bindgen` time which is not currently known. Since the closure descriptor shims and such are already constant values, this commit moves the descriptor function indices into the *descriptor* for a closure/function pointer. This way we can learn about these values at `wasm-bindgen` time instead of only knowing them at runtime. This should have no semantic change on users of `wasm-bindgen`, although some closure invocations may be slightly speedier because there's less arguments being transferred over the boundary. Overall though this will help #1002 as the closure shims that the Rust compiler generates may not be the exact ones we hand out to JS, but rather wrappers around them which do `anyref` business things.
418 lines
15 KiB
Rust
418 lines
15 KiB
Rust
//! Support for long-lived closures in `wasm-bindgen`
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//!
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//! This module defines the `Closure` type which is used to pass "owned
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//! closures" from Rust to JS. Some more details can be found on the `Closure`
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//! type itself.
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#[cfg(feature = "nightly")]
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use std::marker::Unsize;
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use std::mem::{self, ManuallyDrop};
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use std::prelude::v1::*;
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use JsValue;
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use convert::*;
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use describe::*;
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use throw_str;
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/// A handle to both a closure in Rust as well as JS closure which will invoke
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/// the Rust closure.
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///
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/// A `Closure` is the primary way that a `'static` lifetime closure is
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/// transferred from Rust to JS. `Closure` currently requires that the closures
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/// it's created with have the `'static` lifetime in Rust for soundness reasons.
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///
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/// This type is a "handle" in the sense that whenever it is dropped it will
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/// invalidate the JS closure that it refers to. Any usage of the closure in JS
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/// after the `Closure` has been dropped will raise an exception. It's then up
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/// to you to arrange for `Closure` to be properly deallocate at an appropriate
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/// location in your program.
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///
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/// The type parameter on `Closure` is the type of closure that this represents.
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/// Currently this can only be the `Fn` and `FnMut` traits with up to 7
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/// arguments (and an optional return value). The arguments/return value of the
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/// trait must be numbers like `u32` for now, although this restriction may be
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/// lifted in the future!
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///
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/// # Example
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///
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/// Sample usage of `Closure` to invoke the `setTimeout` API.
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///
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/// ```rust,no_run
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/// #[wasm_bindgen]
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/// extern "C" {
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/// fn setTimeout(closure: &Closure<FnMut()>, time: u32);
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///
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/// #[wasm_bindgen(js_namespace = console)]
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/// fn log(s: &str);
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/// }
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///
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/// #[wasm_bindgen]
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/// pub struct ClosureHandle(Closure<FnMut()>);
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///
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/// #[wasm_bindgen]
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/// pub fn run() -> ClosureHandle {
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/// // First up we use `Closure::wrap` to wrap up a Rust closure and create
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/// a JS closure.
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/// let cb = Closure::wrap(Box::new(move || {
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/// log("timeout elapsed!");
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/// }) as Box<FnMut()>);
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///
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/// // Next we pass this via reference to the `setTimeout` function, and
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/// // `setTimeout` gets a handle to the corresponding JS closure.
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/// setTimeout(&cb, 1_000);
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///
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/// // If we were to drop `cb` here it would cause an exception to be raised
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/// // when the timeout elapses. Instead we *return* our handle back to JS
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/// // so JS can tell us later when it would like to deallocate this handle.
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/// ClosureHandle(cb)
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/// }
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/// ```
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///
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/// Sample usage of the same example as above except using `web_sys` instead
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///
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/// ```rust,no_run
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/// extern crate wasm_bindgen;
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/// extern crate web_sys;
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///
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/// use wasm_bindgen::JsCast;
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///
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/// #[wasm_bindgen]
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/// pub struct ClosureHandle(Closure<FnMut()>);
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///
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/// #[wasm_bindgen]
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/// pub fn run() -> ClosureHandle {
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/// let cb = Closure::wrap(Box::new(move || {
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/// web_sys::console::log_1(&"timeout elapsed!".into());
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/// }) as Box<FnMut()>);
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///
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/// let window = web_sys::window().unwrap();
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/// window.set_timeout_with_callback_and_timeout_and_arguments_0(
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/// // Note this method call, which uses `as_ref()` to get a `JsValue`
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/// // from our `Closure` which is then converted to a `&Function`
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/// // using the `JsCast::unchecked_ref` function.
