From: zed.0xff@...
Date: 2014-07-02T22:20:28+00:00
Subject: [ruby-core:63523] [ruby-trunk - Bug #10007] [Open] IO#read does not respect String Copy-On-Write in some cases

Issue #10007 has been reported by Andrey Zaikin.

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Bug #10007: IO#read does not respect String Copy-On-Write in some cases
https://bugs.ruby-lang.org/issues/10007

* Author: Andrey Zaikin
* Status: Open
* Priority: High
* Assignee: 
* Category: 
* Target version: 
* ruby -v: ruby 2.0.0p481 (2014-05-08 revision 45883) [x86_64-linux]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
~~~
#!/usr/bin/env ruby

CHUNK_SIZE = 29            # no problems with CHUNK_SIZE = 28

File.open(__FILE__,'r') do |f|
  d0 = ''
  a = []

  f.read(CHUNK_SIZE, d0)
  a << d0[15..-2]
  a << d0[5..-2]
  a << d0[-5..-2]
  a << d0[15..-1]
  a << d0[5..-1]           # <-- only this string will change
  a << d0[-5..-1]
  p a
  f.read(CHUNK_SIZE, d0)   # <-- 2nd read
  p a
end
~~~

output on ruby 1.9.3p448: **(all strings are similar before and after 2nd read)** 

~~~
["ruby\n\nCHUNK_S", "r/bin/env ruby\n\nCHUNK_S", "NK_S", "ruby\n\nCHUNK_SI", "r/bin/env ruby\n\nCHUNK_SI", "NK_SI"]
["ruby\n\nCHUNK_S", "r/bin/env ruby\n\nCHUNK_S", "NK_S", "ruby\n\nCHUNK_SI", "r/bin/env ruby\n\nCHUNK_SI", "NK_SI"]
~~~

output on ruby 2.0.0p481, 2.0.0p353, 2.0.0p247: **(5th string changed unexpectedly!)**

~~~
["ruby\n\nCHUNK_S", "r/bin/env ruby\n\nCHUNK_S", "NK_S", "ruby\n\nCHUNK_SI", "r/bin/env ruby\n\nCHUNK_SI", "NK_SI"]
["ruby\n\nCHUNK_S", "r/bin/env ruby\n\nCHUNK_S", "NK_S", "ruby\n\nCHUNK_SI", "29\n\nFile.open(__FILE__,'", "NK_SI"]
~~~



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