Merge bitcoin/bitcoin#32466: threading: drop CSemaphore in favor of c++20 std::counting_semaphore

6f7052a threading: semaphore: move CountingSemaphoreGrant to its own header (Cory Fields)
fd15469 threading: semaphore: remove temporary convenience types (Cory Fields)
1f89e2a scripted-diff: threading: semaphore: use direct types rather than the temporary convenience ones (Cory Fields)
f21365c threading: replace CountingSemaphore with std::counting_semaphore (Cory Fields)
1acacfb threading: make CountingSemaphore/CountingSemaphoreGrant template types (Cory Fields)
e6ce5f9 scripted-diff: rename CSemaphore and CSemaphoreGrant (Cory Fields)
793166d wallet: change the write semaphore to a BinarySemaphore (Cory Fields)
6790ad2 scripted-diff: rename CSemaphoreGrant and CSemaphore for net (Cory Fields)
d870bc9 threading: add temporary semaphore aliases (Cory Fields)
7b816c4 threading: rename CSemaphore methods to match std::semaphore (Cory Fields)

Pull request description:

  This is relatively simple, but done in a bunch of commits to enable scripted diffs.

  I wanted to add a semaphore in a branch I've been working on, but it was unclear if I should use `std::counting_semaphore` or stick with our old `CSemaphore`. I couldn't decide, so I just decided to remove all doubt and get rid of ours :)

  This replaces our old `CSemaphore` with `std::counting_semaphore` everywhere we used it. `CSemaphoreGrant` is still there as an RAII wrapper, but is now called `CountingSemaphoreGrant` and `BinarySemaphoreGrant` to match. Those have been moved out of `sync.h` to their own file.

ACKs for top commit:
  purpleKarrot:
    ACK 6f7052a
  achow101:
    ACK 6f7052a
  TheCharlatan:
    ACK 6f7052a
  hebasto:
    ACK 6f7052a, I have reviewed the code and it looks OK.

Tree-SHA512: 5975d13aa21739174e3a22c544620ae3f36345f172b51612346d3b7baf0a07c39ef6fd54f647c87878c21a67951b347a5d4a5f90e897f3f6c0db360a3779d0df

Author: achow101

src/net.cpp

@@ -1886,7 +1886,7 @@ bool CConnman::AddConnection(const std::string& address, ConnectionType conn_typ
     if (max_connections != std::nullopt && existing_connections >= max_connections) return false;
 
     // Max total outbound connections already exist
-    CSemaphoreGrant grant(*semOutbound, true);
+    CountingSemaphoreGrant<> grant(*semOutbound, true);
     if (!grant) return false;
 
     OpenNetworkConnection(CAddress(), false, std::move(grant), address.c_str(), conn_type, /*use_v2transport=*/use_v2transport);
@@ -2402,7 +2402,7 @@ void CConnman::ProcessAddrFetch()
     // peer doesn't support it or immediately disconnects us for another reason.
     const bool use_v2transport(GetLocalServices() & NODE_P2P_V2);
     CAddress addr;
-    CSemaphoreGrant grant(*semOutbound, /*fTry=*/true);
+    CountingSemaphoreGrant<> grant(*semOutbound, /*fTry=*/true);
     if (grant) {
         OpenNetworkConnection(addr, false, std::move(grant), strDest.c_str(), ConnectionType::ADDR_FETCH, use_v2transport);
     }
@@ -2576,7 +2576,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect, std
 
         PerformReconnections();
 
-        CSemaphoreGrant grant(*semOutbound);
+        CountingSemaphoreGrant<> grant(*semOutbound);
         if (interruptNet)
             return;
 
@@ -2954,7 +2954,7 @@ void CConnman::ThreadOpenAddedConnections()
     AssertLockNotHeld(m_reconnections_mutex);
     while (true)
     {
-        CSemaphoreGrant grant(*semAddnode);
+        CountingSemaphoreGrant<> grant(*semAddnode);
         std::vector<AddedNodeInfo> vInfo = GetAddedNodeInfo(/*include_connected=*/false);
         bool tried = false;
         for (const AddedNodeInfo& info : vInfo) {
@@ -2967,7 +2967,7 @@ void CConnman::ThreadOpenAddedConnections()
             CAddress addr(CService(), NODE_NONE);
             OpenNetworkConnection(addr, false, std::move(grant), info.m_params.m_added_node.c_str(), ConnectionType::MANUAL, info.m_params.m_use_v2transport);
             if (!interruptNet.sleep_for(std::chrono::milliseconds(500))) return;
-            grant = CSemaphoreGrant(*semAddnode, /*fTry=*/true);
+            grant = CountingSemaphoreGrant<>(*semAddnode, /*fTry=*/true);
         }
         // See if any reconnections are desired.
         PerformReconnections();
@@ -2978,7 +2978,7 @@ void CConnman::ThreadOpenAddedConnections()
 }
 
