From: Yusuke ENDOH <mame@...>
Date: 2009-02-04T22:56:11+09:00
Subject: [ruby-core:21833] Re: 1.9 Bug Report

Hi,

2009/2/4 Yehuda Katz <wycats@gmail.com>:
> When going through the RubySpecs for 1.9, I found that the following code
> will hang the 1.9 VM:
>
> hsh = {1 => 2, 3 => 4, 5 => 6}
> hsh.merge!(hsh) {|x, y, z| p [x, y, z]; hsh.shift; true }
> p hsh
>
> The output in 1.8:
> [5, 6, 6]
> [1, 2, 2]
> [3, 4, 4]
> {3=>true}

It's by chance that your code stops in 1.8, I think.
In 1.8, an iteration order of hash is unspecified.  So, when you insert
an new key into a hash during its iteration, it is also unspecified
whether the new key will appear on the remaining iteration or not.

On the other hand, 1.9's hash is insertion-ordered.  An newly inserted
key will certainly appear at the last of the order.
So inserting new keys during an iteration makes the iteration infinite
loop.


$ cat test.rb
h = {1 => nil, 2 => nil}
h.each_key {|x| p [h, x]; h.delete(x); h[x] = nil }

$ ruby18 test.rb
[{1=>nil, 2=>nil}, 1]
[{1=>nil, 2=>nil}, 2]

$ ruby19 test.rb
[{1=>nil, 2=>nil}, 1]
[{2=>nil, 1=>nil}, 2]
[{1=>nil, 2=>nil}, 1]
[{2=>nil, 1=>nil}, 2]
[{1=>nil, 2=>nil}, 1]
[{2=>nil, 1=>nil}, 2]
[{1=>nil, 2=>nil}, 1]
[{2=>nil, 1=>nil}, 2]
[{1=>nil, 2=>nil}, 1]
[{2=>nil, 1=>nil}, 2]
...

-- 
Yusuke ENDOH <mame@tsg.ne.jp>