Assertion failure in mrb_write_barrier

[clayton-shopify]

The following input demonstrates an assertion failure:

GC.start
ObjectSpace.each_object{ GC.generational_mode = nil }
a

Assertion failed: (((gc)->generational) || gc->state != MRB_GC_STATE_ROOT), function mrb_write_barrier, file /Users/clayton/git/mruby/src/gc.c, line 1304.

This issue was reported by Dinko Galetic & Denis Kasak (https://hackerone.com/dgaletic).

[matz]

It turns out it's (almost) impossible to keep consistency in GC after changing generational mode during iteration. I will make it impossible by raising an exception.