Playing with the Ruby Garbage Collector

playing_with_gc.rb

require "objspace"

class TestWithFinalizer

  def self.finalizer
    proc {|o_id| puts "Collected #{o_id}"}
  end

  def test
    3.times.each_with_object(mark []) {|i,a|
      a << mark({ i => i })
    }
  end

  def mark(x)
    ObjectSpace.define_finalizer(x, TestWithFinalizer.finalizer)
    puts "Marked #{x.object_id} - #{x.class}"
    x
  end

end

pry-session-01.rb

# This is the confusing session. It consistently never collects the last reference until after I `exit`.
# I can also successfully get the object from the object_id via `ObjectSpace._id2ref()`

t = TestWithFinalizer.new
o = t.test; nil  # follow with 'nil' or Pry stores the output ref
# Marked 18869680 - Array
# Marked 18869420 - Hash
# Marked 18869220 - Hash
# Marked 18869020 - Hash

o = nil

ObjectSpace.garbage_collect
# Collected 18869680
# Collected 18869420
# Collected 18869220

exit
# Collected 18869020

pry-session-02.rb

# This is the way I would expect it to behave.

t = TestWithFinalizer.new

o1 = t.test; nil
# Marked 22159300
# Marked 22159280
# Marked 22159260
# Marked 22159240

o2 = t.test; nil
# Marked 22268960
# Marked 22268940
# Marked 22268920
# Marked 22268900

o1 = nil
ObjectSpace.garbage_collect
# Collected 22159300
# Collected 22159280
# Collected 22159260
# Collected 22159240

o2 = nil

ObjectSpace.garbage_collect
# Collected 22268960
# Collected 22268940
# Collected 22268920
# Collected 22268900

exit