bpo-38009: Do not call weakref callbacks that are being collected

Include/cpython/objimpl.h

@@ -97,6 +97,7 @@ typedef struct {
 #define _PyGC_SET_FINALIZED(o) \
     _PyGCHead_SET_FINALIZED(_Py_AS_GC(o))
 
+PyAPI_FUNC(int) _PyObject_GC_IS_COLLECTING(PyObject *op);
 
 PyAPI_FUNC(PyObject *) _PyObject_GC_Malloc(size_t size);
 PyAPI_FUNC(PyObject *) _PyObject_GC_Calloc(size_t size);

Modules/gcmodule.c

@@ -69,10 +69,22 @@ module gc
 /* Get the object given the GC head */
 #define FROM_GC(g) ((PyObject *)(((PyGC_Head *)g)+1))
 
+#define _PyGCHead_IS_COLLECTING(o) \
+    (((o)->_gc_prev & PREV_MASK_COLLECTING) != 0)
+
+int _PyObject_GC_IS_COLLECTING(PyObject *op){
+    if (PyObject_IS_GC(op)) {
+        PyGC_Head *gc = AS_GC(op);
+        return _PyGCHead_IS_COLLECTING(gc);
+    } else {
+        return 0;
+    }
+}
+
 static inline int
 gc_is_collecting(PyGC_Head *g)
 {
-    return (g->_gc_prev & PREV_MASK_COLLECTING) != 0;
+    return _PyGCHead_IS_COLLECTING(g);
 }
 
 static inline void

Objects/weakrefobject.c

@@ -1,4 +1,5 @@
 #include "Python.h"
+#include "objimpl.h"
 #include "structmember.h"
 
 
@@ -874,6 +875,35 @@ PyWeakref_GetObject(PyObject *ref)
 static void
 handle_callback(PyWeakReference *ref, PyObject *callback)
 {
+    /* If the garbage collector support is not properly implemented on
+     * some classes that are involved in a reference cycle, a weak
+     * reference may try to invoke a callback object that is being
+     * cleaned (tp_clear) by the garbage collector and it may be in an
+     * inconsistent state. As the garbage collector explicitly does
+     * not invoke callbacks that are part of the same cycle isolate (check
+     * PEP 442 for references about this terminology) as the weak reference
+     * (pretending that the weak reference was * destroyed first), we
+     * should act in the same way here.
+     *
+     * When running the garbage collector pass over a generation, is
+     * possible to end in this function if the tp_clear of a function
+     * decrements the references of some internal object that is
+     * weak-referenced, invoking the weak reference callback that will
+     * try to call the function, which is in an incosistent state as
+     * is in the middle of its tp_clear and some internal fields may
+     * be NULL. */
+
+    if (PyObject_IS_GC(callback) && _PyObject_GC_IS_COLLECTING(callback)) {
+        PyErr_WarnEx(PyExc_RuntimeWarning, "A weak reference"
+                " was trying to execute a callback to a function that is being cleared by"
+                " the garbage collector.\n A C extension class in the dependence"
+                " chain is probably not implementing the garbage collector support"
+                " correctly.", 1);
+        /* Return to avoid a potential crash when calling the callback that is in
+         * an invalid state */
+        return;
+    }
+
     PyObject *cbresult = _PyObject_CallOneArg(callback, (PyObject *)ref);
 
     if (cbresult == NULL)