bpo-9263: Dump Python object on GC assertion failure (GH-10062)

Changes:

* Add _PyObject_AssertFailed() function.
* Add _PyObject_ASSERT() and _PyObject_ASSERT_WITH_MSG() macros.
* gc_decref(): replace assert() with _PyObject_ASSERT_WITH_MSG() to
  dump the faulty object if the assertion fails.

_PyObject_AssertFailed() calls:

* _PyMem_DumpTraceback(): try to log the traceback where the object
  memory has been allocated if tracemalloc is enabled.
* _PyObject_Dump(): log repr(obj).
* Py_FatalError(): log the current Python traceback.

_PyObject_AssertFailed() uses _PyObject_IsFreed() heuristic to check
if the object memory has been freed by a debug hook on Python memory
allocators.

Initial patch written by David Malcolm.

Co-Authored-By: David Malcolm <dmalcolm@redhat.com>

2 files changed, 13 insertions, 9 deletions

diff --git a/Modules/_tracemalloc.c b/Modules/_tracemalloc.c
index d736b24..1005f2e 100644
--- a/Modules/_tracemalloc.c
+++ b/Modules/_tracemalloc.c
@@ -1436,10 +1436,12 @@ _tracemalloc__get_object_traceback(PyObject *module, PyObject *obj)
     traceback_t *traceback;
 
     type = Py_TYPE(obj);
-    if (PyType_IS_GC(type))
+    if (PyType_IS_GC(type)) {
         ptr = (void *)((char *)obj - sizeof(PyGC_Head));
-    else
+    }
+    else {
         ptr = (void *)obj;
+    }
 
     traceback = tracemalloc_get_traceback(DEFAULT_DOMAIN, (uintptr_t)ptr);
     if (traceback == NULL)
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index 7cddaba..2e19fe4 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -62,6 +62,12 @@ module gc
 // most gc_list_* functions for it.
 #define NEXT_MASK_UNREACHABLE  (1)
 
+/* Get an object's GC head */
+#define AS_GC(o) ((PyGC_Head *)(o)-1)
+
+/* Get the object given the GC head */
+#define FROM_GC(g) ((PyObject *)(((PyGC_Head *)g)+1))
+
 static inline int
 gc_is_collecting(PyGC_Head *g)
 {
@@ -98,16 +104,12 @@ gc_reset_refs(PyGC_Head *g, Py_ssize_t refs)
 static inline void
 gc_decref(PyGC_Head *g)
 {
-    assert(gc_get_refs(g) > 0);
+    _PyObject_ASSERT_WITH_MSG(FROM_GC(g),
+                              gc_get_refs(g) > 0,
+                              "refcount is too small");
     g->_gc_prev -= 1 << _PyGC_PREV_SHIFT;
 }
 
-/* Get an object's GC head */
-#define AS_GC(o) ((PyGC_Head *)(o)-1)
-
-/* Get the object given the GC head */
-#define FROM_GC(g) ((PyObject *)(((PyGC_Head *)g)+1))
-
 /* Python string to use if unhandled exception occurs */
 static PyObject *gc_str = NULL;