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/// cb.as_ref().unchecked_ref(),
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/// 1_000,
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/// );
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///
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/// // same as above
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/// ClosureHandle(cb)
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/// }
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/// ```
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pub struct Closure<T: ?Sized> {
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js: ManuallyDrop<JsValue>,
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data: ManuallyDrop<Box<T>>,
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}
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union FatPtr<T: ?Sized> {
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ptr: *mut T,
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fields: (usize, usize),
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}
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impl<T> Closure<T>
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where T: ?Sized + WasmClosure,
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{
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/// Creates a new instance of `Closure` from the provided Rust closure.
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///
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/// Note that the closure provided here, `F`, has a few requirements
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/// associated with it:
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///
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/// * It must implement `Fn` or `FnMut`
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/// * It must be `'static`, aka no stack references (use the `move` keyword)
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/// * It can have at most 7 arguments
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/// * Its arguments and return values are all wasm types like u32/f64.
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///
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/// This is unfortunately pretty restrictive for now but hopefully some of
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/// these restrictions can be lifted in the future!
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///
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/// *This method requires the `nightly` feature of the `wasm-bindgen` crate
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/// to be enabled, meaning this is a nightly-only API. Users on stable
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/// should use `Closure::wrap`.*
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#[cfg(feature = "nightly")]
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pub fn new<F>(t: F) -> Closure<T>
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where F: Unsize<T> + 'static
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{
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Closure::wrap(Box::new(t) as Box<T>)
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}
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/// A mostly internal function to wrap a boxed closure inside a `Closure`
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/// type.
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///
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/// This is the function where the JS closure is manufactured.
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pub fn wrap(mut data: Box<T>) -> Closure<T> {
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assert_eq!(mem::size_of::<*const T>(), mem::size_of::<FatPtr<T>>());
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let (a, b) = unsafe {
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FatPtr { ptr: &mut *data as *mut T }.fields
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};
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// Here we need to create a `JsValue` with the data and `T::invoke()`
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// function pointer. To do that we... take a few unconventional turns.
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// In essence what happens here is this:
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//
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// 1. First up, below we call a function, `breaks_if_inlined`. This
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// function, as the name implies, does not work if it's inlined.
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// More on that in a moment.
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// 2. This function internally calls a special import recognized by the
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// `wasm-bindgen` CLI tool, `__wbindgen_describe_closure`. This
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// imported symbol is similar to `__wbindgen_describe` in that it's
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// not intended to show up in the final binary but it's an
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// intermediate state for a `wasm-bindgen` binary.
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// 3. The `__wbindgen_describe_closure` import is namely passed a
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// descriptor function, monomorphized for each invocation.
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//
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// Most of this doesn't actually make sense to happen at runtime! The
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// real magic happens when `wasm-bindgen` comes along and updates our
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// generated code. When `wasm-bindgen` runs it performs a few tasks:
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//
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// * First, it finds all functions that call
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// `__wbindgen_describe_closure`. These are all `breaks_if_inlined`
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// defined below as the symbol isn't called anywhere else.
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// * Next, `wasm-bindgen` executes the `breaks_if_inlined`
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// monomorphized functions, passing it dummy arguments. This will
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// execute the function just enough to invoke the special import,
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// namely telling us about the function pointer that is the describe
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// shim.
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// * This knowledge is then used to actually find the descriptor in the
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// function table which is then executed to figure out the signature
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// of the closure.
