bpo-40887: Don't use finalized free lists (GH-20700)

In debug mode, ensure that free lists are no longer used after being
finalized. Set numfree to -1 in finalization functions
(eg. _PyList_Fini()), and then check that numfree is not equal to -1
before using a free list (e.g list_dealloc()).

1 files changed, 12 insertions, 0 deletions

diff --git a/Python/context.c b/Python/context.c
index 3cf8db4c90c..dedbca99384 100644
--- a/Python/context.c
+++ b/Python/context.c
@@ -335,6 +335,10 @@ _context_alloc(void)
     PyInterpreterState *interp = _PyInterpreterState_GET();
     struct _Py_context_state *state = &interp->context;
     PyContext *ctx;
+#ifdef Py_DEBUG
+    // _context_alloc() must not be called after _PyContext_Fini()
+    assert(state->numfree != -1);
+#endif
     if (state->numfree) {
         state->numfree--;
         ctx = state->freelist;
@@ -460,6 +464,10 @@ context_tp_dealloc(PyContext *self)
 
     PyInterpreterState *interp = _PyInterpreterState_GET();
     struct _Py_context_state *state = &interp->context;
+#ifdef Py_DEBUG
+    // _context_alloc() must not be called after _PyContext_Fini()
+    assert(state->numfree != -1);
+#endif
     if (state->numfree < CONTEXT_FREELIST_MAXLEN) {
         state->numfree++;
         self->ctx_weakreflist = (PyObject *)state->freelist;
@@ -1290,6 +1298,10 @@ _PyContext_Fini(PyThreadState *tstate)
 {
     Py_CLEAR(_token_missing);
     _PyContext_ClearFreeList(tstate);
+#ifdef Py_DEBUG
+    struct _Py_context_state *state = &tstate->interp->context;
+    state->numfree = -1;
+#endif
     _PyHamt_Fini();
 }