Method tryGenerate

src/Processors/Sources/RemoteSource.cpp:157–236

}

std::optional<Chunk> RemoteSource::tryGenerate()
{
    /// onCancel() will do the cancel if the query was sent.
    if (isCancelled())
        return {};

    if (!was_query_sent)
    {
        if (async_query_sending)
        {
            int fd_ = query_executor->sendQueryAsync();
            if (fd_ >= 0)
            {
                fd = fd_;
                is_async_state = true;
                return Chunk();
            }

            is_async_state = false;
        }
        else
        {
            query_executor->sendQuery();
        }

        was_query_sent = true;
    }

    Block block;

    if (async_read)
    {
        auto res = query_executor->readAsync();

        if (res.getType() == RemoteQueryExecutor::ReadResult::Type::Nothing)
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Got an empty packet from the RemoteQueryExecutor. This is a bug");

        if (res.getType() == RemoteQueryExecutor::ReadResult::Type::FileDescriptor)
        {
            fd = res.getFileDescriptor();
            is_async_state = true;
            return Chunk();
        }

        if (res.getType() == RemoteQueryExecutor::ReadResult::Type::ParallelReplicasToken)
        {
            is_async_state = false;
            return Chunk();
        }

        is_async_state = false;

        block = res.getBlock();
    }
    else
        block = query_executor->readBlock();

    if (block.empty())