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
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
use std::ptr::null_mut;
use std::mem::drop;
#[repr(usize)]
enum SingletonState {
Initial = 0,
Loading = 1,
Ready = 2,
Finalized = 3,
}
pub struct Singleton<T: Send + Sync> {
#[doc(hidden)]
pub state: AtomicUsize,
#[doc(hidden)]
pub ptr: AtomicPtr<T>,
}
#[macro_export]
macro_rules! make_singleton {
() => {
$crate::Singleton {
state: ::std::sync::atomic::AtomicUsize::new(0),
ptr: ::std::sync::atomic::AtomicPtr::new(::std::ptr::null_mut())
}
};
}
impl<T: Send + Sync> Default for Singleton<T> {
fn default() -> Self {
make_singleton!()
}
}
impl<T: Send + Sync> Singleton<T> {
#[cfg(feature = "const_fn")]
pub const fn new() -> Self {
make_singleton!()
}
#[cfg(not(feature = "const_fn"))]
pub fn new() -> Self {
make_singleton!()
}
pub fn get(&self) -> &T
where
T: Default,
{
self.get_or_insert_with(<T as Default>::default)
}
pub fn get_opt(&self) -> Option<&T> {
unsafe { self.ptr.load(Ordering::SeqCst).as_ref() }
}
fn error_stateshift() {
panic!("singleton: state shifted during singleton initialization. Maybe caused by unsafe finalized() calling. ");
}
fn error_finalized() {
panic!("singleton: trying to access a finalized singleton. Maybe caused by unsafe finalized() calling. ");
}
pub fn get_or_insert_with<F>(&self, f: F) -> &T
where
F: FnOnce() -> T,
{
if let Some(v) = unsafe { self.ptr.load(Ordering::SeqCst).as_ref() } {
return v;
}
let mut cur_state = self.state.compare_and_swap(
SingletonState::Initial as _,
SingletonState::Loading as _,
Ordering::SeqCst,
);
'spin: loop {
if cur_state == SingletonState::Loading as _ {
cur_state = self.state.load(Ordering::SeqCst);
continue 'spin;
} else if cur_state == SingletonState::Initial as _
|| cur_state == SingletonState::Ready as _
{
if cur_state == SingletonState::Initial as _ {
let v = Box::into_raw(Box::new(f()));
self.ptr.store(v, Ordering::SeqCst);
cur_state = self.state.compare_and_swap(
SingletonState::Loading as _,
SingletonState::Ready as _,
Ordering::SeqCst,
);
if cur_state != SingletonState::Loading as _ {
Self::error_stateshift();
unreachable!();
}
}
if let Some(v) = unsafe { self.ptr.load(Ordering::SeqCst).as_ref() } {
return v;
} else {
Self::error_stateshift();
unreachable!();
}
}
Self::error_finalized();
unreachable!();
}
}
pub unsafe fn finalize(&self) {
self.state
.store(SingletonState::Finalized as _, Ordering::SeqCst);
let old_ptr = self.ptr.swap(null_mut(), Ordering::SeqCst);
if old_ptr.is_null() {
return;
}
drop(Box::from_raw(old_ptr));
}
}
impl<T: Send + Sync> Drop for Singleton<T> {
fn drop(&mut self) {
unsafe {
self.finalize();
}
}
}
#[cfg(test)]
mod tests {
use super::Singleton;
struct A(usize);
impl Default for A {
fn default() -> Self {
A(42)
}
}
struct B(usize);
impl Default for B {
fn default() -> Self {
B(100)
}
}
static SINGLETON_A: Singleton<A> = make_singleton!();
static SINGLETON_B: Singleton<B> = make_singleton!();
#[test]
fn it_works() {
assert!(SINGLETON_A.get_opt().is_none());
assert!(SINGLETON_B.get_opt().is_none());
let a1 = SINGLETON_A.get();
assert!(!SINGLETON_A.get_opt().is_none());
let a2 = SINGLETON_A.get();
assert_eq!(a1 as *const _, a2 as *const _);
let _b = SINGLETON_B.get();
assert!(!SINGLETON_B.get_opt().is_none());
}
}