Dynamic borrow checking for stackrefs

[mpage]

Feature or enhancement

Proposal:

#130708 introduces stackrefs whose lifetime must be shorter than the lifetime of another stackref. We should verify this property in the debug build of stackrefs: warn anytime a stackref with outstanding borrowed references is closed.

Here is a sketch of a possible implementation:

• Add a counter, borrows, to each stackref that records the number of borrows.
• Add a borrowed_from field to each stackref that records the index of the stackref it was borrowed from, if any. This field is preserved by PyStackRef_DUP.
• PyStackRef_Borrow(from) sets the the borrowed_from field on the returned stackref and increments the borrows field on from. We should follow chains of borrows until we reach the unborrowed reference at the head.
• PyStackRef_CLOSE on a borrowed stackref decrements the borrows field on the borrowed-from stackref.
• PyStackRef_CLOSE warns if borrows is non-zero.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response