Fix accessing greenlet.gr_frame.f_back on Python 3.12.

Previously it crashed.

CHANGES.rst

@@ -15,9 +15,14 @@
   Building greenlet from source defaults to the shared library. Set
   the environment variable ``GREENLET_STATIC_RUNTIME=1`` at build time
   to change that.
-
 - Build binary wheels for Python 3.12 on macOS.
+- Fix compiling greenlet on a debug build of CPython 3.12.
+- Python 3.12: Fix walking the frame stack of suspended greenlets.
+  Previously accessing ``glet.gr_frame.f_back`` would crash due to
+  `changes in CPython's undocumented internal frame handling <https://github.com/python/cpython/commit/1e197e63e21f77b102ff2601a549dda4b6439455>`_.
 
+Platforms
+---------
 - Now, greenlet *may* compile and work on Windows ARM64 using
   llvm-mingw, but this is untested and unsupported. See `PR
   <https://github.com/python-greenlet/greenlet/pull/224>`_ by Adrian
@@ -162,7 +167,7 @@
 ====================
 
 Platforms
----------
+~~~~~~~~~
 
 - Add experimental, untested support for 64-bit Windows on ARM using
   MSVC. See `PR 271 <https://github.com/python-greenlet/greenlet/pull/271>`_.
@@ -185,7 +190,7 @@ Platforms
   thanks to Brandt Bucher and the CPython developers.
 
 Fixes
------
+~~~~~
 
 - Fix several leaks that could occur when using greenlets from
   multiple threads. For example, it is no longer necessary to call
@@ -205,7 +210,7 @@ Fixes
   platforms. This work is ongoing.
 
 Changes
--------
+~~~~~~~
 
 - The repr of some greenlets has changed. In particular, if the
   greenlet object was running in a thread that has exited, the repr
@@ -312,7 +317,7 @@ Changes
 - (Documentation) Publish the change log to https://greenlet.readthedocs.io
 
 Supported Platforms
--------------------
+~~~~~~~~~~~~~~~~~~~
 
 - Drop support for Python 2.4, 2.5, 2.6, 3.0, 3.1, 3.2 and 3.4.
   The project metadata now includes the ``python_requires`` data to
@@ -322,7 +327,7 @@ Supported Platforms
   <https://github.com/python-greenlet/greenlet/pull/197>`_.
 
 Packaging Changes
------------------
+~~~~~~~~~~~~~~~~~
 
 - Require setuptools to build from source.
 - Stop asking setuptools to build both .tar.gz and .zip

src/greenlet/greenlet.cpp

@@ -2209,7 +2209,9 @@ green_switch(PyGreenlet* self, PyObject* args, PyObject* kwargs)
         // It's possible it's never been switched to.
         assert(!current->args());
 #endif
-        return result.relinquish_ownership();
+        PyObject* p = result.relinquish_ownership();
+        assert(p || PyErr_Occurred());
+        return p;
     }
     catch(const PyErrOccurred&) {
         return nullptr;
@@ -2760,6 +2762,7 @@ static PyMethodDef green_methods[] = {
 };
 
 static PyGetSetDef green_getsets[] = {
+    /* name, getter, setter, doc, context pointer */
     {"__dict__", (getter)green_getdict, (setter)green_setdict, /*XXX*/ NULL},
     {"run", (getter)green_getrun, (setter)green_setrun, /*XXX*/ NULL},
     {"parent", (getter)green_getparent, (setter)green_setparent, /*XXX*/ NULL},

src/greenlet/greenlet_cpython_compat.hpp

@@ -57,9 +57,15 @@ Greenlet won't compile on anything older than Python 3.11 alpha 4 (see
 #ifndef _Py_DEC_REFTOTAL
 /* _Py_DEC_REFTOTAL macro has been removed from Python 3.9 by:
   https://github.com/python/cpython/commit/49932fec62c616ec88da52642339d83ae719e924
+
+  The symbol we use to replace it was removed by at least 3.12.
 */
 #    ifdef Py_REF_DEBUG
+#      if GREENLET_PY312
+#         define _Py_DEC_REFTOTAL
+#      else
 #        define _Py_DEC_REFTOTAL _Py_RefTotal--
+#      endif
 #    else
 #        define _Py_DEC_REFTOTAL
 #    endif

src/greenlet/greenlet_greenlet.hpp

@@ -22,6 +22,10 @@ using greenlet::refs::BorrowedGreenlet;
 #  define _PyInterpreterFrame _interpreter_frame
 #endif
 
