1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145

// 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;

/// Something that can be created from a ffi value.
///
/// # Safety
///
/// It is unsafe behavior to call `Something::into_ffi_value().get()` and take this as input for
/// `from_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` from the given ffi value.
	fn from_ffi_value(arg: Self::FFIType) -> Self;
}

/// Something that can be converted into a ffi value.
pub trait IntoFFIValue: RIType {
	/// The owned rust type that is stored with the ffi value in [`WrappedFFIValue`].
	///
	/// If no owned value is required, `()` can be used as a type.
	type Owned;

	/// Convert `self` into a [`WrappedFFIValue`].
	fn into_ffi_value(&self) -> WrappedFFIValue<Self::FFIType, Self::Owned>;
}

/// Represents a wrapped ffi value.
///
/// It is either the ffi value itself or the ffi value plus some other owned value. By providing
/// support for storing another owned value besides the actual ffi value certain performance
/// optimizations can be applied. For example using the pointer to a `Vec<u8>`, while using the
/// pointer to a SCALE encoded `Vec<u8>` that is stored in this wrapper for any other `Vec<T>`.
pub enum WrappedFFIValue<T, O = ()> {
	Wrapped(T),
	WrappedAndOwned(T, O),
}

impl<T: Copy, O> WrappedFFIValue<T, O> {
	/// Returns the wrapped ffi value.
	pub fn get(&self) -> T {
		match self {
			Self::Wrapped(data) | Self::WrappedAndOwned(data, _) => *data,
		}
	}
}

impl<T, O> From<T> for WrappedFFIValue<T, O> {
	fn from(val: T) -> Self {
		WrappedFFIValue::Wrapped(val)
	}
}

impl<T, O> From<(T, O)> for WrappedFFIValue<T, O> {
	fn from(val: (T, O)) -> Self {
		WrappedFFIValue::WrappedAndOwned(val.0, val.1)
	}
}

/// 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"));
	}
}