test.rb

require 'open3'

command = "tail"
target  = "x" * 65535
out     = ""

Open3.popen3(command) do |stdin, stdout, _|
    stdin.puts target
    stdin.close
    out = stdout.read
end

puts out.length

target  = "x" * 65536*10
out     = ""

puts "1"
Open3.popen3(command) do |stdin, stdout, _|
    puts "2"
    stdin.puts target
    puts "3"
    stdin.close
    out = stdout.read
end

puts out.length