+#if GREENLET_PY312
+#  include "internal/pycore_frame.h"
+#endif
+
 // XXX: TODO: Work to remove all virtual functions
 // for speed of calling and size of objects (no vtable).
 // One pattern is the Curiously Recurring Template
@@ -113,6 +117,9 @@ namespace greenlet
         PyObject** datastack_top;
         PyObject** datastack_limit;
 #endif
+#if GREENLET_PY312
+        _PyInterpreterFrame* _prev_frame;
+#endif
 
     public:
         PythonState();
@@ -721,6 +728,9 @@ PythonState::PythonState()
     ,datastack_top(nullptr)
     ,datastack_limit(nullptr)
 #endif
+#if GREENLET_PY312
+    ,_prev_frame(nullptr)
+#endif
 {
 #if GREENLET_USE_CFRAME
     /*
@@ -809,6 +819,12 @@ void PythonState::operator<<(const PyThreadState *const tstate) noexcept
     PyFrameObject *frame = PyThreadState_GetFrame((PyThreadState *)tstate);
     Py_XDECREF(frame);  // PyThreadState_GetFrame gives us a new reference.
     this->_top_frame.steal(frame);
+  #if GREENLET_PY312
+    if (frame) {
+        this->_prev_frame = frame->f_frame->previous;
+        frame->f_frame->previous = nullptr;
+    }
+  #endif
   #if GREENLET_PY312
     this->trash_delete_nesting = tstate->trash.delete_nesting;
   #else // not 312
@@ -843,9 +859,16 @@ void PythonState::operator>>(PyThreadState *const tstate) noexcept
   #if GREENLET_PY312
     tstate->py_recursion_remaining = tstate->py_recursion_limit - this->py_recursion_depth;
     tstate->c_recursion_remaining = C_RECURSION_LIMIT - this->c_recursion_depth;
-  #else // not 3.12
+    // We're just going to throw this object away anyway, go ahead and
+    // do it now.
+    PyFrameObject* frame = this->_top_frame.relinquish_ownership();
+    if (frame && frame->f_frame) {
+      frame->f_frame->previous = this->_prev_frame;
+    }
+    this->_prev_frame = nullptr;
+  #else // \/ 3.11
     tstate->recursion_remaining = tstate->recursion_limit - this->recursion_depth;
-  #endif
+  #endif // GREENLET_PY312
     tstate->cframe->current_frame = this->current_frame;
     tstate->datastack_chunk = this->datastack_chunk;
     tstate->datastack_top = this->datastack_top;

src/greenlet/tests/test_greenlet.py

@@ -838,6 +838,25 @@ def test_switch_to_dead_greenlet_reparent(self):
         self.assertIsNone(result)
         self.assertEqual(seen, [42, 24])
 
+    def test_can_access_f_back_of_suspended_greenlet(self):
+        # On Python 3.12, they added a ->previous field to
+        # _PyInterpreterFrame that has to be cleared when a frame is inactive.
+        # If we got that wrong, this immediately crashes.
+        main = greenlet.getcurrent()
+
+        def Hub():
+            main.switch()
+
+        hub = greenlet(Hub)
+        # start it
+        hub.switch()
+        # now it is suspended
+        self.assertIsNotNone(hub.gr_frame)
+        # The next line is what would crash
+        self.assertIsNone(hub.gr_frame.f_back)
+
+
+
 
 class TestGreenletSetParentErrors(TestCase):
     def test_threaded_reparent(self):

src/greenlet/tests/test_tracing.py

@@ -276,3 +276,8 @@ def g2_run():
             ('call', '__exit__'),
             ('c_call', '__exit__'),
         ])
+
+
+if __name__ == '__main__':
+    import unittest
+    unittest.main()