ClickHouse/dbms/include/DB/IO/ReadBufferFromFileDescriptor.h

#pragma once

#include <unistd.h>

#include <DB/IO/ReadBufferFromFileBase.h>
#include <DB/IO/ReadBuffer.h>


namespace DB
{

/** Use ready file descriptor. Does not open or close a file.
  */
class ReadBufferFromFileDescriptor : public ReadBufferFromFileBase
{
protected:
	int fd;
	off_t pos_in_file; /// What offset in file corresponds to working_buffer.end().

	bool nextImpl() override;

	/// Name or some description of file.
	std::string getFileName() const override;

public:
	ReadBufferFromFileDescriptor(int fd_, size_t buf_size = DBMS_DEFAULT_BUFFER_SIZE, char * existing_memory = nullptr, size_t alignment = 0)
		: ReadBufferFromFileBase(buf_size, existing_memory, alignment), fd(fd_), pos_in_file(0) {}

	int getFD() const override
	{
		return fd;
	}

	off_t getPositionInFile() override
	{
		return pos_in_file - (working_buffer.end() - pos);
	}

private:
	/// If 'offset' is small enough to stay in buffer after seek, then true seek in file does not happen.
	off_t doSeek(off_t offset, int whence) override;

	/// Assuming file descriptor supports 'select', check that we have data to read or wait until timeout.
	bool poll(size_t timeout_microseconds);
};

}