Common: Add TransferableSharedMutex class and unit tests.

Source/Core/Common/CMakeLists.txt

@@ -151,6 +151,7 @@ add_library(common
   Timer.h
   TimeUtil.cpp
   TimeUtil.h
+  TransferableSharedMutex.h
   TraversalClient.cpp
   TraversalClient.h
   TraversalProto.h

Source/Core/Common/TransferableSharedMutex.h

@@ -0,0 +1,92 @@
+// Copyright 2025 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <atomic>
+#include <cassert>
+#include <cstdint>
+
+namespace Common
+{
+// Behaves like `std::shared_mutex` but locks and unlocks may come from different threads.
+class TransferableSharedMutex
+{
+public:
+  void lock()
+  {
+    while (true)
+    {
+      CounterType old_value{};
+      if (m_counter.compare_exchange_strong(old_value, EXCLUSIVE_LOCK_VALUE,
+                                            std::memory_order_acquire, std::memory_order_relaxed))
+      {
+        return;
+      }
+
+      // lock() or lock_shared() is already held.
+      // Wait for an unlock notification and try again.
+      m_counter.wait(old_value, std::memory_order_relaxed);
+    }
+  }
+
+  bool try_lock()
+  {
+    CounterType old_value{};
+    return m_counter.compare_exchange_weak(old_value, EXCLUSIVE_LOCK_VALUE,
+                                           std::memory_order_acquire, std::memory_order_relaxed);
+  }
+
+  void unlock()
+  {
+    m_counter.store(0, std::memory_order_release);
+    m_counter.notify_all();  // Notify potentially multiple wait()ers in lock_shared().
+  }
+
+  void lock_shared()
+  {
+    while (true)
+    {
+      auto old_value = m_counter.load(std::memory_order_relaxed);
+      while (old_value < LAST_SHARED_LOCK_VALUE)
+      {
+        if (m_counter.compare_exchange_strong(old_value, old_value + 1, std::memory_order_acquire,
+                                              std::memory_order_relaxed))
+        {
+          return;
+        }
+      }
+
+      // Something has gone very wrong if m_counter is nearly saturated with shared_lock().
+      assert(old_value != LAST_SHARED_LOCK_VALUE);
+
+      // lock() is already held.
+      // Wait for an unlock notification and try again.
+      m_counter.wait(old_value, std::memory_order_relaxed);
+    }
+  }
+
+  bool try_lock_shared()
+  {
+    auto old_value = m_counter.load(std::memory_order_relaxed);
+    return (old_value < LAST_SHARED_LOCK_VALUE) &&
+           m_counter.compare_exchange_weak(old_value, old_value + 1, std::memory_order_acquire,
+                                           std::memory_order_relaxed);
+  }
+
+  void unlock_shared()
+  {
+    if (m_counter.fetch_sub(1, std::memory_order_release) == 1)
+      m_counter.notify_one();  // Notify one of the wait()ers in lock().
+  }
+
+private:
+  using CounterType = std::uintptr_t;
+
+  static constexpr auto EXCLUSIVE_LOCK_VALUE = CounterType(-1);
+  static constexpr auto LAST_SHARED_LOCK_VALUE = EXCLUSIVE_LOCK_VALUE - 1;
+
+  std::atomic<CounterType> m_counter{};
+};
+
+}  // namespace Common

Source/Core/DolphinLib.props

@@ -171,6 +171,7 @@
     <ClInclude Include="Common\Thread.h" />
     <ClInclude Include="Common\Timer.h" />
     <ClInclude Include="Common\TimeUtil.h" />
+    <ClInclude Include="Common\TransferableSharedMutex.h" />
     <ClInclude Include="Common\TraversalClient.h" />
     <ClInclude Include="Common\TraversalProto.h" />
     <ClInclude Include="Common\TypeUtils.h" />