ClickHouse/dbms/include/DB/Common/ThreadPool.h

#pragma once

#include <cstdint>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <functional>
#include <queue>
#include <vector>


/** Very simple thread pool similar to boost::threadpool.
  * Advantages:
  * - catches exceptions and rethrows on wait.
  */

class ThreadPool
{
private:
	using Job = std::function<void()>;

public:
	/// Size is constant, all threads are created immediately.
	ThreadPool(size_t m_size);

	/// Add new job. Locks until free thread in pool become available or exception in one of threads was thrown.
	/// If an exception in some thread was thrown, method silently returns, and exception will be rethrown only on call to 'wait' function.
	void schedule(Job job);

	/// Wait for all currently active jobs to be done.
	/// You may call schedule and wait many times in arbitary order.
	/// If any thread was throw an exception, it will be rethrown from this method.
	/// List of exceptions is not cleared: on subsequent calls to wait, same exception will be rethrown.
	void wait();

	/// Waits for all threads. Doesn't rethrow exceptions (use 'wait' method to rethrow exceptions).
	/// You should not destroy object while calling schedule or wait methods from another threads.
	~ThreadPool();

	size_t size() const { return m_size; }

	/// Returns number of active jobs.
	size_t active() const;

private:
	mutable std::mutex mutex;
	std::condition_variable has_free_thread;
	std::condition_variable has_new_job_or_shutdown;

	const size_t m_size;
	size_t active_jobs = 0;
	bool shutdown = false;

	std::queue<Job> jobs;
	std::vector<std::thread> threads;
	std::vector<std::exception_ptr> exceptions;		/// NOTE Saving many exceptions but we rethrow just first one.


	void worker();
};