GH-97002: Prevent `_PyInterpreterFrame`s from backing more than one `PyFrameObject` (GH-97996)

1 files changed, 23 insertions, 6 deletions

diff --git a/Python/frame.c b/Python/frame.c
index 96566de..89f084b 100644
--- a/Python/frame.c
+++ b/Python/frame.c
@@ -37,14 +37,31 @@ _PyFrame_MakeAndSetFrameObject(_PyInterpreterFrame *frame)
         Py_XDECREF(error_type);
         Py_XDECREF(error_value);
         Py_XDECREF(error_traceback);
+        return NULL;
     }
-    else {
-        assert(frame->owner != FRAME_OWNED_BY_FRAME_OBJECT);
-        assert(frame->owner != FRAME_CLEARED);
-        f->f_frame = frame;
-        frame->frame_obj = f;
-        PyErr_Restore(error_type, error_value, error_traceback);
+    PyErr_Restore(error_type, error_value, error_traceback);
+    if (frame->frame_obj) {
+        // GH-97002: How did we get into this horrible situation? Most likely,
+        // allocating f triggered a GC collection, which ran some code that
+        // *also* created the same frame... while we were in the middle of
+        // creating it! See test_sneaky_frame_object in test_frame.py for a
+        // concrete example.
+        //
+        // Regardless, just throw f away and use that frame instead, since it's
+        // already been exposed to user code. It's actually a bit tricky to do
+        // this, since we aren't backed by a real _PyInterpreterFrame anymore.
+        // Just pretend that we have an owned, cleared frame so frame_dealloc
+        // doesn't make the situation worse:
+        f->f_frame = (_PyInterpreterFrame *)f->_f_frame_data;
+        f->f_frame->owner = FRAME_CLEARED;
+        f->f_frame->frame_obj = f;
+        Py_DECREF(f);
+        return frame->frame_obj;
     }
+    assert(frame->owner != FRAME_OWNED_BY_FRAME_OBJECT);
+    assert(frame->owner != FRAME_CLEARED);
+    f->f_frame = frame;
+    frame->frame_obj = f;
     return f;
 }