stream2.rb

class Loader
  def load
    Enumerator.new { |main_enum| stream(main_enum) }
  end

  private

  def stream(main_enum)
    reader = nil
    file_uri.open do |file|
      reader = Zlib::GzipReader.new(file)
      reader.each_line.lazy.drop(1).each do |line|
        main_enum << preprocess_row(line)
      end
    end
  ensure
    reader&.close
  end

  def file_uri
    URI.parse("ftp://user:[email protected]/file.csv.gz")
  end

  def preprocess_row(row)
    row.chomp.gsub('"', "").split(",")
  end
end

Why are you stripping out the main quote character for CSVs, this absolutely the wrong way to parse CSV data except the most basic input

"single, column value containing quotes "" , still in first col",second column,"third"

is going to be parsed completely incorrectly by your preprocess_row function

the correct output would be:

[
  'single column value containing quotes ", still in first col',
  'second column',
  'third'
]

What you're going to get

[
  'single column value containing quotes',
  'still in first col',
  'second column',
  'third'
]

building a proper streaming CSV parser, you would actually open an IO object, pass that into CSV.foreach, and then feed each line into the IO

what about CSVs containing quoted newlines, nested quotes, etc?