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// * Finally, and probably most heinously, the call to
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// `breaks_if_inlined` is rewritten to call an otherwise globally
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// imported function. This globally imported function will generate
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// the `JsValue` for this closure specialized for the signature in
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// question.
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//
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// Later on `wasm-gc` will clean up all the dead code and ensure that
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// we don't actually call `__wbindgen_describe_closure` at runtime. This
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// means we will end up not actually calling `breaks_if_inlined` in the
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// final binary, all calls to that function should be pruned.
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//
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// See crates/cli-support/src/js/closures.rs for a more information
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// about what's going on here.
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extern "C" fn describe<T: WasmClosure + ?Sized>() {
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inform(CLOSURE);
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T::describe()
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}
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#[inline(never)]
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unsafe fn breaks_if_inlined<T: WasmClosure + ?Sized>(
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a: usize,
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b: usize,
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) -> u32 {
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super::__wbindgen_describe_closure(
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a as u32,
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b as u32,
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describe::<T> as u32,
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)
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}
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let idx = unsafe {
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breaks_if_inlined::<T>(a, b)
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};
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Closure {
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js: ManuallyDrop::new(JsValue::_new(idx)),
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data: ManuallyDrop::new(data),
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}
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}
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/// Leaks this `Closure` to ensure it remains valid for the duration of the
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/// entire program.
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///
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/// > **Note**: this function will leak memory. It should be used sparingly
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/// > to ensure the memory leak doesn't affect the program too much.
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///
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/// When a `Closure` is dropped it will invalidate the associated JS
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/// closure, but this isn't always desired. Some callbacks are alive for
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/// the entire duration of the program, so this can be used to conveniently
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/// leak this instance of `Closure` while performing as much internal
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/// cleanup as it can.
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pub fn forget(self) {
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unsafe {
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super::__wbindgen_cb_forget(self.js.idx);
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mem::forget(self);
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}
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}
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}
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impl<T: ?Sized> AsRef<JsValue> for Closure<T> {
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fn as_ref(&self) -> &JsValue {
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&self.js
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}
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}
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impl<T> WasmDescribe for Closure<T>
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where T: WasmClosure + ?Sized,
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{
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fn describe() {
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inform(ANYREF);
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}
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}
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// `Closure` can only be passed by reference to imports.
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impl<'a, T> IntoWasmAbi for &'a Closure<T>
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where T: WasmClosure + ?Sized,
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{
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type Abi = u32;
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fn into_abi(self, extra: &mut Stack) -> u32 {
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(&*self.js).into_abi(extra)
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}
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}
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fn _check() {
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fn _assert<T: IntoWasmAbi>() {}
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_assert::<&Closure<Fn()>>();
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_assert::<&Closure<Fn(String)>>();
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_assert::<&Closure<Fn() -> String>>();
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_assert::<&Closure<FnMut()>>();
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_assert::<&Closure<FnMut(String)>>();
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_assert::<&Closure<FnMut() -> String>>();
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}
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impl<T> Drop for Closure<T>
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where T: ?Sized,
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{
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fn drop(&mut self) {
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unsafe {
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// this will implicitly drop our strong reference in addition to
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// invalidating all future invocations of the closure
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if super::__wbindgen_cb_drop(self.js.idx) != 0 {
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ManuallyDrop::drop(&mut self.data);
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}
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}
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}
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}
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/// An internal trait for the `Closure` type.
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///
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/// This trait is not stable and it's not recommended to use this in bounds or
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/// implement yourself.
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#[doc(hidden)]
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pub unsafe trait WasmClosure: 'static {
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fn describe();
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}
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// The memory safety here in these implementations below is a bit tricky. We
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// want to be able to drop the `Closure` object from within the invocation of a
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// `Closure` for cases like promises. That means that while it's running we
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// might drop the `Closure`, but that shouldn't invalidate the environment yet.
