DM-21314: Investigate GC problems with Storable

[kfindeisen]

This PR adds a custom holder type that blocks Python garbage collection of the associated object for the lifetime of the holder. It must be used for any class that supports Python subclasses.

Despite my best efforts, I only partially understand how PySharedPtr works. In particular, while I know that all C++ classes with direct Python subclasses need to use PySharedPtr as their holder type, I'm not sure if the same goes for their superclasses, or whether that depends on, e.g., the static types used in function signatures.

[TallJimbo]

while I know that all C++ classes with direct Python subclasses need to use PySharedPtr as their holder type, I'm not sure if the same goes for their superclasses

This seems the same as the question of whether one could use unique_ptr as the holder for one class while using shared_ptr for another class with which it has any kind of inheritance relationship, and I do know the answer to that is definitely "no".

[kfindeisen]

This seems the same as the question of whether one could use unique_ptr as the holder for one class while using shared_ptr for another class with which it has any kind of inheritance relationship, and I do know the answer to that is definitely "no".

Can you elaborate? I thought mixing unique_ptr or shared_ptr, or relying on their one-way implicit conversion, does not compile, whereas the only problem I've run into so far is that you don't get PySharedPtr's keep-alive reference if you declare a subclass with shared_ptr.

[TallJimbo]

Can you elaborate? I thought mixing unique_ptr or shared_ptr, or relying on their one-way implicit conversion, does not compile, whereas the only problem I've run into so far is that you don't get PySharedPtr's keep-alive reference if you declare a subclass with shared_ptr.

Interesting that the holder types are treated differently w.r.t. whether the code compiles; I wonder if the implicit conversions are what make the difference. In any case, I don't have anything to add - I just wanted to make sure you knew what I knew about shared_ptr and unique_ptr, but you already knew that and then some.

[kfindeisen]

Ok. I don't want to add PySharedPtr to every single subclass of Persistable if I don't have to, so I suggest we don't go that far until we know there's a problem.