This module allows you to see if an object is retained "still alive" or if it is freed "dead".

There be dragons, see LivingDead.gc_start to see some of the hacks we have to do, for who knows why, to get this to work.

In your Gemfile add:

gem 'living_dead'

Then run

$ bundle install

Before you use this you should understand how it works. This gem is a c-extension. It hooks into the Ruby tracepoint API and registers a hook for the RUBY_INTERNAL_EVENT_FREEOBJ event. This event gets called when an object is freed (i.e. it is not retained and garbage collection has been called).

When you call LivingDead.trace(obj) we store the object_id of the thing you are "tracing" in a hash. Then inside of our c-extension hook we listen for when an object is freed. When this happens we check to see if that object's object_id matches one in our hash. If it does we mark it down in a separate "freed" hash.

We don't retain the objects you are tracing but we do keep a copy of the object_id, we can then use this same number to check the freed hash to see if it was recorded as being freed.

WARNING: Seriously, see LivingDead.gc_start. This library isn't bullet proof.

Require the library and use LivingDead.trace to "trace" an object. Later use LivingDead.traced_objects to iterate through "tracers" of each object.

Here is an example of tracing an object that is not retained:

require 'living_dead'

def run
  obj = Object.new
  LivingDead.trace(obj)

  return nil
end

run

puts LivingDead.traced_objects.select {|tracer| tracer.retained? }.count
# => 0

Note: Calling LivingDead.traced_objects auto calls GC.start, you don't need to do it manually. However you should look at the implementation of LivingDead.gc_start to understand the depth of hacks you're playing with.

Here is an example of tracing an object that IS retained:

require 'living_dead'

def run
  obj = Object.new
  LivingDead.trace(obj)

  return obj
end

@retained_here = run

puts LivingDead.traced_objects.select {|tracer| tracer.retained? }.count
# => 1

You can get more ways of interacting with a tracer by looking at LivingDead::ObjectTrace.

Compile the code:

$ rake compile

Run the tests:

$ rake spec

or "why not both":

$ rake compile spec

MIT

Copyright Richard Schneeman 2016