ClickHouse/src/DataStreams/AsynchronousBlockInputStream.h

#pragma once

#include <Poco/Event.h>

#include <DataStreams/IBlockInputStream.h>
#include <Common/CurrentMetrics.h>
#include <Common/ThreadPool.h>


namespace CurrentMetrics
{
    extern const Metric QueryThread;
}

namespace DB
{

/** Executes another BlockInputStream in a separate thread.
  * This serves two purposes:
  * 1. Allows you to make the different stages of the query execution pipeline work in parallel.
  * 2. Allows you not to wait until the data is ready, and periodically check their readiness without blocking.
  *    This is necessary, for example, so that during the waiting period you can check if a packet
  *     has come over the network with a request to interrupt the execution of the query.
  *    It also allows you to execute multiple queries at the same time.
  */
class AsynchronousBlockInputStream : public IBlockInputStream
{
public:
    AsynchronousBlockInputStream(const BlockInputStreamPtr & in)
    {
        children.push_back(in);
    }

    String getName() const override { return "Asynchronous"; }

    void waitInnerThread()
    {
        if (started)
            pool.wait();
    }

    void readPrefix() override
    {
        /// Do not call `readPrefix` on the child, so that the corresponding actions are performed in a separate thread.
        if (!started)
        {
            next();
            started = true;
        }
    }

    void readSuffix() override
    {
        if (started)
        {
            pool.wait();
            if (exception)
                std::rethrow_exception(exception);
            children.back()->readSuffix();
            started = false;
        }
    }


    /** Wait for the data to be ready no more than the specified timeout. Start receiving data if necessary.
      * If the function returned true - the data is ready and you can do `read()`; You can not call the function just at the same moment again.
      */
    bool poll(UInt64 milliseconds)
    {
        if (!started)
        {
            next();
            started = true;
        }

        return ready.tryWait(milliseconds);
    }


    Block getHeader() const override { return children.at(0)->getHeader(); }

    void cancel(bool kill) override
    {
        IBlockInputStream::cancel(kill);

        /// Wait for some background calculations to be sure,
        ///  that after end of stream nothing is being executing.
        if (started)
            pool.wait();
    }

    ~AsynchronousBlockInputStream() override
    {
        if (started)
            pool.wait();
    }

protected:
    ThreadPool pool{1};
    Poco::Event ready;
    bool started = false;
    bool first = true;

    Block block;
    std::exception_ptr exception;

    Block readImpl() override;

    void next();

    /// Calculations that can be performed in a separate thread
    void calculate();
};

}