Handles / Opaque linear references

[encukou]

Reference counting (incref/decref) is replaced by what HPy calls handles, and Mark's proposal Opaque, linear references:

• Each handle has exctly one owner.
• Handles are destroyed by PyRef_Close.
• Handles are duplicated using PyRef_Dup. Each must be destroyed (i.e. you can't Close one of them twice).
• There can be multiple handles for a single object.
• Object identity is compared using PyRef_Is, not by a pointer comparison

HPy shows that this enables a moving GC. It does need all users to update all their code, so CPython probably can't ever require this. But we could provide PyRef_Dup/PyRef_Close/PyRef_Is (implemented as Py_INCREF/Py_DECREF/Py_Is) to allow users to signal that they're using handle semantics, and are (theoretically) compatible with implementations that use a moving GC.

[gvanrossum]

Why are these called "linear references"? Is there a common definition of that term?

[encukou]

@markshannon, there's some academic background for the term, right? I hope I'm using it correctly :)

[jeanas]

Why are these called "linear references"? Is there a common definition of that term?

See https://en.wikipedia.org/wiki/Substructural_type_system#Linear_type_systems

[vstinner]

See also https://github.com/capi-workgroup/problems/issues/37