https://github.com/taf2/curb/issues/277

curb-multi-connections.rb

require 'curb'
require 'socket'

URL = 'https://google.com'

# 0123456 default & reserved RubyVM. It will probably include 7 from Dir.glob
FDS_OPEN_BEFORE_START = Dir.glob("/proc/self/fd/*").map do |fd_path|
  File.basename(fd_path).to_i
end

def run!
  multi = Curl::Multi.new
  5.times do |n|
    easy = Curl::Easy.new(URL) do |curl|
      curl.on_complete { puts "Transfer #{n+1} completed. Open FDs (#{open_fds.size}): #{open_fds.inspect}"; inspect_open_sockets }
    end
    multi.add(easy)
  end

  multi.max_connects = 1
  multi.perform
  multi
end

def open_fds
  fds = Dir.glob("/proc/self/fd/*").map do |fd_path|
    File.basename(fd_path).to_i
  end
  (fds - FDS_OPEN_BEFORE_START).sort
end

def inspect_open_sockets
  open_fds.each do |fd|
    io = IO.for_fd(fd) rescue nil # discard on race condition
    next unless io
    stat = io.stat
    next unless stat.ftype == "socket" || stat.ftype == "file"

    link = case stat.ftype
           when "socket" then get_socket_address_for_fd(fd)
           when "file" then File.readlink("/proc/self/fd/#{fd}")
           else ""
           end
    puts "FD #{fd}; #{stat.ftype}: #{link}"
  end
end

def get_socket_address_for_fd(fd)
  soc = BasicSocket.for_fd(fd)
  from = "#{soc.connect_address.ip_address}:#{soc.connect_address.ip_port}"
  to = "#{soc.remote_address.ip_address}:#{soc.remote_address.ip_port}"
  "#{from} => #{to}"
end

puts "System FDs: #{FDS_OPEN_BEFORE_START.uniq.inspect}"
puts "These FDs will be ignored."
puts "Additional FDs open before start (#{open_fds.size}): #{open_fds.inspect}"
inspect_open_sockets
puts "Starting multi"
multi = run!
puts "All transfers completed"
puts "Open FDs after multi completed (#{open_fds.size}): #{open_fds.inspect}"
inspect_open_sockets
puts "Sleeping 10 seconds..."
10.times { sleep 1; print '.' }
puts
puts "Open FDs after sleep (#{open_fds.size}): #{open_fds.inspect}"
inspect_open_sockets
puts
puts "Add 5 new transfers"
puts
5.times do |n|
  easy = Curl::Easy.new(URL) do |curl|
    curl.on_complete { puts "Transfer #{n+1} completed. Open FDs (#{open_fds.size}): #{open_fds.inspect}"; inspect_open_sockets }
  end
  multi.add(easy)
end
puts "FDs open before start (#{open_fds.size}): #{open_fds.inspect}"
inspect_open_sockets
puts "Starting multi"
multi.perform
puts "All transfers completed"
puts "Open FDs after multi completed (#{open_fds.size}): #{open_fds.inspect}"
inspect_open_sockets
puts