[PATCH 0/4] Disallow the return of borrowed references

[Tom Tromey <tom@tromey.com>]

While working on a different Python patch, I misread
pspace_to_pspace_object and thought it was not handling reference
counts properly.

It is -- but I found it difficult to reason about because it returns a
borrowed reference, but uses gdbpy_ref as well.

This series changes gdb so that none of the Python functions will
return a borrowed reference.  These functions are modified instead to
return a new reference using gdbpy_ref<>, which not only makes the
intent clear, but also makes it more difficult to have a buggy caller.

This could go further and make all functions return gdbpy_ref<>.
(Even Python-facing ones could be done via template fuction wrappers.)
However I have not done this.

I think now gdb should disallow Python functions returning borrowed
references.  Accepting a borrowed reference is still ok, and I think
should continue to be.

I think this series found at least one bug:
infpy_thread_from_thread_handle could return None without incref'ing
it.

Let me know what you think.  I tested this on x86-64 Fedora 28.

Tom