プロセスリソースに制限があろうとも僕は Process.#setrlimit で故意に落ちる

皆さん、恋してますか？僕は故意に恋い焦がれて故意に泣きたい心境です。

きっかけ

Fukuoka.rb #109 でもくもくとなるほどUnixプロセス ― Rubyで学ぶUnixの基礎を読み進めていたのだが、 Process.#setrlimit の項目で意図せず irb が落ちた。なぜ落ちたのか自分にはさっぱり分からなかった。

動作環境はこんな感じ。

• Debian 9 (VirtualBox)
• Ruby 2.5.1
• irb 0.9.6

事象

先述の書籍に以下のような実行例があった。

Process.setrlimit(:NOFILE, 3)
File.open('/dev/null')

これを実行すると Errno::EMFILE: Too many open files - /dev/null が出力されると書いてあったので、素直に irb で試してみた。

$ irb
irb(main):001:0> Process.setrlimit(:NOFILE, 3)
=> nil
Traceback (most recent call last):
    21: from /home/jinroq/.rbenv/versions/2.5.1/bin/irb:11:in `<main>'
    20: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:383:in `start'
    19: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:427:in `run'
    18: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:427:in `catch'
    17: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:428:in `block in run'
    16: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:487:in `eval_input'
    15: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:231:in `each_top_level_statement'
    14: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:231:in `catch'
    13: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:232:in `block in each_top_level_statement'
    12: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:232:in `loop'
    11: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:236:in `block (2 levels) in each_top_level_statement'
    10: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:266:in `lex'
     9: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:292:in `token'
     8: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/slex.rb:76:in `match'
     7: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/slex.rb:206:in `match_io'
     6: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:104:in `getc'
     5: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:189:in `buf_input'
     4: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:470:in `block in eval_input'
     3: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:623:in `signal_status'
     2: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:471:in `block (2 levels) in eval_input'
     1: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/input-method.rb:149:in `gets'
/home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/input-method.rb:149:in `input=': Invalid argument - dup (Errno::EINVAL)

$ 

Process.#setrlimit を実行しただけで irb が落ちた。それだけならまだしもエラーメッセージが　Invalid argument と全く異なるものが出力された。

検証

Process.#setrlimitを見てみると何故かメソッドが 2 通り載っているが、これは
setrlimit(resource, cur_limit, max_limit = cur_limit)
ってことだろうか？ひとまず第二引数の値を色々いじって確認した。

$ irb
irb(main):001:0> Process.setrlimit(:NOFILE, 4)
=> nil
Traceback (most recent call last):
    21: from /home/jinroq/.rbenv/versions/2.5.1/bin/irb:11:in `<main>'
    20: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:383:in `start'
    19: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:427:in `run'
    18: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:427:in `catch'
    17: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:428:in `block in run'
    16: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:487:in `eval_input'
    15: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:231:in `each_top_level_statement'
    14: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:231:in `catch'
    13: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:232:in `block in each_top_level_statement'
    12: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:232:in `loop'
    11: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:236:in `block (2 levels) in each_top_level_statement'
    10: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:266:in `lex'
     9: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:292:in `token'
     8: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/slex.rb:76:in `match'
     7: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/slex.rb:206:in `match_io'
     6: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:104:in `getc'
     5: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:189:in `buf_input'
     4: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:470:in `block in eval_input'
     3: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:623:in `signal_status'
     2: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:471:in `block (2 levels) in eval_input'
     1: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/input-method.rb:149:in `gets'
/home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/input-method.rb:149:in `input=': Too many open files - dup (Errno::EMFILE)

$

$ irb
irb(main):007:0> Process.setrlimit(:NOFILE, 8)
=> nil
Traceback (most recent call last):
    21: from /home/jinroq/.rbenv/versions/2.5.1/bin/irb:11:in `<main>'
    20: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:383:in `start'
    19: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:427:in `run'
    18: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:427:in `catch'
    17: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:428:in `block in run'
    16: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:487:in `eval_input'
    15: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:231:in `each_top_level_statement'
    14: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:231:in `catch'
    13: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:232:in `block in each_top_level_statement'
    12: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:232:in `loop'
    11: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:236:in `block (2 levels) in each_top_level_statement'
    10: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:266:in `lex'
     9: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:292:in `token'
     8: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/slex.rb:76:in `match'
     7: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/slex.rb:206:in `match_io'
     6: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:104:in `getc'
     5: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/ruby-lex.rb:189:in `buf_input'
     4: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:470:in `block in eval_input'
     3: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:623:in `signal_status'
     2: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb.rb:471:in `block (2 levels) in eval_input'
     1: from /home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/input-method.rb:150:in `gets'
/home/jinroq/.rbenv/versions/2.5.1/lib/ruby/2.5.0/irb/input-method.rb:150:in `output=': Too many open files - dup (Errno::EMFILE)

$

$ irb
irb(main):001:0> Process.setrlimit(:NOFILE, 9)
=> nil
irb(main):002:0> p Process.getrlimit(:NOFILE)
[9, 9]
=> [9, 9]
irb(main):003:0> 

まとめるとこんな感じ。

cur_limit の値	実行結果
3	Invalid argument - dup (Errno::EINVAL)
4 以上 8 以下	Too many open files - dup (Errno::EMFILE)
9 以上	正常に制限値が設定される

ただし cur_limit = 8 の場合だけ、エラー箇所が input-method.rb:150 となっていた。

分かりそうな人に聞いてみた

という訳で近くにいた nagachika さんに聞いてみた。非常に親切丁寧に解説していただいたのだが、正直お話がディープすぎてあまり理解できていない。記憶を探って書いてみる。

• 「irb が落ちている」のではなく、「irb が何かしらの理由で処理を停止させている」
• irb の --noreadline オプションを付けると停止しないはず
• irb 起動中のファイルディスクリプターの最小値は 3 ではなく、実は他にも動いている

確かめてみた

$ irb --noreadline
irb(main):001:0> Process.setrlimit(:NOFILE, 3)
=> nil
irb(main):002:0> File.open('/dev/null')
Traceback (most recent call last):
        4: from /home/jinroq/.rbenv/versions/2.5.1/bin/irb:11:in `<main>'
        3: from (irb):2
        2: from (irb):2:in `open'
        1: from (irb):2:in `initialize'
Errno::EMFILE (Too many open files @ rb_sysopen - /dev/null)
irb(main):003:0>

おお！

$ irb
irb(main):001:0> require 'objspace'
=> true
irb(main):002:0> ObjectSpace.each_object(IO).each{ |i| p i unless i.closed? }
#<IO:<STDERR>>
#<IO:<STDOUT>>
#<IO:<STDIN>>
#<IO:fd 1>
#<IO:fd 0>
=> 6
irb(main):003:0> 

おおー！

まとめ

• readline を使わなければ期待通りに動く

おまけ

$ irb
irb(main):001:0> passwd = File.open('/etc/passwd')
=> #<File:/etc/passwd>
irb(main):002:0> puts passwd.fileno
9
=> nil
irb(main):003:0>

どうも 9 が最小値っぽい。また本の訳注には「ruby 1.9.3-p392/2.0.0-p0 では最小値 5。3 と 4 は YARV に予約されていた」とあった。つまり、

irb 起動中のファイルディスクリプター番号	予約している人
0, 1	fd
2	STDIN
3	STDOUT
4	STDERR
5, 6, 7	YARV

なんじゃないかな、と思った。（ツッコミ待ち）