 // if successful, this moves the passed grant to the constructed node
-void CConnman::OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant&& grant_outbound, const char *pszDest, ConnectionType conn_type, bool use_v2transport)
+void CConnman::OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CountingSemaphoreGrant<>&& grant_outbound, const char *pszDest, ConnectionType conn_type, bool use_v2transport)
 {
     AssertLockNotHeld(m_unused_i2p_sessions_mutex);
     assert(conn_type != ConnectionType::INBOUND);
@@ -3329,11 +3329,11 @@ bool CConnman::Start(CScheduler& scheduler, const Options& connOptions)
 
     if (semOutbound == nullptr) {
         // initialize semaphore
-        semOutbound = std::make_unique<CSemaphore>(std::min(m_max_automatic_outbound, m_max_automatic_connections));
+        semOutbound = std::make_unique<std::counting_semaphore<>>(std::min(m_max_automatic_outbound, m_max_automatic_connections));
     }
     if (semAddnode == nullptr) {
         // initialize semaphore
-        semAddnode = std::make_unique<CSemaphore>(m_max_addnode);
+        semAddnode = std::make_unique<std::counting_semaphore<>>(m_max_addnode);
     }
 
     //
@@ -3421,13 +3421,13 @@ void CConnman::Interrupt()
 
     if (semOutbound) {
         for (int i=0; i<m_max_automatic_outbound; i++) {
-            semOutbound->post();
+            semOutbound->release();
         }
     }
 
     if (semAddnode) {
         for (int i=0; i<m_max_addnode; i++) {
-            semAddnode->post();
+            semAddnode->release();
         }
     }
 }

src/net.h

@@ -24,6 +24,7 @@
 #include <policy/feerate.h>
 #include <protocol.h>
 #include <random.h>
+#include <semaphore_grant.h>
 #include <span.h>
 #include <streams.h>
 #include <sync.h>
@@ -729,7 +730,7 @@ class CNode
     // Setting fDisconnect to true will cause the node to be disconnected the
     // next time DisconnectNodes() runs
     std::atomic_bool fDisconnect{false};
-    CSemaphoreGrant grantOutbound;
+    CountingSemaphoreGrant<> grantOutbound;
     std::atomic<int> nRefCount{0};
 
     const uint64_t nKeyedNetGroup;
@@ -1136,7 +1137,7 @@ class CConnman
     bool GetNetworkActive() const { return fNetworkActive; };
     bool GetUseAddrmanOutgoing() const { return m_use_addrman_outgoing; };
     void SetNetworkActive(bool active);
-    void OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant&& grant_outbound, const char* strDest, ConnectionType conn_type, bool use_v2transport) EXCLUSIVE_LOCKS_REQUIRED(!m_unused_i2p_sessions_mutex);
+    void OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CountingSemaphoreGrant<>&& grant_outbound, const char* strDest, ConnectionType conn_type, bool use_v2transport) EXCLUSIVE_LOCKS_REQUIRED(!m_unused_i2p_sessions_mutex);
     bool CheckIncomingNonce(uint64_t nonce);
     void ASMapHealthCheck();
 
@@ -1491,8 +1492,8 @@ class CConnman
      */
     std::atomic<ServiceFlags> m_local_services;
 
-    std::unique_ptr<CSemaphore> semOutbound;
-    std::unique_ptr<CSemaphore> semAddnode;
+    std::unique_ptr<std::counting_semaphore<>> semOutbound;
+    std::unique_ptr<std::counting_semaphore<>> semAddnode;
 
     /**
      * Maximum number of automatic connections permitted, excluding manual
@@ -1614,7 +1615,7 @@ class CConnman
     struct ReconnectionInfo
     {
         CAddress addr_connect;
-        CSemaphoreGrant grant;
+        CountingSemaphoreGrant<> grant;
         std::string destination;
         ConnectionType conn_type;
         bool use_v2transport;

src/semaphore_grant.h

@@ -0,0 +1,93 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_SEMAPHORE_GRANT_H
+#define BITCOIN_SEMAPHORE_GRANT_H
+
+#include <semaphore>
+
+/** RAII-style semaphore lock */
+template <std::ptrdiff_t LeastMaxValue = std::counting_semaphore<>::max()>
+class CountingSemaphoreGrant
+{
+private:
+    std::counting_semaphore<LeastMaxValue>* sem;
+    bool fHaveGrant;
+
+public:
+    void Acquire() noexcept
+    {
+        if (fHaveGrant) {
+            return;
+        }
+        sem->acquire();
+        fHaveGrant = true;
+    }
+
+    void Release() noexcept
+    {
+        if (!fHaveGrant) {
+            return;
+        }
+        sem->release();
+        fHaveGrant = false;
+    }
+
+    bool TryAcquire() noexcept
+    {
+        if (!fHaveGrant && sem->try_acquire()) {
+            fHaveGrant = true;
+        }
+        return fHaveGrant;
+    }
+
+    // Disallow copy.
+    CountingSemaphoreGrant(const CountingSemaphoreGrant&) = delete;
+    CountingSemaphoreGrant& operator=(const CountingSemaphoreGrant&) = delete;
+
+    // Allow move.
+    CountingSemaphoreGrant(CountingSemaphoreGrant&& other) noexcept
+    {
+        sem = other.sem;
+        fHaveGrant = other.fHaveGrant;
+        other.fHaveGrant = false;
+        other.sem = nullptr;
+    }
+
+    CountingSemaphoreGrant& operator=(CountingSemaphoreGrant&& other) noexcept
+    {
+        Release();
+        sem = other.sem;
+        fHaveGrant = other.fHaveGrant;
+        other.fHaveGrant = false;
+        other.sem = nullptr;
+        return *this;
+    }
+
+    CountingSemaphoreGrant() noexcept : sem(nullptr), fHaveGrant(false) {}
+
+    explicit CountingSemaphoreGrant(std::counting_semaphore<LeastMaxValue>& sema, bool fTry = false) noexcept : sem(&sema), fHaveGrant(false)
+    {
+        if (fTry) {
+            TryAcquire();
+        } else {
+            Acquire();
+        }
+    }
+
+    ~CountingSemaphoreGrant()
+    {
+        Release();
+    }
+
+    explicit operator bool() const noexcept
+    {
+        return fHaveGrant;
+    }
+};
+
+using BinarySemaphoreGrant = CountingSemaphoreGrant<1>;
+
+#endif // BITCOIN_SEMAPHORE_GRANT_H

