Assertion failure in mrb_gc_mark

[clayton-shopify]

The following input demonstrates a crash:

f = Fiber.new do
    m = Fiber.current
    Fiber.yield Proc.new {}
end

f = f.resume
GC.start

Result:

Assertion failed: ((obj)->tt != MRB_TT_FREE), function mrb_gc_mark, file /Users/clayton/git/mruby/src/gc.c, line 698.
Abort trap: 6

This issue was reported by Dinko Galetic and Denis Kasak (https://hackerone.com/dgaletic). They believe the issue was introduced by ecee8c5.

[matz]

I couldn't reproduce the problem (even with GC_STRESS). Hopefully it's already fixed.

[clayton-shopify]

@matz This still crashes for me on the latest master. I just tested on OS X and Ubuntu.

[clayton-shopify]

Building with MRB_GC_STRESS makes the crash go away, so you need to build without that.