python3/00170-gc-assertions.patch

diff --git Include/object.h Include/object.h
index 50d9747..9b7e89e 100644
--- Include/object.h
+++ Include/object.h
@@ -1059,6 +1059,49 @@ PyAPI_FUNC(void)
 _PyObject_DebugTypeStats(FILE *out);
 #endif /* ifndef Py_LIMITED_API */
 
+/* 
+   Define a pair of assertion macros.
+
+   These work like the regular C assert(), in that they will abort the
+   process with a message on stderr if the given condition fails to hold,
+   but compile away to nothing if NDEBUG is defined.
+
+   However, before aborting, Python will also try to call _PyObject_Dump() on
+   the given object.  This may be of use when investigating bugs in which a
+   particular object is corrupt (e.g. buggy a tp_visit method in an extension
+   module breaking the garbage collector), to help locate the broken objects.
+
+   The WITH_MSG variant allows you to supply an additional message that Python
+   will attempt to print to stderr, after the object dump.
+*/
+#ifdef NDEBUG
+/* No debugging: compile away the assertions: */
+#define PyObject_ASSERT_WITH_MSG(obj, expr, msg) ((void)0)
+#else
+/* With debugging: generate checks: */
+#define PyObject_ASSERT_WITH_MSG(obj, expr, msg) \
+  ((expr)                                           \
+   ? (void)(0)                                      \
+   : _PyObject_AssertFailed((obj),                  \
+                            (msg),                  \
+                            (__STRING(expr)),       \
+                            (__FILE__),             \
+                            (__LINE__),             \
+                            (__PRETTY_FUNCTION__)))
+#endif
+
+#define PyObject_ASSERT(obj, expr) \
+  PyObject_ASSERT_WITH_MSG(obj, expr, NULL)
+
+/* 
+   Declare and define the entrypoint even when NDEBUG is defined, to avoid
+   causing compiler/linker errors when building extensions without NDEBUG
+   against a Python built with NDEBUG defined
+*/
+PyAPI_FUNC(void) _PyObject_AssertFailed(PyObject *,  const char *,
+                                        const char *, const char *, int,
+                                        const char *);
+
 #ifdef __cplusplus
 }
 #endif
diff --git Objects/object.c Objects/object.c
index d21fd90..d07b49c 100644
--- Objects/object.c
+++ Objects/object.c
@@ -2034,6 +2034,35 @@ _PyTrash_thread_destroy_chain(void)
     }
 }
 
+PyAPI_FUNC(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.\n",
+            file, line, function, expr);
+    if (msg) {
+        fprintf(stderr, "%s\n", msg);
+    }
+
+    fflush(stderr);
+
+    if (obj) {
+        /* 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);
+    }
+    else {
+        fprintf(stderr, "NULL object\n");
+    }
+
+    fflush(stdout);
+    fflush(stderr);
+
+    /* Terminate the process: */
+    abort();
+}
+
 #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. */
diff --git Lib/test/test_gc.py Lib/test/test_gc.py
index a4d684b..e77029e 100644
--- Lib/test/test_gc.py
+++ Lib/test/test_gc.py
@@ -1,10 +1,11 @@
 import unittest
 from test.support import (verbose, refcount_test, run_unittest,
                           strip_python_stderr, cpython_only, start_threads,
-                          temp_dir, requires_type_collecting)
+                          temp_dir, requires_type_collecting, import_module)
 from test.support.script_helper import assert_python_ok, make_script
 
 import sys
+import sysconfig
 import time
 import gc
 import weakref
@@ -50,6 +51,8 @@ class GC_Detector(object):
         # gc collects it.
         self.wr = weakref.ref(C1055820(666), it_happened)
 
+BUILD_WITH_NDEBUG = ('-DNDEBUG' in sysconfig.get_config_vars()['PY_CFLAGS'])
+
 @with_tp_del
 class Uncollectable(object):
     """Create a reference cycle with multiple __del__ methods.
@@ -864,6 +867,50 @@ class GCCallbackTests(unittest.TestCase):
         self.assertEqual(len(gc.garbage), 0)
 
