On Thu, May 27, 2021 at 8:24 PM Pablo Galindo Salgado <pablog...@gmail.com> wrote: > > And if a type pointer is the only thing being visited, then there's no > > point unless the object can itself be reachable from the type object. > > But that could happen easily for heap types as they are mutable by default. > For instance, you set the instance in a global: > > type -> module -> globals -> instance -> type
Immutable heap types have the same leak problem. Making a mutable heap immutable doesn't solve the problem. Example of my msg385297: "If one of these conditions is not met, the GC can fail to destroy a type during a GC collection. If an instance is kept alive late while a Python interpreter is being deleted, it's possible that the type is never deleted, which can keep indirectly many objects alive and so don't delete them neither." For a more concrete example, read the "_thread lock traverse" section of my article on these problems: https://vstinner.github.io/subinterpreter-leaks.html There were two reference cycles, and both were "connected" with a lock object in the middle (look at my drawing). The lock object was *not* directly part of any ref cycle. But because it had no traverse function, the GC failed to break both cycles in a single collection. A second manual GC collection was needed to break the second cycles, to *work around* the issue. The problem is that a small reference cycle can keep tons of Python objects alive way longer than expected. If a heap cycle doesn't fully implement the GC protocol, in the worst case, these objects are never deleted. Victor -- Night gathers, and now my watch begins. It shall not end until my death. _______________________________________________ python-committers mailing list -- python-committers@python.org To unsubscribe send an email to python-committers-le...@python.org https://mail.python.org/mailman3/lists/python-committers.python.org/ Message archived at https://mail.python.org/archives/list/python-committers@python.org/message/BM55YS7NBNWAHTGEFJS4HSK7UJNRZIDX/ Code of Conduct: https://www.python.org/psf/codeofconduct/
