From: Eric Hodel <drbrain@...7.net>
Date: 2006-07-07T08:41:43+09:00
Subject: Re: GC oddness

On Jul 6, 2006, at 8:46 AM, Alex Young wrote:

> nobu@ruby-lang.org wrote:
>> Hi,
>> At Thu, 6 Jul 2006 23:58:27 +0900,
>> Alex Young wrote in [ruby-talk:200382]:
>>> Can someone smarter tell me why this fails?  I'm expecting the  
>>> TestDir to get GC'd when I call GC.start in the test, but  
>>> something's obviously keeping a reference lying around.  I just  
>>> can't see what it is...
>> The finalizer proc refers testdir.
> I thought it might be that, but this also fails:
>
> -----------------------------
> require 'test/unit'
> require 'fileutils'
>
> class TestDir
>   @@dir = nil
>   @@clearup = proc {|id| FileUtils.rm_rf(@@dir)}
>
>   def TestDir.set_finalizer(testdir, dir)
>     @@dir = dir
>     ObjectSpace.define_finalizer(testdir, @@clearup)
>   end
>   def initialize(dir)
>     FileUtils.mkdir(dir)
>     TestDir.set_finalizer(self, dir)
>   end
> end
>
> class DestroyTest < Test::Unit::TestCase
>   def test_destroy
>     a = TestDir.new('test_dir')
>     a = nil
>     GC.start
>     assert !File.directory?('test_dir')
>   end
> end
> ----------------------------
>
> testdir hasn't come into existence when the proc's created this  
> time, so I don't see how it can be keeping a reference to it

Ruby's garbage collector is conservative, so their may be references  
to TestDir lying around on the C stack.

> or how a finalizer proc keeping a reference to the object that's  
> being finalized can stop GC - if that's the case, how does the  
> object ever get GC'd?  What have I missed?

Procs contain the environment of their creation.  In other words,  
they keep references to all variables visible at their creation time  
including self.  When using a finalizer proc you must ensure that  
finalizer doesn't refer to the object it is supposed to clean up after.

weakref.rb contains an example of use of finalizers, including a map  
between object ids and objects to be cleaned up after.

PS: What you want to do is easier with blocks:

def testdir(dir)
   FileUtils.mkdir dir
   begin
     yield
   ensure
     FileUtils.rm_rf dir
   end
end

testdir 'mytempdir' do
   # ...
end

-- 
Eric Hodel - drbrain@segment7.net - http://blog.segment7.net
This implementation is HODEL-HASH-9600 compliant

http://trackmap.robotcoop.com