I opened this issue, because I was convinced that it would be easy to implement checks faster than gc.setthreshold(), but I failed to write efficient tests which detect the bugs that I listed above.

My approach was basically to check all objects tracked by the GC every N memory allocations (PyGC_Malloc): too slow. I tried to put thresholds per generation: it was still too slow.

Maybe recent objects should be checked often, but old objects should be checked less often. For example, only check objects in generation 0 and scan new objects, and then remember the size of generation 0. At the next check, ignore objects already checked.

I failed to find time and interest to implement this approach. I abandon this issue and my PR.

In the meanwhile, gc.set_threshold(5) can be used. It isn't too slow and is quite good to find most bugs listed in this issue.