Function selectReadBuffer

src/Storages/StorageFile.cpp:419–493

}

std::unique_ptr<ReadBuffer> selectReadBuffer(
    const String & current_path,
    bool use_table_fd,
    int table_fd,
    const struct stat & file_stat,
    ContextPtr context)
{
    auto read_method = context->getSettingsRef()[Setting::storage_file_read_method];

    /** Using mmap on server-side is unsafe for the following reasons:
      * - concurrent modifications of a file will result in SIGBUS;
      * - IO error from the device will result in SIGBUS;
      * - recovery from this signal is not feasible even with the usage of siglongjmp,
      *   as it might require stack unwinding from arbitrary place;
      * - arbitrary slowdown due to page fault in arbitrary place in the code is difficult to debug.
      *
      * But we keep this mode for clickhouse-local as it is not so bad for a command line tool.
      */
    if (context->getApplicationType() == Context::ApplicationType::SERVER && read_method == LocalFSReadMethod::mmap)
        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Using storage_file_read_method=mmap is not safe in server mode. Consider using pread.");

    /// Avoid zero-length mmap: empty regular files still read as empty via `pread`, while
    /// pseudo-files such as `/proc` and `/sys` may report `st_size == 0` despite readable content.
    /// See #69070.
    if (S_ISREG(file_stat.st_mode) && read_method == LocalFSReadMethod::mmap && file_stat.st_size > 0)
    {
        try
        {
            std::unique_ptr<ReadBufferFromFileBase> res;
            if (use_table_fd)
                res = std::make_unique<MMapReadBufferFromFileDescriptor>(table_fd, 0);
            else
                res = std::make_unique<MMapReadBufferFromFile>(current_path, 0);

            ProfileEvents::increment(ProfileEvents::CreatedReadBufferMMap);
            return res;
        }
        catch (const ErrnoException &)
        {
            /// Fallback if mmap is not supported.
            ProfileEvents::increment(ProfileEvents::CreatedReadBufferMMapFailed);
        }
    }

    std::unique_ptr<ReadBufferFromFileBase> res;
    if (S_ISREG(file_stat.st_mode) && (read_method == LocalFSReadMethod::pread || read_method == LocalFSReadMethod::mmap))
    {
        if (use_table_fd)
            res = std::make_unique<ReadBufferFromFileDescriptorPRead>(table_fd, context->getSettingsRef()[Setting::max_read_buffer_size]);
        else
            res = std::make_unique<ReadBufferFromFilePRead>(current_path, context->getSettingsRef()[Setting::max_read_buffer_size]);

        ProfileEvents::increment(ProfileEvents::CreatedReadBufferOrdinary);
    }
    else if (read_method == LocalFSReadMethod::io_uring && !use_table_fd)
    {
#if USE_LIBURING
        auto & reader = getIOUringReaderOrThrow(context);