src/sync.h

@@ -300,131 +300,4 @@ inline MutexType* MaybeCheckNotHeld(MutexType* m) LOCKS_EXCLUDED(m) LOCK_RETURNE
 //! gcc and the -Wreturn-stack-address flag in clang, both enabled by default.
 #define WITH_LOCK(cs, code) (MaybeCheckNotHeld(cs), [&]() -> decltype(auto) { LOCK(cs); code; }())
 
-/** An implementation of a semaphore.
- *
- * See https://en.wikipedia.org/wiki/Semaphore_(programming)
- */
-class CSemaphore
-{
-private:
-    std::condition_variable condition;
-    std::mutex mutex;
-    int value;
-
-public:
-    explicit CSemaphore(int init) noexcept : value(init) {}
-
-    // Disallow default construct, copy, move.
-    CSemaphore() = delete;
-    CSemaphore(const CSemaphore&) = delete;
-    CSemaphore(CSemaphore&&) = delete;
-    CSemaphore& operator=(const CSemaphore&) = delete;
-    CSemaphore& operator=(CSemaphore&&) = delete;
-
-    void wait() noexcept
-    {
-        std::unique_lock<std::mutex> lock(mutex);
-        condition.wait(lock, [&]() { return value >= 1; });
-        value--;
-    }
-
-    bool try_wait() noexcept
-    {
-        std::lock_guard<std::mutex> lock(mutex);
-        if (value < 1) {
-            return false;
-        }
-        value--;
-        return true;
-    }
-
-    void post() noexcept
-    {
-        {
-            std::lock_guard<std::mutex> lock(mutex);
-            value++;
-        }
-        condition.notify_one();
-    }
-};
-
-/** RAII-style semaphore lock */
-class CSemaphoreGrant
-{
-private:
-    CSemaphore* sem;
-    bool fHaveGrant;
-
-public:
-    void Acquire() noexcept
-    {
-        if (fHaveGrant) {
-            return;
-        }
-        sem->wait();
-        fHaveGrant = true;
-    }
-
-    void Release() noexcept
-    {
-        if (!fHaveGrant) {
-            return;
-        }
-        sem->post();
-        fHaveGrant = false;
-    }
-
-    bool TryAcquire() noexcept
-    {
-        if (!fHaveGrant && sem->try_wait()) {
-            fHaveGrant = true;
-        }
-        return fHaveGrant;
-    }
-
-    // Disallow copy.
-    CSemaphoreGrant(const CSemaphoreGrant&) = delete;
-    CSemaphoreGrant& operator=(const CSemaphoreGrant&) = delete;
-
-    // Allow move.
-    CSemaphoreGrant(CSemaphoreGrant&& other) noexcept
-    {
-        sem = other.sem;
-        fHaveGrant = other.fHaveGrant;
-        other.fHaveGrant = false;
-        other.sem = nullptr;
-    }
-
-    CSemaphoreGrant& operator=(CSemaphoreGrant&& other) noexcept
-    {
-        Release();
-        sem = other.sem;
-        fHaveGrant = other.fHaveGrant;
-        other.fHaveGrant = false;
-        other.sem = nullptr;
-        return *this;
-    }
-
-    CSemaphoreGrant() noexcept : sem(nullptr), fHaveGrant(false) {}
-
-    explicit CSemaphoreGrant(CSemaphore& sema, bool fTry = false) noexcept : sem(&sema), fHaveGrant(false)
-    {
-        if (fTry) {
-            TryAcquire();
-        } else {
-            Acquire();
-        }
-    }
-
-    ~CSemaphoreGrant()
-    {
-        Release();
-    }
-
-    explicit operator bool() const noexcept
-    {
-        return fHaveGrant;
-    }
-};
-
 #endif // BITCOIN_SYNC_H