pythongh-108634: Py_TRACE_REFS uses a hash table

Python built with "configure --with-trace-refs" (tracing references)
is now ABI compatible with Python built in release mode. Moreover, it
now also supports the Limited API.

Change Py_TRACE_REFS build:

* Remove _PyObject_EXTRA_INIT macro.
* The PyObject structure no longer has two extra members (_ob_prev
  and _ob_next).
* Use a hash table (_Py_hashtable_t) to trace references (all
  objects): PyInterpreterState.object_state.refchain.
* test_sys now tests sys.getobjects().
* Limited C API extensions can now be built with Py_TRACE_REFS:
  xxlimited, xxlimited_35, _testclinic_limited.
* Py_TRACE_REFS build is now ABI compatible with release build and
  debug build.
* No longer rename PyModule_Create2() and PyModule_FromDefAndSpec2()
  functions to PyModule_Create2TraceRefs() and
  PyModule_FromDefAndSpec2TraceRefs().
* Add test.support.Py_TRACE_REFS constant.
* test_exceptions skips test_recursion_normalizing_with_no_memory()
  and test_memory_error_in_PyErr_PrintEx() if Python is built with
  Py_TRACE_REFS.
* test_repl skips test_no_memory().
* test_capi skisp test_set_nomemory().

Doc/c-api/typeobj.rst

@@ -528,28 +528,6 @@ type objects) *must* have the :c:member:`~PyVarObject.ob_size` field.
    This field is inherited by subtypes.
 
 
-.. c:member:: PyObject* PyObject._ob_next
-             PyObject* PyObject._ob_prev
-
-   These fields are only present when the macro ``Py_TRACE_REFS`` is defined
-   (see the :option:`configure --with-trace-refs option <--with-trace-refs>`).
-
-   Their initialization to ``NULL`` is taken care of by the
-   ``PyObject_HEAD_INIT`` macro.  For :ref:`statically allocated objects
-   <static-types>`, these fields always remain ``NULL``.  For :ref:`dynamically
-   allocated objects <heap-types>`, these two fields are used to link the
-   object into a doubly linked list of *all* live objects on the heap.
-
-   This could be used for various debugging purposes; currently the only uses
-   are the :func:`sys.getobjects` function and to print the objects that are
-   still alive at the end of a run when the environment variable
-   :envvar:`PYTHONDUMPREFS` is set.
-
-   **Inheritance:**
-
-   These fields are not inherited by subtypes.
-
-
 PyVarObject Slots
 -----------------
 

Doc/using/configure.rst

@@ -408,8 +408,7 @@ See also the :ref:`Python Development Mode <devmode>` and the
 .. versionchanged:: 3.8
    Release builds and debug builds are now ABI compatible: defining the
    ``Py_DEBUG`` macro no longer implies the ``Py_TRACE_REFS`` macro (see the
-   :option:`--with-trace-refs` option), which introduces the only ABI
-   incompatibility.
+   :option:`--with-trace-refs` option).
 
 
 Debug options
@@ -430,8 +429,14 @@ Debug options
    * Add :func:`sys.getobjects` function.
    * Add :envvar:`PYTHONDUMPREFS` environment variable.
 
-   This build is not ABI compatible with release build (default build) or debug
-   build (``Py_DEBUG`` and ``Py_REF_DEBUG`` macros).
+   The :envvar:`PYTHONDUMPREFS` environment variable can be used to dump
+   objects and reference counts still alive at Python exit.
+
+   :ref:`Statically allocated objects <static-types>` are not traced.
+
+   .. versionchanged:: 3.13
+      This build is now ABI compatible with release build and :ref:`debug build
+      <debug-build>`.
 
    .. versionadded:: 3.8
 

Doc/whatsnew/3.13.rst

@@ -828,6 +828,11 @@ Build Changes
 * SQLite 3.15.2 or newer is required to build the :mod:`sqlite3` extension module.
   (Contributed by Erlend Aasland in :gh:`105875`.)
 
+* Python built with :file:`configure` :option:`--with-trace-refs` (tracing
+  references) is now ABI compatible with Python release build and and
+  :ref:`debug build <debug-build>`.
+  (Contributed by Victor Stinner in :gh:`108634`.)
+
 
 C API Changes
 =============
@@ -900,6 +905,10 @@ New Features
   (with an underscore prefix).
   (Contributed by Victor Stinner in :gh:`108014`.)
 
