Memoize Virtus attribute and fix memory leak.

[marshall-lee]

This one fixes a serious problem we faced in production under a heavy load. Investigation led us to the fact that it happens only when params are defined with type: Array[SomeClass]. In this case Virtus dynamically create some virtual classes (with Class.new) which inherits from base class with DescendantsTracker added which saves descendant classes into the array. Here we have a leak because garbage collector doesn't free these classes because an array still holds a reference to them.

So at least I can say that Virtus isn't designed for creating things dynamically. They should be stored somehow statically.

@dblock
Look, this is funny! Remember ruby-grape/grape-entity#161 ? This is why it is bad to store descendant classes in the array. Look at the implementation of DescendantsTracker — is it similar, right?

[marshall-lee]

Some demonstrating examples:

Non-leaking:

require 'virtus'

type = Integer

10.times do |i|
  ObjectSpace.garbage_collect
  puts ObjectSpace.count_objects[:T_CLASS]
  Virtus::Attribute.build(type)
end

outputs:

854
858
858
858
858
858
858
858
858
858

Leaking:

type = Array[String]

10.times do |i|
  ObjectSpace.garbage_collect
  puts ObjectSpace.count_objects[:T_CLASS]
  Virtus::Attribute.build(type)
end

outputs:

854
865
867
869
871
873
875
877
879
881

[dblock]

This is solid work @marshall-lee, thanks.