 
+    @unittest.skipIf(BUILD_WITH_NDEBUG,
+                     'built with -NDEBUG')
+    def test_refcount_errors(self):
+        self.preclean()
+        # Verify the "handling" of objects with broken refcounts
+        import_module("ctypes") #skip if not supported
+
+        import subprocess
+        code = '''if 1:
+        a = []
+        b = [a]
+
+        # Simulate the refcount of "a" being too low (compared to the
+        # references held on it by live data), but keeping it above zero
+        # (to avoid deallocating it):
+        import ctypes
+        ctypes.pythonapi.Py_DecRef(ctypes.py_object(a))
+
+        # The garbage collector should now have a fatal error when it reaches
+        # the broken object:
+        import gc
+        gc.collect()
+        '''
+        p = subprocess.Popen([sys.executable, "-c", code],
+                             stdout=subprocess.PIPE,
+                             stderr=subprocess.PIPE)
+        stdout, stderr = p.communicate()
+        p.stdout.close()
+        p.stderr.close()
+        # Verify that stderr has a useful error message:
+        self.assertRegex(stderr,
+            b'Modules/gcmodule.c:[0-9]+: visit_decref: Assertion "\(\(gc\)->gc.gc_refs >> \(1\)\) != 0" failed.')
+        self.assertRegex(stderr,
+            b'refcount was too small')
+        self.assertRegex(stderr,
+            b'object  : \[\]')
+        self.assertRegex(stderr,
+            b'type    : list')
+        self.assertRegex(stderr,
+            b'refcount: 1')
+        self.assertRegex(stderr,
+            b'address : 0x[0-9a-f]+')
+
+
 class GCTogglingTests(unittest.TestCase):
     def setUp(self):
         gc.enable()
diff --git Modules/gcmodule.c Modules/gcmodule.c
index cb7222d..6a9be80 100644
--- Modules/gcmodule.c
+++ Modules/gcmodule.c
@@ -341,7 +341,8 @@ update_refs(PyGC_Head *containers)
 {
     PyGC_Head *gc = containers->gc.gc_next;
     for (; gc != containers; gc = gc->gc.gc_next) {
-        assert(_PyGCHead_REFS(gc) == GC_REACHABLE);
+        PyObject_ASSERT(FROM_GC(gc),
+                        _PyGCHead_REFS(gc) == GC_REACHABLE);
         _PyGCHead_SET_REFS(gc, Py_REFCNT(FROM_GC(gc)));
         /* Python's cyclic gc should never see an incoming refcount
          * of 0:  if something decref'ed to 0, it should have been
@@ -361,7 +362,8 @@ update_refs(PyGC_Head *containers)
          * so serious that maybe this should be a release-build
          * check instead of an assert?
          */
-        assert(_PyGCHead_REFS(gc) != 0);
+        PyObject_ASSERT(FROM_GC(gc),
+                        _PyGCHead_REFS(gc) != 0);
     }
 }
 
@@ -376,7 +378,9 @@ visit_decref(PyObject *op, void *data)
          * generation being collected, which can be recognized
          * because only they have positive gc_refs.
          */
-        assert(_PyGCHead_REFS(gc) != 0); /* else refcount was too small */
+        PyObject_ASSERT_WITH_MSG(FROM_GC(gc),
+                        _PyGCHead_REFS(gc) != 0,
+                        "refcount was too small"); /* else refcount was too small */
         if (_PyGCHead_REFS(gc) > 0)
             _PyGCHead_DECREF(gc);
     }
@@ -436,9 +440,10 @@ visit_reachable(PyObject *op, PyGC_Head *reachable)
          * If gc_refs == GC_UNTRACKED, it must be ignored.
          */
          else {
-            assert(gc_refs > 0
-                   || gc_refs == GC_REACHABLE
-                   || gc_refs == GC_UNTRACKED);
+             PyObject_ASSERT(FROM_GC(gc),
+                             gc_refs > 0
+                             || gc_refs == GC_REACHABLE
+                             || gc_refs == GC_UNTRACKED);
          }
     }
     return 0;
@@ -480,7 +485,7 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
              */
             PyObject *op = FROM_GC(gc);
             traverseproc traverse = Py_TYPE(op)->tp_traverse;
-            assert(_PyGCHead_REFS(gc) > 0);
+            PyObject_ASSERT(op, _PyGCHead_REFS(gc) > 0);
             _PyGCHead_SET_REFS(gc, GC_REACHABLE);
             (void) traverse(op,
                             (visitproc)visit_reachable,
@@ -543,7 +548,7 @@ move_legacy_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
     for (gc = unreachable->gc.gc_next; gc != unreachable; gc = next) {
         PyObject *op = FROM_GC(gc);
 