+* Python built with :file:`configure` :option:`--with-trace-refs` (tracing
+  references) now supports the :ref:`Limited API <limited-c-api>`.
+  (Contributed by Victor Stinner in :gh:`108634`.)
+
 Porting to Python 3.13
 ----------------------
 

Include/internal/pycore_object.h

@@ -44,7 +44,6 @@ PyAPI_FUNC(int) _PyObject_IsFreed(PyObject *);
    backwards compatible solution */
 #define _PyObject_HEAD_INIT(type)         \
     {                                     \
-        _PyObject_EXTRA_INIT              \
         .ob_refcnt = _Py_IMMORTAL_REFCNT, \
         .ob_type = (type)                 \
     },
@@ -173,6 +172,8 @@ _PyType_HasFeature(PyTypeObject *type, unsigned long feature) {
 extern void _PyType_InitCache(PyInterpreterState *interp);
 
 extern void _PyObject_InitState(PyInterpreterState *interp);
+extern void _PyObject_FiniState(PyInterpreterState *interp);
+extern bool _PyRefchain_IsTraced(PyInterpreterState *interp, PyObject *obj);
 
 /* Inline functions trading binary compatibility for speed:
    _PyObject_Init() is the fast version of PyObject_Init(), and

Include/internal/pycore_object_state.h

@@ -8,6 +8,8 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
+#include "pycore_hashtable.h"     // _Py_hashtable_t
+
 struct _py_object_runtime_state {
 #ifdef Py_REF_DEBUG
     Py_ssize_t interpreter_leaks;
@@ -20,11 +22,10 @@ struct _py_object_state {
     Py_ssize_t reftotal;
 #endif
 #ifdef Py_TRACE_REFS
-    /* Head of circular doubly-linked list of all objects.  These are linked
-     * together via the _ob_prev and _ob_next members of a PyObject, which
-     * exist only in a Py_TRACE_REFS build.
-     */
-    PyObject refchain;
+    // Hash table storing all objects. The key is the object pointer
+    // (PyObject*) and the value is just the number 1 (as uintptr_t).
+    // See _PyRefchain_IsTraced() and _PyRefchain_Trace() functions.
+    _Py_hashtable_t *refchain;
 #endif
     int _not_used;
 };

Include/internal/pycore_runtime_init.h

@@ -192,7 +192,7 @@ extern PyTypeObject _PyExc_MemoryError;
 #ifdef Py_TRACE_REFS
 # define _py_object_state_INIT(INTERP) \
     { \
-        .refchain = {&INTERP.object_state.refchain, &INTERP.object_state.refchain}, \
+        .refchain = NULL, \
     }
 #else
 # define _py_object_state_INIT(INTERP) \

Include/modsupport.h

@@ -111,14 +111,6 @@ PyAPI_FUNC(int) PyModule_ExecDef(PyObject *module, PyModuleDef *def);
 #define PYTHON_ABI_VERSION 3
 #define PYTHON_ABI_STRING "3"
 
-#ifdef Py_TRACE_REFS
- /* When we are tracing reference counts, rename module creation functions so
-    modules compiled with incompatible settings will generate a
-    link-time error. */
- #define PyModule_Create2 PyModule_Create2TraceRefs
- #define PyModule_FromDefAndSpec2 PyModule_FromDefAndSpec2TraceRefs
-#endif
-
 PyAPI_FUNC(PyObject *) PyModule_Create2(PyModuleDef*, int apiver);
 
 #ifdef Py_LIMITED_API

Include/object.h

@@ -58,23 +58,6 @@ whose size is determined when the object is allocated.
 #  define Py_REF_DEBUG
 #endif
 
-#if defined(Py_LIMITED_API) && defined(Py_TRACE_REFS)
-#  error Py_LIMITED_API is incompatible with Py_TRACE_REFS
-#endif
-
-#ifdef Py_TRACE_REFS
-/* Define pointers to support a doubly-linked list of all live heap objects. */
-#define _PyObject_HEAD_EXTRA            \
-    PyObject *_ob_next;           \
-    PyObject *_ob_prev;
-
-#define _PyObject_EXTRA_INIT _Py_NULL, _Py_NULL,
-
-#else
-#  define _PyObject_HEAD_EXTRA
-#  define _PyObject_EXTRA_INIT
-#endif
-
 /* PyObject_HEAD defines the initial segment of every PyObject. */
 #define PyObject_HEAD                   PyObject ob_base;
 
@@ -130,14 +113,12 @@ check by comparing the reference count field to the immortality reference count.
 #ifdef Py_BUILD_CORE
 #define PyObject_HEAD_INIT(type)    \
     {                               \
-        _PyObject_EXTRA_INIT        \
         { _Py_IMMORTAL_REFCNT },    \
         (type)                      \
     },
 #else
 #define PyObject_HEAD_INIT(type) \
     {                            \
-        _PyObject_EXTRA_INIT     \
         { 1 },                   \
         (type)                   \
     },
@@ -164,8 +145,6 @@ check by comparing the reference count field to the immortality reference count.
  * in addition, be cast to PyVarObject*.
  */
 struct _object {
-    _PyObject_HEAD_EXTRA
-
 #if (defined(__GNUC__) || defined(__clang__)) \
         && !(defined __STDC_VERSION__ && __STDC_VERSION__ >= 201112L)
     // On C99 and older, anonymous union is a GCC and clang extension

