Python 3.11+: Add __notes__ to error_already_set::what() output.

[rwgk]

Description

The original motivation for this PR was to account for a a fairly major behavior change in Python 3.12:

• Suspected PyErr_Fetch() behavior change python/cpython#102594

Python 3.12 PyErr_Fetch() normalizes exceptions immediately and tracks any normalization errors under __notes__:

• https://github.com/python/cpython/pull/102675/files

__notes__ were introduced already with Python 3.11, but this was not reflected in pybind11 until now. Adding them to the error_already_set::what() output therefore catches up with Python developments in general, and ensures that exception normalization errors continue to be obvious.

A highly technical detail for completeness:

The FAILURE obtaining and FAILURE formatting conditions are impractical to exercise in the usual way via unit tests, but a manual pass was made exercising them with temporary tweaks.

Suggested changelog entry:

Python exception ``__notes__`` (introduced with Python 3.11) are now added to the ``error_already_set::what()`` output.

[EthanSteinberg]

Looks mostly good, with two possible flaws

[EthanSteinberg]

Shouldn't this be reinterpret_borrow?

[rwgk]

This is copy-pasted from the existing code further up (btw I didn't see enough meat to factor out).

https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_AsEncodedString

returns a new reference, we have to take ownership.

(Unfortuantely IMO) it's called "steal" instead of "take_ownership", I think this is correct.

[EthanSteinberg]

Oops, nvm. Misread this and didn't notice the PyUnicode_AsEncodedString. You are 100% right.

[EthanSteinberg]

Is this version check necessary? I know it should only be true for python 3.11 and above, but the hasattrstring is implicitly a version check.

[rwgk]

I'm thinking of it as a very easy runtime optimization, eliding the runtime overhead for the attribute lookup. — A while ago I looked at the implementation and it's actually quite involved. It basically does a full getattr() equivalent with PyErr_Clear().

[rwgk]

Thanks for the review @lalaland!

I just tested this PR also with Python 3.12.0b1 + two patches to make that possible (references below). I'll merge this now so that it'll be easier to keep up with 3.12 developments.

---

1. Locally applied https://patch-diff.githubusercontent.com/raw/python/cpython/pull/104742.diff (unfortunately that came too late for the Python 3.12.0b1 cut).

2. Related to gh-94673: More Per-Interpreter Fields for Builtin Static Types python/cpython#103912:

commit b66198c8345614df7ddf34402cbf5670ae298a6f (HEAD -> wrk)
Author: Ralf W. Grosse-Kunstleve <rwgk@google.com>
Date:   Tue May 23 09:46:48 2023 -0700

    Check for `t->tp_bases == nullptr`

diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index 16387506..eb7ec7a3 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -104,6 +104,10 @@ all_type_info_get_cache(PyTypeObject *type);

 // Populates a just-created cache entry.
 PYBIND11_NOINLINE void all_type_info_populate(PyTypeObject *t, std::vector<type_info *> &bases) {
+    if (t->tp_bases == nullptr) {
+        // Since https://github.com/python/cpython/pull/103912 (Python 3.12.0b1)
+        return;
+    }
     std::vector<PyTypeObject *> check;
     for (handle parent : reinterpret_borrow<tuple>(t->tp_bases)) {
         check.push_back((PyTypeObject *) parent.ptr());
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 28ebc222..db1c5a99 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1363,6 +1363,10 @@ protected:

     /// Helper function which tags all parents of a type using mult. inheritance
     void mark_parents_nonsimple(PyTypeObject *value) {
+        if (value->tp_bases == nullptr) {
+            // Since https://github.com/python/cpython/pull/103912 (Python 3.12.0b1)
+            return;
+        }
         auto t = reinterpret_borrow<tuple>(value->tp_bases);
         for (handle h : t) {
             auto *tinfo2 = get_type_info((PyTypeObject *) h.ptr());