py: fix reference count bug in From(Py<T>) for PyObject

[davidhewitt]

I have just discovered a reference counting issue in one of our From implementations. It leads to a reference count being decreased when it should not. Unfortunately this was an implementation introduced by me since 0.12 🤦

The offending line is here:

pyo3/src/instance.rs

Line 477 in 32be8d9

let Py(ptr, _) = other;

I incorrectly thought the destructuring would mean that Py::drop() is not run for other. What actually happens is that ptr implements Copy, so it is copied from other, and then other is dropped.

This means ptr has a reference count decreased when it should not.

This PR corrects the issue and adds a test. The test fails on master.

cc @kngwyu - I think I will have to backport a bugfix 0.12.4 and yank the rest of the 0.12 releases.

[kngwyu]

Good catch, thank you!

What actually happens is that ptr implements Copy, so it is copied from other, and then other is dropped.

Oops, I also didn't notice this 😨

I think I will have to backport a bugfix 0.12.4 and yank the rest of the 0.12 releases.

👍