Okay that makes sense. :)

I looked a bit at the gc code. A consumer object always has one
reference to a memoryview with an export, which isn't visited. So
it looks to me that the gc_refs of that memoryview cannot fall to 0.

So memory_clear() isn't called in that case, but mbuf_clear() is, which
is known and expected to handle mbuf->exports >= 0.

Indeed let's perhaps just add "if (self->exports > 0) return 0" to memory_clear() if those assumptions are too complex.