ClickHouse/src/IO/ReadBufferFromFileDescriptor.cpp

#include <errno.h>
#include <time.h>
#include <optional>
#include <Common/ProfileEvents.h>
#include <Common/Stopwatch.h>
#include <Common/Exception.h>
#include <Common/CurrentMetrics.h>
#include <IO/ReadBufferFromFileDescriptor.h>
#include <IO/WriteHelpers.h>


namespace ProfileEvents
{
    extern const Event ReadBufferFromFileDescriptorRead;
    extern const Event ReadBufferFromFileDescriptorReadFailed;
    extern const Event ReadBufferFromFileDescriptorReadBytes;
    extern const Event DiskReadElapsedMicroseconds;
    extern const Event Seek;
}

namespace CurrentMetrics
{
    extern const Metric Read;
}

namespace DB
{

namespace ErrorCodes
{
    extern const int CANNOT_READ_FROM_FILE_DESCRIPTOR;
    extern const int ARGUMENT_OUT_OF_BOUND;
    extern const int CANNOT_SEEK_THROUGH_FILE;
    extern const int CANNOT_SELECT;
}


std::string ReadBufferFromFileDescriptor::getFileName() const
{
    return "(fd = " + toString(fd) + ")";
}


bool ReadBufferFromFileDescriptor::nextImpl()
{
    size_t bytes_read = 0;
    while (!bytes_read)
    {
        ProfileEvents::increment(ProfileEvents::ReadBufferFromFileDescriptorRead);

        Stopwatch watch(profile_callback ? clock_type : CLOCK_MONOTONIC);

        ssize_t res = 0;
        {
            CurrentMetrics::Increment metric_increment{CurrentMetrics::Read};
            res = ::read(fd, internal_buffer.begin(), internal_buffer.size());
        }
        if (!res)
            break;

        if (-1 == res && errno != EINTR)
        {
            ProfileEvents::increment(ProfileEvents::ReadBufferFromFileDescriptorReadFailed);
            throwFromErrnoWithPath("Cannot read from file " + getFileName(), getFileName(),
                                   ErrorCodes::CANNOT_READ_FROM_FILE_DESCRIPTOR);
        }

        if (res > 0)
            bytes_read += res;

        /// It reports real time spent including the time spent while thread was preempted doing nothing.
        /// And it is Ok for the purpose of this watch (it is used to lower the number of threads to read from tables).
        /// Sometimes it is better to use taskstats::blkio_delay_total, but it is quite expensive to get it
        /// (TaskStatsInfoGetter has about 500K RPS).
        watch.stop();
        ProfileEvents::increment(ProfileEvents::DiskReadElapsedMicroseconds, watch.elapsedMicroseconds());

        if (profile_callback)
        {
            ProfileInfo info;
            info.bytes_requested = internal_buffer.size();
            info.bytes_read = res;
            info.nanoseconds = watch.elapsed();
            profile_callback(info);
        }
    }

    file_offset_of_buffer_end += bytes_read;

    if (bytes_read)
    {
        ProfileEvents::increment(ProfileEvents::ReadBufferFromFileDescriptorReadBytes, bytes_read);
        working_buffer = internal_buffer;
        working_buffer.resize(bytes_read);
    }
    else
        return false;

    return true;
}


/// If 'offset' is small enough to stay in buffer after seek, then true seek in file does not happen.
off_t ReadBufferFromFileDescriptor::seek(off_t offset, int whence)
{
    size_t new_pos;
    if (whence == SEEK_SET)
    {
        assert(offset >= 0);
        new_pos = offset;
    }
    else if (whence == SEEK_CUR)
    {
        new_pos = file_offset_of_buffer_end - (working_buffer.end() - pos) + offset;
    }
    else
    {
        throw Exception("ReadBufferFromFileDescriptor::seek expects SEEK_SET or SEEK_CUR as whence", ErrorCodes::ARGUMENT_OUT_OF_BOUND);
    }

    /// Position is unchanged.
    if (new_pos + (working_buffer.end() - pos) == file_offset_of_buffer_end)
        return new_pos;

    // file_offset_of_buffer_end corresponds to working_buffer.end(); it's a past-the-end pos,
    // so the second inequality is strict.
    if (file_offset_of_buffer_end - working_buffer.size() <= static_cast<size_t>(new_pos)
        && new_pos < file_offset_of_buffer_end)
    {
        /// Position is still inside buffer.
        pos = working_buffer.end() - file_offset_of_buffer_end + new_pos;
        assert(pos >= working_buffer.begin());
        assert(pos < working_buffer.end());

        return new_pos;
    }
    else
    {
        ProfileEvents::increment(ProfileEvents::Seek);
        Stopwatch watch(profile_callback ? clock_type : CLOCK_MONOTONIC);

        pos = working_buffer.end();
        off_t res = ::lseek(fd, new_pos, SEEK_SET);
        if (-1 == res)
            throwFromErrnoWithPath("Cannot seek through file " + getFileName(), getFileName(),
                                   ErrorCodes::CANNOT_SEEK_THROUGH_FILE);
        file_offset_of_buffer_end = new_pos;

        watch.stop();
        ProfileEvents::increment(ProfileEvents::DiskReadElapsedMicroseconds, watch.elapsedMicroseconds());

        return res;
    }
}


/// Assuming file descriptor supports 'select', check that we have data to read or wait until timeout.
bool ReadBufferFromFileDescriptor::poll(size_t timeout_microseconds)
{
    fd_set fds;
    FD_ZERO(&fds);
    FD_SET(fd, &fds);
    timeval timeout = { time_t(timeout_microseconds / 1000000), suseconds_t(timeout_microseconds % 1000000) };

    int res = select(1, &fds, nullptr, nullptr, &timeout);

    if (-1 == res)
        throwFromErrno("Cannot select", ErrorCodes::CANNOT_SELECT);

    return res > 0;
}

}