STDOUT.close

13 Oct 2014

Closing STDOUT and then creating another ruby process has some interesting behaviors across different versions and implementations of ruby. Consider the following two files:

close_and_redirect.rb

STDERR.puts "STDOUT closed?: #{STDOUT.closed?}"
STDERR.puts "sleeping... try lsof -p #{$$}\n"
sleep 10

STDOUT.close
STDERR.puts "STDOUT closed?: #{STDOUT.closed?}"
STDERR.puts "sleeping... try lsof -p #{$$}\n"
sleep 10

exec "ruby", "test_stdout.rb"

test_stdout.rb

STDERR.puts "STDOUT closed?: #{STDOUT.closed?}"
STDERR.puts "sleeping... try lsof -p #{$$}"
sleep 10

puts "STDOUT says hello"

MRI 2.0

$ ruby -v close_and_redirect.rb
ruby 2.0.0p481 (2014-05-08 revision 45883) [universal.x86_64-darwin13]
STDOUT closed?: false
sleeping... try lsof -p 59100

$ lsof -p 59100 | grep 1u
ruby  59100 sshao  1u  CHR 16,2  0t866 773 /dev/ttys002

$ (continued) ruby -v close_and_redirect.rb
STDOUT closed?: true
sleeping... try lsof -p 59100

$ lsof -p 59100 | grep 1u
ruby  59100 sshao  1u  CHR 16,2  0t917 773 /dev/ttys002
# stdout is still open!

$ (continued) ruby -v close_and_redirect.rb
STDOUT closed?: false
sleeping... try lsof -p 59100
STDOUT says hello

$ lsof -p 59100 | grep 1u
ruby  59100 sshao  1u  CHR 16,2  0t969 773 /dev/ttys002

stdout (fd 1) is never actually closed -- even when ruby says it is.

RBX 2.x

$ ruby -v close_and_redirect.rb
rubinius 2.2.10 (2.1.0 bf61ae2e 2014-06-27 JI) [x86_64-darwin13.4.0]
STDOUT closed?: false
sleeping... try lsof -p 30091

$ lsof -p 30091 | grep 1u
rbx 30091 sshao  1u  CHR 16,2  0t5050  773 /dev/ttys002

$ (continued) ruby -v close_and_redirect.rb
STDOUT closed?: true
sleeping... try lsof -p 30091

$ lsof -p 30091 | grep 1u
# stdout is closed

$ (continued) ruby -v close_and_redirect.rb
ERROR: the VM is exiting improperly running test_stdout.rb
intended operation: :exception
associated value: nil
destination scope: unknown
An exception occurred

    closed stream (IOError)

Backtrace:
  IO#fileno at kernel/common/io.rb:1560
  IO.setup at kernel/common/io.rb:977
  IO(File)#initialize at kernel/common/io.rb:995
  File#initialize at kernel/common/file.rb:1142
  Class#new at kernel/alpha.rb:94
  IO.open at kernel/common/io.rb:624
  Rubinius::Loader#write_last_error at kernel/loader.rb:794
  Rubinius::Loader#main at kernel/loader.rb:854

Unlike MRI 2.0, Rubinius actually closes stdout -- and propagates its closed status to the exec'd process, which fails in Rubinius::Loader due to the closed stream.

MRI 1.8

$ ruby -v close_and_redirect.rb
ruby 1.8.7 (2012-10-12 patchlevel 371) [i686-darwin13.4.0]
STDOUT closed?: false
sleeping... try lsof -p 59166

$ lsof -p 59166 | grep 1u
ruby  59166 sshao  1u  CHR 16,1 0t3231295  771 /dev/ttys001

STDOUT closed?: true
sleeping... try lsof -p 59166

$ lsof -p 59166 | grep 1u
# stdout is closed!

STDOUT closed?: false
sleeping... try lsof -p 59166

$ lsof -p 59166 | grep 1u
# stdout is still closed!

Although lsof shows us that stdout is closed in the last process (after the exec), ruby tells us that STDOUT.closed? is false! Furthermore, the puts "STDOUT says hello" is never output.