Include/pyport.h

@@ -684,12 +684,6 @@ extern char * _getpty(int *, int, mode_t, int);
 #  endif
 #endif
 
-/* Check that ALT_SOABI is consistent with Py_TRACE_REFS:
-   ./configure --with-trace-refs should must be used to define Py_TRACE_REFS */
-#if defined(ALT_SOABI) && defined(Py_TRACE_REFS)
-#  error "Py_TRACE_REFS ABI is not compatible with release and debug ABI"
-#endif
-
 #if defined(__ANDROID__) || defined(__VXWORKS__)
    // Use UTF-8 as the locale encoding, ignore the LC_CTYPE locale.
    // See _Py_GetLocaleEncoding(), PyUnicode_DecodeLocale()

Lib/test/support/__init__.py

@@ -779,9 +779,6 @@ def python_is_optimized():
 
 _header = 'nP'
 _align = '0n'
-if hasattr(sys, "getobjects"):
-    _header = '2P' + _header
-    _align = '0P'
 _vheader = _header + 'n'
 
 def calcobjsize(fmt):
@@ -2469,3 +2466,5 @@ def adjust_int_max_str_digits(max_digits):
 #Windows doesn't have os.uname() but it doesn't support s390x.
 skip_on_s390x = unittest.skipIf(hasattr(os, 'uname') and os.uname().machine == 's390x',
                                 'skipped on s390x')
+
+Py_TRACE_REFS = hasattr(sys, 'getobjects')

Lib/test/test_capi/test_mem.py

@@ -112,6 +112,9 @@ def test_pyobject_forbidden_bytes_is_freed(self):
     def test_pyobject_freed_is_freed(self):
         self.check_pyobject_is_freed('check_pyobject_freed_is_freed')
 
+    # Python built with Py_TRACE_REFS fail with a fatal error in
+    # _PyRefchain_Trace() on memory allocation error.
+    @unittest.skipIf(support.Py_TRACE_REFS, 'cannot test Py_TRACE_REFS build')
     def test_set_nomemory(self):
         code = """if 1:
             import _testcapi

Lib/test/test_exceptions.py

@@ -1484,6 +1484,9 @@ def recurse_in_body_and_except():
 
 
     @cpython_only
+    # Python built with Py_TRACE_REFS fail with a fatal error in
+    # _PyRefchain_Trace() on memory allocation error.
+    @unittest.skipIf(support.Py_TRACE_REFS, 'cannot test Py_TRACE_REFS build')
     def test_recursion_normalizing_with_no_memory(self):
         # Issue #30697. Test that in the abort that occurs when there is no
         # memory left and the size of the Python frames stack is greater than
@@ -1652,6 +1655,9 @@ def test_unhandled(self):
                 self.assertTrue(report.endswith("\n"))
 
     @cpython_only
+    # Python built with Py_TRACE_REFS fail with a fatal error in
+    # _PyRefchain_Trace() on memory allocation error.
+    @unittest.skipIf(support.Py_TRACE_REFS, 'cannot test Py_TRACE_REFS build')
     def test_memory_error_in_PyErr_PrintEx(self):
         code = """if 1:
             import _testcapi

Lib/test/test_import/__init__.py

@@ -29,7 +29,7 @@
 from test.support import os_helper
 from test.support import (
     STDLIB_DIR, swap_attr, swap_item, cpython_only, is_emscripten,
-    is_wasi, run_in_subinterp, run_in_subinterp_with_config)
+    is_wasi, run_in_subinterp, run_in_subinterp_with_config, Py_TRACE_REFS)
 from test.support.import_helper import (
     forget, make_legacy_pyc, unlink, unload, DirsOnSysPath, CleanImport)
 from test.support.os_helper import (
@@ -2556,7 +2556,7 @@ def test_basic_multiple_interpreters_main_no_reset(self):
     def test_basic_multiple_interpreters_deleted_no_reset(self):
         # without resetting; already loaded in a deleted interpreter
 
-        if hasattr(sys, 'getobjects'):
+        if Py_TRACE_REFS:
             # It's a Py_TRACE_REFS build.
             # This test breaks interpreter isolation a little,
             # which causes problems on Py_TRACE_REF builds.

