multi-phart.rb

#!/usr/bin/env ruby

require 'tempfile'
require 'securerandom'

endpoint = ARGV.shift || 'http://localhost:3000/'
parts = Integer(ARGV.shift || 10_000)
boundary = SecureRandom.hex(20)

data = (1..parts).map do |i|
  <<~STR.gsub(/\n/, "\r\n")
    --"#{boundary}"
    Content-Disposition: form-data; name="a"
    1
  STR
end.join("\r\n")
data << "--#{boundary}--\r\n"

Tempfile.open('request-body') do |f|
  f.write(data)
  system(
    'curl', '-s', '-o', '/dev/null', '-v', '--trace-time',
    '-H', "Content-Type: multipart/form-data; boundary=#{boundary}",
    '--data-binary', "@#{f.path}",
    endpoint,
  )
end