ClickHouse/src/IO/ReadBufferFromFileDescriptor.h

#pragma once

#include <IO/ReadBufferFromFileBase.h>
#include <Interpreters/Context.h>

#include <unistd.h>


namespace DB
{

/** Use ready file descriptor. Does not open or close a file.
  */
class ReadBufferFromFileDescriptor : public ReadBufferFromFileBase
{
protected:
    const size_t required_alignment = 0;    /// For O_DIRECT both file offsets and memory addresses have to be aligned.
    size_t file_offset_of_buffer_end = 0;       /// What offset in file corresponds to working_buffer.end().
    int fd;

    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), required_alignment(alignment), fd(fd_) {}

    int getFD() const
    {
        return fd;
    }

    off_t getPosition() override
    {
        return file_offset_of_buffer_end - (working_buffer.end() - pos);
    }

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

    /// Seek to the beginning, discarding already read data if any. Useful to reread file that changes on every read.
    void rewind();

    off_t size();

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

}