test_asyncio started to crash in https://github.com/python/cpython/pull/7487

I debugged the crash with Yury: _PyHAMT_New() triggers indirectly a GC collection at "o->h_root = hamt_node_bitmap_new(0);". Problem: the object that is being created is already tracked by the GC, whereas its h_root field is not set. Then the GC does crash because the field is a random pointer.

hamt_alloc() must initialize h_root and h_count before tracking the object in the GC.

Thinking about the GC when writing an object constructor is hard :-(

Maybe we should add a debug option to trigger the GC more often to make such random bug more likely. contextvars has been implemented a few months ago, and we only spotted the bug a few days before Python 3.7.0 final release...