[ruby-core:39221] [Ruby 1.9 - Bug #5261][Open] Symbol#to_proc memory leak in 1.9.x

Issue #5261 has been reported by Ken Simon.

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Bug #5261: Symbol#to_proc memory leak in 1.9.x
http://redmine.ruby-lang.org/issues/5261

Author: Ken Simon
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 1.9.x
ruby -v: ruby 1.9.2p290 (2011-07-09 revision 32553) [x86_64-linux]


=begin
It appears that running an array through .map(&:foo) leaks the array's contents, and they don't get picked up by the Garbage Collector.

Given a simple class:

 class C
   def foo
    "foo"
   end
 end

The following appears to leave references around (1.9.3-preview1 irb session shown, ruby -v gives ruby -v
ruby 1.9.3dev (2011-07-31 revision 32789) [x86_64-darwin11.1.0]):

 ruby-1.9.3-preview1 :001 > a = 10.times.map{C.new}
  => [... snip ...]
 ruby-1.9.3-preview1 :002 > b = a.map(&:foo)
  => ["foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo"] 
 ruby-1.9.3-preview1 :003 > a = b = nil
  => nil 
 ruby-1.9.3-preview1 :004 > GC.start
  => nil 
 ruby-1.9.3-preview1 :005 > ObjectSpace.each_object(C){}
  => 10

If I instead run a through the block form of map, the GC collects the objects as expected:

 ruby-1.9.3-preview1 :001 > a = 10.times.map{C.new}
  => [... snip ...] 
 ruby-1.9.3-preview1 :002 > b = a.map{|x| x.foo}
  => ["foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo"] 
 ruby-1.9.3-preview1 :003 > a = b = nil
  => nil 
 ruby-1.9.3-preview1 :004 > GC.start
  => nil 
 ruby-1.9.3-preview1 :005 > ObjectSpace.each_object(C){}
  => 0

The same issue happens in 1.9.2-p180 and 1.9.2-p290, Linux and Darwin, but *not* in any 1.8 releases I've tried.

Also, as Niklas reported in the StackOverflow post I made about this (http://stackoverflow.com/questions/7263268/ruby-symbolto-proc-leaks-references-in-1-9-2-p180), replacing Symbol#to_proc with a pure-ruby equivalent solves the issue just fine:

 class Symbol
   def to_proc
     lambda { |x| x.send(self) }
   end
 end

The above has no memory leaks with a.map(&:foo).   Also, as Niklas said, calling a.map(&:foo.to_proc) explicitly doesn't involve a leak either.  The issue seems to me to be with ruby's sym_proc_cache global in string.c... when that code path is avoided, nothing seems to leak.

What I would expect is for a.map(&:foo) and a.map{|x| x.foo} to work identically, but the (&:foo) form seems to leak memory.

This issue is important to me because we had a very high-memory using codebase on our production servers and the items in my array are each a few hundred megs in size, and such memory leaks ran our servers out of memory fairly quickly.  (The explicit block way of using map works fine for now, but I want to make sure others don't hit this issue.) 


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