Can someone explain the "write barrier" stuff? It's hard to glean what the problem is that they're solving here... And what they're saying about Arrays, Strings, Hash, etc, in relation to WB protected vs. WB unprotected.
I've not read the code, so someone may have to correct me, but I think this is the gist of it:
Ruby 2.1 introduces a generational garbage collector, this divides all objects into young and old generations. A regular GC run will only look at the young generation, with the old being collected less frequently. An object is promoted to the old generation when it survives a young generation run.
If you have objects in the old generation referring objects in the young generation, but you're only looking at the young generation it may seem like an object doesn't have any references, and you might incorrectly GC an in-use object. Write barriers prevent this by adding old generation objects to a 'remember set' when they are modified to refer to a young generation object (eg old_array.push(young_string)). This 'remember set' is then taken in to account when collecting the young generation.
Most generational garbage collectors need these write barriers on all objects, but with the many 3rd party C extensions available for Ruby this isn't possible, so a workaround was devised whereby objects that aren't write barrier protected won't ever be promoted to the old generation. This isn't ideal as you won't get the full benefit of the generational GC, but it does maximise backwards compatibility.
