gh-131527: Stackref debug borrow checker

[efimov-mikhail]

• Issue: Dynamic borrow checking for stackrefs #131527

[efimov-mikhail]

It works fine and generally follows the sketch in the issue.
But I don't follow chains of borrows and check only the nearest borrowed_from stackref.

There are 17 failed tests with this PR in Py_STACKREF_DEBUG mode.

17 tests failed:
test.test_gdb.test_backtrace test.test_gdb.test_misc
test.test_gdb.test_pretty_print test.test_io.test_general
test.test_multiprocessing_fork.test_misc
test.test_multiprocessing_forkserver.test_misc
test.test_multiprocessing_spawn.test_manager
test.test_multiprocessing_spawn.test_misc test_external_inspection
test_faulthandler test_frame test_interpreters test_profiling
test_pyrepl test_regrtest test_repl test_threading

Compare with
#139475 (comment)

Here test.test_io.test_general is a bit flaky, but test_frame is a real find by new borrow checker.

-> % ./python -m unittest -v test.test_frame.TestFrameLocals.test_overwrite_locals
test_overwrite_locals (test.test_frame.TestFrameLocals.test_overwrite_locals) ... Fatal Python error: _Py_stackref_close: StackRef with ID 2401080 closed while borrows 1 refs at Objects/frameobject.c:288. Opened at Python/generated_cases.c.h:1239

There's no real problem here but we can rewrite it a little to prevent this error and make code related to frame->f_overwritten_fast_locals clearer.

[mpage]

Nice! Just some minor inline comments/suggestions. Feel free to push back if you don't agree.

[mpage]

LGTM - thanks!

[efimov-mikhail]

Thanks a lot for merge, @mpage!

Maybe you'll be interesting in another stackref related PR?
#139717