sp_runtime_interface/wasm.rs
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// This file is part of Substrate.
// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! Traits required by the runtime interface from the wasm side.
use crate::RIType;
use core::cell::Cell;
/// A type used as a return value in a host function. Can be created from an FFI value.
///
/// Implementations are safe to assume that the `arg` given to `from_ffi_value`
/// is only generated by the corresponding [`host::IntoFFIValue`](crate::host::IntoFFIValue)
/// implementation.
pub trait FromFFIValue: Sized + RIType {
/// Create `Self::Inner` from the given FFI value.
fn from_ffi_value(arg: Self::FFIType) -> Self::Inner;
}
/// A type used as a parameter in a host function. Can be turned into an FFI value.
pub trait IntoFFIValue: RIType {
/// Destructor for the value passed into `into_ffi_value`.
type Destructor;
/// Convert `Self::Inner` into an FFI type, with an optional destructor.
fn into_ffi_value(value: &mut Self::Inner) -> (Self::FFIType, Self::Destructor);
}
/// The state of an exchangeable function.
#[derive(Clone, Copy)]
enum ExchangeableFunctionState {
/// Original function is present
Original,
/// The function has been replaced.
Replaced,
}
/// A function which implementation can be exchanged.
///
/// Internally this works by swapping function pointers.
pub struct ExchangeableFunction<T>(Cell<(T, ExchangeableFunctionState)>);
impl<T> ExchangeableFunction<T> {
/// Create a new instance of `ExchangeableFunction`.
pub const fn new(impl_: T) -> Self {
Self(Cell::new((impl_, ExchangeableFunctionState::Original)))
}
}
impl<T: Copy> ExchangeableFunction<T> {
/// Replace the implementation with `new_impl`.
///
/// # Panics
///
/// Panics when trying to replace an already replaced implementation.
///
/// # Returns
///
/// Returns the original implementation wrapped in [`RestoreImplementation`].
pub fn replace_implementation(&'static self, new_impl: T) -> RestoreImplementation<T> {
if let ExchangeableFunctionState::Replaced = self.0.get().1 {
panic!("Trying to replace an already replaced implementation!")
}
let old = self.0.replace((new_impl, ExchangeableFunctionState::Replaced));
RestoreImplementation(self, Some(old.0))
}
/// Restore the original implementation.
fn restore_orig_implementation(&self, orig: T) {
self.0.set((orig, ExchangeableFunctionState::Original));
}
/// Returns the internal function pointer.
pub fn get(&self) -> T {
self.0.get().0
}
}
// Wasm does not support threads, so this is safe; qed.
unsafe impl<T> Sync for ExchangeableFunction<T> {}
/// Restores a function implementation on drop.
///
/// Stores a static reference to the function object and the original implementation.
pub struct RestoreImplementation<T: 'static + Copy>(&'static ExchangeableFunction<T>, Option<T>);
impl<T: Copy> Drop for RestoreImplementation<T> {
fn drop(&mut self) {
self.0
.restore_orig_implementation(self.1.take().expect("Value is only taken on drop; qed"));
}
}