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>

1 files changed, 52 insertions, 0 deletions

diff --git a/Objects/object.c b/Objects/object.c
index 8256071..2252f98 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -10,6 +10,9 @@
 extern "C" {
 #endif
 
+/* Defined in tracemalloc.c */
+extern void _PyMem_DumpTraceback(int fd, const void *ptr);
+
 _Py_IDENTIFIER(Py_Repr);
 _Py_IDENTIFIER(__bytes__);
 _Py_IDENTIFIER(__dir__);
@@ -2212,6 +2215,55 @@ _PyTrash_thread_destroy_chain(void)
     --tstate->trash_delete_nesting;
 }
 
+
+void
+_PyObject_AssertFailed(PyObject *obj, const char *msg, const char *expr,
+                       const char *file, int line, const char *function)
+{
+    fprintf(stderr,
+            "%s:%d: %s: Assertion \"%s\" failed",
+            file, line, function, expr);
+    fflush(stderr);
+
+    if (msg) {
+        fprintf(stderr, "; %s.\n", msg);
+    }
+    else {
+        fprintf(stderr, ".\n");
+    }
+    fflush(stderr);
+
+    if (obj == NULL) {
+        fprintf(stderr, "<NULL object>\n");
+    }
+    else if (_PyObject_IsFreed(obj)) {
+        /* It seems like the object memory has been freed:
+           don't access it to prevent a segmentation fault. */
+        fprintf(stderr, "<Freed object>\n");
+    }
+    else {
+        /* Diplay the traceback where the object has been allocated.
+           Do it before dumping repr(obj), since repr() is more likely
+           to crash than dumping the traceback. */
+        void *ptr;
+        PyTypeObject *type = Py_TYPE(obj);
+        if (PyType_IS_GC(type)) {
+            ptr = (void *)((char *)obj - sizeof(PyGC_Head));
+        }
+        else {
+            ptr = (void *)obj;
+        }
+        _PyMem_DumpTraceback(fileno(stderr), ptr);
+
+        /* This might succeed or fail, but we're about to abort, so at least
+           try to provide any extra info we can: */
+        _PyObject_Dump(obj);
+    }
+    fflush(stderr);
+
+    Py_FatalError("_PyObject_AssertFailed");
+}
+
 #ifndef Py_TRACE_REFS
 /* For Py_LIMITED_API, we need an out-of-line version of _Py_Dealloc.
    Define this here, so we can undefine the macro. */