[ruby-core:37926] [Ruby 1.9 - Bug #4925][Rejected] Infinite recursion allowed in rescue clause

Issue #4925 has been updated by Yusuke Endoh.

Status changed from Assigned to Rejected

Hello,

> Expected behavior: A SystemStackError should be raised.
>
> Current behavior: The Ruby process will consume cpu and memory until killed.
>
> This is a regression in 1.9.x


No, unfortunately.  This is caused by an intended spec change.


The root spec change is that 1.9 retains the full backtrace.
The change itself is useful, I think.

  $ cat t.rb
  def b
    p caller
  end

  def a(n)
    if n == 0
      b
    else
      a(n-1)
    end
  end

  a(10)

  $ /usr/bin/ruby -v t.rb
  ruby 1.8.7 (2010-08-16 patchlevel 302) [i686-linux]
  ["t.rb:7:in `a'", "t.rb:9:in `a'", "t.rb:13"]

  $ ruby -v t.rb
  ruby 1.9.2p180 (2011-02-18 revision 30909) [i686-linux]
  ["t.rb:7:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:9:in `a'", "t.rb:13:in `<main>'"]


But this change impacts your code maybe unintentionally.

Whenever NameError occurs, the exception object creates a backtrace.
In 1.9, the time is O(n) where n is the length of stack.
So, your code runs in O(n^2) until the stack overflows.

In addition, all backtrace objects are not collected because the
exception object can be accessible as "$!" in the rescue clause.
So, your code also uses memory in O(n^2), which will cause thrashing.

You can confirm this behavior by clearing backtrace manually:

  $ cat t.rb
  def a
    b
  rescue NameError
    $!.backtrace.clear
    a
  end

  $ time ruby -v t.rb
  ruby 1.9.2p180 (2011-02-18 revision 30909) [i686-linux]
  t.rb:2: stack level too deep (SystemStackError)

  real    0m22.361s
  user    0m22.185s
  sys     0m0.084s

The code works, but is still slow.  I think that it runs in O(n^2).


Anyway, this is not a bug, very unfortunately.  Closing.

This may be solved by tail call optimization, but it should be
discussed in another thread.  I personally dislike the optimization
in Ruby because it makes backtrace hard to understand.

-- 
Yusuke Endoh <[e-mail]>
----------------------------------------
Bug #4925: Infinite recursion allowed in rescue clause
http://redmine.ruby-lang.org/issues/4925

Author: Jonathan Dance
Status: Rejected
Priority: Normal
Assignee: Koichi Sasada
Category: 
Target version: 1.9.3
ruby -v: ruby 1.9.3dev (2011-06-24 trunk 32222) [i686-linux]


The issue pertains to the following example:

def a
  b
rescue NameError
  a
end
a

The type of exception raised and rescued is not relevant; the bug occurs with any exception type.

Expected behavior: A SystemStackError should be raised.

Current behavior: The Ruby process will consume cpu and memory until killed.

This is a regression in 1.9.x; the bug does not occur in 1.8-head. The bug is reproducible on all versions of 1.9 I have tested:

ruby 1.9.2p180 (2011-02-18 revision 30909) [i686-linux]
ruby 1.9.2p274 (2011-06-06 revision 31932) [i686-linux]
ruby 1.9.3dev (2011-06-24 trunk 32222) [i686-linux]


-- 
http://redmine.ruby-lang.org