railties/lib/rails.rb specifies default internal and encodings of UTF_8. If that ever gets required before a test using a pipe runs writing the data to the pipe will fail unless we set the read and write of the pipe to binmode.

pipe_encoding.rb

def pipetest(set_binmode = false)
    rd, wr = IO.pipe

    if set_binmode
      rd.binmode
      wr.binmode
    end
    puts "rd.internal_encoding:#{rd.internal_encoding}"
    puts "rd.external_encoding:#{rd.external_encoding}"
    puts "wr.internal_encoding:#{wr.internal_encoding}"
    puts "wr.external_encoding:#{wr.external_encoding}"
    pid = fork {
      rd.close
      wr.write Marshal.dump 12356234
      wr.close
      exit!
    }

    wr.close

    Process.waitpid pid
    puts object_id.inspect
    puts Marshal.load(rd.read).inspect
    rd.close
end

puts "==binmode=true=="
pipetest(true)
puts "==binmode=false=="
pipetest
require 'rails'
puts "===After Rails Required==="
puts "==binmode=true=="
pipetest(true)
puts "==binmode=false=="
pipetest


===============the output===================

==binmode=true==
rd.internal_encoding:
rd.external_encoding:ASCII-8BIT
wr.internal_encoding:
wr.external_encoding:ASCII-8BIT
70277031773820
12356234
==binmode=false==
rd.internal_encoding:
rd.external_encoding:UTF-8
wr.internal_encoding:
wr.external_encoding:
70277031773820
12356234
===After Rails Required===
==binmode=true==
rd.internal_encoding:
rd.external_encoding:ASCII-8BIT
wr.internal_encoding:
wr.external_encoding:ASCII-8BIT
70277031773820
12356234
==binmode=false==
rd.internal_encoding:
rd.external_encoding:UTF-8
wr.internal_encoding:
wr.external_encoding:UTF-8
/Users/acarey/Desktop/pipetest.rb:14:in `write': "\x8A" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)
    from /Users/acarey/Desktop/pipetest.rb:14:in `block in pipetest'
    from /Users/acarey/Desktop/pipetest.rb:12:in `fork'
    from /Users/acarey/Desktop/pipetest.rb:12:in `pipetest'
    from /Users/acarey/Desktop/pipetest.rb:38:in `<main>'
70277031773820
/Users/acarey/Desktop/pipetest.rb:23:in `load': marshal data too short (ArgumentError)
    from /Users/acarey/Desktop/pipetest.rb:23:in `pipetest'
    from /Users/acarey/Desktop/pipetest.rb:38:in `<main>'