Scope-owned permits for Bitcoin concurrency limits

1// Copyright (c) 2009-2010 Satoshi Nakamoto
2// Copyright (c) 2009-present The Bitcoin Core developers
3// Distributed under the MIT software license, see the accompanying
4// file COPYING or http://www.opensource.org/licenses/mit-license.php.
5
6#ifndef BITCOIN_SEMAPHORE_GRANT_H
7#define BITCOIN_SEMAPHORE_GRANT_H
8
9#include <semaphore>
10
11/** RAII-style semaphore lock */
12template <std::ptrdiff_t LeastMaxValue = std::counting_semaphore<>::max()>
13class CountingSemaphoreGrant
14{
15private:
16    std::counting_semaphore<LeastMaxValue>* sem;
17    bool fHaveGrant;
18
19public:
20    void Acquire() noexcept
21    {
22        if (fHaveGrant) {
23            return;
24        }
25        sem->acquire();
26        fHaveGrant = true;
27    }
28
29    void Release() noexcept
30    {
31        if (!fHaveGrant) {
32            return;
33        }
34        sem->release();
35        fHaveGrant = false;
36    }
37
38    bool TryAcquire() noexcept
39    {
40        if (!fHaveGrant && sem->try_acquire()) {
41            fHaveGrant = true;
42        }
43        return fHaveGrant;
44    }
45
46    // Disallow copy.
47    CountingSemaphoreGrant(const CountingSemaphoreGrant&) = delete;
48    CountingSemaphoreGrant& operator=(const CountingSemaphoreGrant&) = delete;
49
50    // Allow move.
51    CountingSemaphoreGrant(CountingSemaphoreGrant&& other) noexcept
52    {
53        sem = other.sem;
54        fHaveGrant = other.fHaveGrant;
55        other.fHaveGrant = false;
56        other.sem = nullptr;
57    }
58
59    CountingSemaphoreGrant& operator=(CountingSemaphoreGrant&& other) noexcept
60    {
61        Release();
62        sem = other.sem;
63        fHaveGrant = other.fHaveGrant;
64        other.fHaveGrant = false;
65        other.sem = nullptr;
66        return *this;
67    }
68
69    CountingSemaphoreGrant() noexcept : sem(nullptr), fHaveGrant(false) {}
70
71    explicit CountingSemaphoreGrant(std::counting_semaphore<LeastMaxValue>& sema, bool fTry = false) noexcept : sem(&sema), fHaveGrant(false)
72    {
73        if (fTry) {
74            TryAcquire();
75        } else {
76            Acquire();
77        }
78    }
79
80    ~CountingSemaphoreGrant()
81    {
82        Release();
83    }
84
85    explicit operator bool() const noexcept
86    {
87        return fHaveGrant;
88    }
89};
90
91using BinarySemaphoreGrant = CountingSemaphoreGrant<1>;
92
93#endif // BITCOIN_SEMAPHORE_GRANT_H
94