Add new Spinlock class.

Author: asfernandes

src/common/classes/Spinlock.h

@@ -0,0 +1,73 @@
+/*
+ *  The contents of this file are subject to the Initial
+ *  Developer's Public License Version 1.0 (the "License");
+ *  you may not use this file except in compliance with the
+ *  License. You may obtain a copy of the License at
+ *  http://www.ibphoenix.com/main.nfs?a=ibphoenix&page=ibp_idpl.
+ *
+ *  Software distributed under the License is distributed AS IS,
+ *  WITHOUT WARRANTY OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing rights
+ *  and limitations under the License.
+ *
+ *  The Original Code was created by Adriano dos Santos Fernandes
+ *  for the Firebird Open Source RDBMS project.
+ *
+ *  Copyright (c) 2025 Adriano dos Santos Fernandes <adrianosf@gmail.com>
+ *  and all contributors signed below.
+ *
+ *  All Rights Reserved.
+ *  Contributor(s): ______________________________________.
+ */
+
+#ifndef CLASSES_SPINLOCK_H
+#define CLASSES_SPINLOCK_H
+
+#include <atomic>
+#include <thread>
+
+namespace Firebird {
+
+
+// Spinlock implementation that can be used in shared memory.
+// Based in example found in https://en.cppreference.com/w/cpp/atomic/atomic_flag.html
+// Compatible with std::lock_guard, std::scoped_lock and std::unique_lock.
+class SpinLock
+{
+	std::atomic_flag atomicFlag = ATOMIC_FLAG_INIT;
+
+public:
+	void lock() noexcept
+	{
+		while (atomicFlag.test_and_set(std::memory_order_acquire))
+		{
+#if defined(__cpp_lib_atomic_wait) && __cpp_lib_atomic_wait >= 201907L
+			// Since C++20, locks can be acquired only after notification in the unlock,
+			// avoiding any unnecessary spinning.
+			// Note that even though wait guarantees it returns only after the value has
+			// changed, the lock is acquired after the next condition check.
+			atomicFlag.wait(true, std::memory_order_relaxed);
+#else
+			std::this_thread::yield();
+#endif
+		}
+	}
+
+	bool try_lock() noexcept
+	{
+		return !atomicFlag.test_and_set(std::memory_order_acquire);
+	}
+
+	void unlock() noexcept
+	{
+		atomicFlag.clear(std::memory_order_release);
+#if defined(__cpp_lib_atomic_wait) && __cpp_lib_atomic_wait >= 201907L
+		atomicFlag.notify_one();
+#endif
+	}
+};
+
+
+} // namespace Firebird
+
+#endif // CLASSES_SPINLOCK_H