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//
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// Instead what we do is to wrap closures in `Rc` variables. The main `Closure`
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// has a strong reference count which keeps the trait object alive. Each
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// invocation of a closure then *also* clones this and gets a new reference
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// count. When the closure returns it will release the reference count.
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//
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// This means that if the main `Closure` is dropped while it's being invoked
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// then destruction is deferred until execution returns. Otherwise it'll
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// deallocate data immediately.
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macro_rules! doit {
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($(
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($($var:ident)*)
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)*) => ($(
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unsafe impl<$($var,)* R> WasmClosure for Fn($($var),*) -> R
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where $($var: FromWasmAbi + 'static,)*
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R: ReturnWasmAbi + 'static,
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{
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fn describe() {
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#[allow(non_snake_case)]
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unsafe extern "C" fn invoke<$($var: FromWasmAbi,)* R: ReturnWasmAbi>(
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a: usize,
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b: usize,
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$($var: <$var as FromWasmAbi>::Abi),*
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) -> <R as ReturnWasmAbi>::Abi {
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if a == 0 {
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throw_str("closure invoked recursively or destroyed already");
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}
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// Make sure all stack variables are converted before we
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// convert `ret` as it may throw (for `Result`, for
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// example)
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let ret = {
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let f: *const Fn($($var),*) -> R =
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FatPtr { fields: (a, b) }.ptr;
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let mut _stack = GlobalStack::new();
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$(
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let $var = <$var as FromWasmAbi>::from_abi($var, &mut _stack);
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)*
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(*f)($($var),*)
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};
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ret.return_abi(&mut GlobalStack::new())
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}
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inform(invoke::<$($var,)* R> as u32);
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unsafe extern fn destroy<$($var: FromWasmAbi,)* R: ReturnWasmAbi>(
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a: usize,
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b: usize,
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) {
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debug_assert!(a != 0);
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drop(Box::from_raw(FatPtr::<Fn($($var,)*) -> R> {
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fields: (a, b)
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}.ptr));
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}
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inform(destroy::<$($var,)* R> as u32);
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<&Self>::describe();
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}
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}
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unsafe impl<$($var,)* R> WasmClosure for FnMut($($var),*) -> R
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where $($var: FromWasmAbi + 'static,)*
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R: ReturnWasmAbi + 'static,
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{
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fn describe() {
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#[allow(non_snake_case)]
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unsafe extern "C" fn invoke<$($var: FromWasmAbi,)* R: ReturnWasmAbi>(
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a: usize,
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b: usize,
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$($var: <$var as FromWasmAbi>::Abi),*
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) -> <R as ReturnWasmAbi>::Abi {
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if a == 0 {
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throw_str("closure invoked recursively or destroyed already");
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}
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// Make sure all stack variables are converted before we
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// convert `ret` as it may throw (for `Result`, for
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// example)
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let ret = {
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let f: *const FnMut($($var),*) -> R =
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FatPtr { fields: (a, b) }.ptr;
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let f = f as *mut FnMut($($var),*) -> R;
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let mut _stack = GlobalStack::new();
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$(
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let $var = <$var as FromWasmAbi>::from_abi($var, &mut _stack);
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)*
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(*f)($($var),*)
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};
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ret.return_abi(&mut GlobalStack::new())
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}
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inform(invoke::<$($var,)* R> as u32);
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unsafe extern fn destroy<$($var: FromWasmAbi,)* R: ReturnWasmAbi>(
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a: usize,
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b: usize,
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) {
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debug_assert!(a != 0);
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drop(Box::from_raw(FatPtr::<FnMut($($var,)*) -> R> {
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fields: (a, b)
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}.ptr));
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}
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inform(destroy::<$($var,)* R> as u32);
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<&mut Self>::describe();
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}
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}
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)*)
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}
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doit! {
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()
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(A)
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(A B)
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(A B C)
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(A B C D)
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(A B C D E)
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(A B C D E F)
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(A B C D E F G)
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}
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