-        assert(IS_TENTATIVELY_UNREACHABLE(op));
+        PyObject_ASSERT(op, IS_TENTATIVELY_UNREACHABLE(op));
         next = gc->gc.gc_next;
 
         if (has_legacy_finalizer(op)) {
@@ -619,7 +624,7 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
         PyWeakReference **wrlist;
 
         op = FROM_GC(gc);
-        assert(IS_TENTATIVELY_UNREACHABLE(op));
+        PyObject_ASSERT(op, IS_TENTATIVELY_UNREACHABLE(op));
         next = gc->gc.gc_next;
 
         if (! PyType_SUPPORTS_WEAKREFS(Py_TYPE(op)))
@@ -640,9 +645,9 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
              * the callback pointer intact.  Obscure:  it also
              * changes *wrlist.
              */
-            assert(wr->wr_object == op);
+            PyObject_ASSERT(wr->wr_object, wr->wr_object == op);
             _PyWeakref_ClearRef(wr);
-            assert(wr->wr_object == Py_None);
+            PyObject_ASSERT(wr->wr_object, wr->wr_object == Py_None);
             if (wr->wr_callback == NULL)
                 continue;                       /* no callback */
 
@@ -676,7 +681,7 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
      */
             if (IS_TENTATIVELY_UNREACHABLE(wr))
                 continue;
-            assert(IS_REACHABLE(wr));
+            PyObject_ASSERT(op, IS_REACHABLE(wr));
 
             /* Create a new reference so that wr can't go away
              * before we can process it again.
@@ -685,7 +690,8 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
 
             /* Move wr to wrcb_to_call, for the next pass. */
             wrasgc = AS_GC(wr);
-            assert(wrasgc != next); /* wrasgc is reachable, but
+            PyObject_ASSERT(op, wrasgc != next);
+                                    /* wrasgc is reachable, but
                                        next isn't, so they can't
                                        be the same */
             gc_list_move(wrasgc, &wrcb_to_call);
@@ -701,11 +707,11 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
 
         gc = wrcb_to_call.gc.gc_next;
         op = FROM_GC(gc);
-        assert(IS_REACHABLE(op));
-        assert(PyWeakref_Check(op));
+        PyObject_ASSERT(op, IS_REACHABLE(op));
+        PyObject_ASSERT(op, PyWeakref_Check(op));
         wr = (PyWeakReference *)op;
         callback = wr->wr_callback;
-        assert(callback != NULL);
+        PyObject_ASSERT(op, callback != NULL);
 
         /* copy-paste of weakrefobject.c's handle_callback() */
         temp = PyObject_CallFunctionObjArgs(callback, wr, NULL);
@@ -822,12 +828,14 @@ check_garbage(PyGC_Head *collectable)
     for (gc = collectable->gc.gc_next; gc != collectable;
          gc = gc->gc.gc_next) {
         _PyGCHead_SET_REFS(gc, Py_REFCNT(FROM_GC(gc)));
-        assert(_PyGCHead_REFS(gc) != 0);
+        PyObject_ASSERT(FROM_GC(gc),
+                        _PyGCHead_REFS(gc) != 0);
     }
     subtract_refs(collectable);
     for (gc = collectable->gc.gc_next; gc != collectable;
          gc = gc->gc.gc_next) {
-        assert(_PyGCHead_REFS(gc) >= 0);
+        PyObject_ASSERT(FROM_GC(gc),
+                        _PyGCHead_REFS(gc) >= 0);
         if (_PyGCHead_REFS(gc) != 0)
             return -1;
     }