ClickHouse/libs/libzkutil/include/zkutil/RWLock.h

#pragma once

#include <zkutil/ZooKeeper.h>
#include <DB/Common/Exception.h>
#include <Poco/Event.h>
#include <string>
#include <type_traits>
#include <functional>

namespace zkutil
{

/** Distributed read/write lock for ZooKeeper.
  * Such a RWLock object may not be shared among threads.
  */
class RWLock final
{
public:
	enum Type
	{
		Read = 0,
		Write
	};

	enum Mode
	{
		Blocking = 0,
		NonBlocking
	};

	using CancellationHook = std::function<void()>;

public:
	RWLock() = default;

	/// Create under the specified ZooKeeper path a queue for lock requests
	/// if it doesn't exist yet.
	RWLock(ZooKeeperPtr & zookeeper_, const std::string & path_);

	RWLock(const RWLock &) = delete;
	RWLock & operator=(const RWLock &) = delete;

	RWLock(RWLock &&) = default;
	RWLock & operator=(RWLock &&) = default;

	operator bool() const;

	/// Register a function that checks whether lock acquisition should be cancelled.
	void setCancellationHook(CancellationHook cancellation_hook_);

	/// Get a read lock.
	void acquireRead(RWLock::Mode mode);

	/// Get a write lock.
	void acquireWrite(RWLock::Mode mode);

	/// Check whether we have acquired the lock.
	bool ownsLock() const;

	/// Release lock.
	void release();

public:
	template <Type, Mode = Blocking> class Guard;

private:
	template <RWLock::Type lock_type> void acquireImpl(RWLock::Mode mode);
	void abortIfRequested();

private:
	ZooKeeperPtr zookeeper;
	EventPtr event = new Poco::Event;
	CancellationHook cancellation_hook;
	/// Path to the lock request queue.
	std::string path;
	/// Identifier of our request for a lock.
	std::string key;
	bool owns_lock = false;
};

/** Scoped version of RWLock.
  */
template <RWLock::Type lock_type, RWLock::Mode lock_mode>
class RWLock::Guard final
{
	static_assert((lock_type == RWLock::Read) || (lock_type == RWLock::Write), "Invalid RWLock type");
	static_assert((lock_mode == RWLock::Blocking) || (lock_mode == RWLock::NonBlocking), "Invalid RWLock mode");

public:
	/// Acquire lock.
	Guard(RWLock & rw_lock_)
		: rw_lock(rw_lock_)
	{
		if (lock_type == RWLock::Read)
			rw_lock.acquireRead(lock_mode);
		else if (lock_type == RWLock::Write)
			rw_lock.acquireWrite(lock_mode);
	}

	/// Release lock.
	~Guard()
	{
		if (rw_lock.ownsLock())
		{
			try
			{
				rw_lock.release();
			}
			catch (...)
			{
				DB::tryLogCurrentException(__PRETTY_FUNCTION__);
			}
		}
	}

	Guard(const Guard &) = delete;
	Guard & operator=(const Guard &) = delete;

private:
	RWLock & rw_lock;
};

}