From: "Ara.T.Howard" <ahoward@...>
Date: 2003-10-07T04:02:16+09:00
Subject: bug in open/flock ?


in the code below, i am able to obtain TWO exclusive locks on files using the
following logic:

  iff flock(LOCK_EX | LOCK_NB) fails, then open again and retry

this should not be possible should it??

note that the following code SHOULD deadlock - but does not.

CODE:

----CUT----
#!/usr/local/ruby-1.8.0/bin/ruby
threads = []
flags = (File::LOCK_EX | File::LOCK_NB)

system 'touch a b' rescue nil

threads << Thread.new do
  %w(a b).each do |path|
    fd = open(path)
    #until((ret = fd.flock flags))        # this works (deadlock)
    until((ret = open(path).flock flags)) # this doesn't
      Thread.pass
    end
    printf "0 LOCK_EX %s <%s>\n", path, ret
    sleep 0.5
  end
end

threads << Thread.new do
  %w(b a).each do |path|
    fd = open(path)
    #until((ret = fd.flock flags))        # this works (deadlock)
    until((ret = open(path).flock flags)) # this doesn't
      Thread.pass
    end
    printf "1 LOCK_EX %s <%s>\n", path, ret
    sleep 0.5
  end
end

Thread.abort_on_exception = true
threads.map{|thread| thread.join}
----CUT----


OUTPUT:

[ahoward@localhost flock]$ ./flock.rb
0 LOCK_EX a <0>
1 LOCK_EX b <0>
0 LOCK_EX b <0>
1 LOCK_EX a <0>


so, two threads are able to obtain exclusive locks on a file?  perhaps i am
making an obvious mistake??  shouldn't each call to open(path).flock be
referring to the same open file table entry and, therefore, not affect the
thread's ability to obtain an LOCK_EX?

-a
  ====================================
  | Ara Howard
  | NOAA Forecast Systems Laboratory
  | Information and Technology Services
  | Data Systems Group
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