sled 0.32.0

a modern embedded database
Documentation 
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

#[cfg(feature = "testing")]
use std::cell::RefCell;
use std::sync::atomic::AtomicBool;

use parking_lot::{RwLockReadGuard, RwLockWriteGuard};

use super::*;

#[cfg(feature = "testing")]
thread_local! {
    pub static COUNT: RefCell<u32> = RefCell::new(0);
}

#[derive(Default)]
pub(crate) struct ConcurrencyControl {
    necessary: AtomicBool,
    active_non_lockers: AtomicUsize,
    upgrade_complete: AtomicBool,
    rw: RwLock<()>,
}

static CONCURRENCY_CONTROL: Lazy<
    ConcurrencyControl,
    fn() -> ConcurrencyControl,
> = Lazy::new(init_cc);

fn init_cc() -> ConcurrencyControl {
    ConcurrencyControl::default()
}

#[derive(Debug)]
#[must_use]
pub(crate) enum Protector<'a> {
    Write(RwLockWriteGuard<'a, ()>),
    Read(RwLockReadGuard<'a, ()>),
    None(&'a AtomicUsize),
}

impl<'a> Drop for Protector<'a> {
    fn drop(&mut self) {
        if let Protector::None(active_non_lockers) = self {
            active_non_lockers.fetch_sub(1, Release);
        }
        #[cfg(feature = "testing")]
        COUNT.with(|c| {
            let mut c = c.borrow_mut();
            *c -= 1;
            assert_eq!(*c, 0);
        });
    }
}

pub(crate) fn read<'a>() -> Protector<'a> {
    CONCURRENCY_CONTROL.read()
}

pub(crate) fn write<'a>() -> Protector<'a> {
    CONCURRENCY_CONTROL.write()
}

impl ConcurrencyControl {
    fn enable(&self) {
        if !self.necessary.load(Acquire) && !self.necessary.swap(true, SeqCst) {
            while self.active_non_lockers.load(Acquire) != 0 {
                std::sync::atomic::spin_loop_hint()
            }
            self.upgrade_complete.store(true, Release);
        }
    }

    fn read(&self) -> Protector<'_> {
        #[cfg(feature = "testing")]
        COUNT.with(|c| {
            let mut c = c.borrow_mut();
            *c += 1;
            assert_eq!(*c, 1);
        });

        if self.necessary.load(Acquire) {
            Protector::Read(self.rw.read())
        } else {
            self.active_non_lockers.fetch_add(1, Release);
            Protector::None(&self.active_non_lockers)
        }
    }

    fn write(&self) -> Protector<'_> {
        #[cfg(feature = "testing")]
        COUNT.with(|c| {
            let mut c = c.borrow_mut();
            *c += 1;
            assert_eq!(*c, 1);
        });
        self.enable();
        while !self.upgrade_complete.load(Acquire) {
            std::sync::atomic::spin_loop_hint()
        }
        Protector::Write(self.rw.write())
    }
}