Lib/test/test_repl.py

@@ -5,6 +5,7 @@
 import unittest
 import subprocess
 from textwrap import dedent
+from test import support
 from test.support import cpython_only, has_subprocess_support, SuppressCrashReport
 from test.support.script_helper import kill_python
 
@@ -59,6 +60,9 @@ def run_on_interactive_mode(source):
 class TestInteractiveInterpreter(unittest.TestCase):
 
     @cpython_only
+    # Python built with Py_TRACE_REFS fail with a fatal error in
+    # _PyRefchain_Trace() on memory allocation error.
+    @unittest.skipIf(support.Py_TRACE_REFS, 'cannot test Py_TRACE_REFS build')
     def test_no_memory(self):
         # Issue #30696: Fix the interactive interpreter looping endlessly when
         # no memory. Check also that the fix does not break the interactive

Lib/test/test_sys.py

@@ -1174,6 +1174,27 @@ def test_stdlib_dir(self):
         self.assertEqual(os.path.normpath(sys._stdlib_dir),
                          os.path.normpath(expected))
 
+    @unittest.skipUnless(hasattr(sys, 'getobjects'), 'need sys.getobjects()')
+    def test_getobjects(self):
+        # sys.getobjects(0)
+        all_objects = sys.getobjects(0)
+        self.assertIsInstance(all_objects, list)
+        self.assertGreater(len(all_objects), 0)
+
+        # sys.getobjects(0, MyType)
+        class MyType:
+            pass
+        size = 100
+        my_objects = [MyType() for _ in range(size)]
+        get_objects = sys.getobjects(0, MyType)
+        self.assertEqual(len(get_objects), size)
+        for obj in get_objects:
+            self.assertIsInstance(obj, MyType)
+
+        # sys.getobjects(3, MyType)
+        get_objects = sys.getobjects(3, MyType)
+        self.assertEqual(len(get_objects), 3)
+
 
 @test.support.cpython_only
 class UnraisableHookTest(unittest.TestCase):

Misc/NEWS.d/next/Build/2023-08-30-02-52-52.gh-issue-108634.3dpBvf.rst

@@ -0,0 +1,3 @@
+Python built with :file:`!configure` :option:`--with-trace-refs` (tracing
+references) is now ABI compatible with Python built in release mode. Patch
+by Victor Stinner.

Misc/NEWS.d/next/C API/2023-08-30-02-54-06.gh-issue-108634.oV3Xzk.rst

@@ -0,0 +1,3 @@
+Python built with :file:`configure` :option:`--with-trace-refs` (tracing
+references) now supports the :ref:`Limited API <limited-c-api>`.  Patch by
+Victor Stinner.

Misc/SpecialBuilds.txt

@@ -43,13 +43,9 @@ Py_TRACE_REFS
 
 Build option: ``./configure --with-trace-refs``.
 
-Turn on heavy reference debugging.  This is major surgery.  Every PyObject grows
-two more pointers, to maintain a doubly-linked list of all live heap-allocated
-objects.  Most built-in type objects are not in this list, as they're statically
-allocated.
-
-Note that because the fundamental PyObject layout changes, Python modules
-compiled with Py_TRACE_REFS are incompatible with modules compiled without it.
+Turn on heavy reference debugging.  This is major surgery.  All live
+heap-allocated objects are traced in a hash table.  Most built-in type objects
+are not in this list, as they're statically allocated.
 
 Special gimmicks:
 

Modules/_testcapi/parts.h

@@ -3,19 +3,10 @@
 
 #include "pyconfig.h"  // for Py_TRACE_REFS
 
-// Figure out if Limited API is available for this build. If it isn't we won't
-// build tests for it.
-// Currently, only Py_TRACE_REFS disables Limited API.
-#ifdef Py_TRACE_REFS
-#undef LIMITED_API_AVAILABLE
-#else
-#define LIMITED_API_AVAILABLE 1
-#endif
-
 // Always enable assertions
 #undef NDEBUG
 
-#if !defined(LIMITED_API_AVAILABLE) && defined(Py_LIMITED_API)
+#if defined(Py_LIMITED_API)
 // Limited API being unavailable means that with Py_LIMITED_API defined
 // we can't even include Python.h.
 // Do nothing; the .c file that defined Py_LIMITED_API should also do nothing.
@@ -44,10 +35,8 @@ int _PyTestCapi_Init_PyOS(PyObject *module);
 int _PyTestCapi_Init_Immortal(PyObject *module);
 int _PyTestCapi_Init_GC(PyObject *mod);
 
-#ifdef LIMITED_API_AVAILABLE
 int _PyTestCapi_Init_VectorcallLimited(PyObject *module);
 int _PyTestCapi_Init_HeaptypeRelative(PyObject *module);
-#endif // LIMITED_API_AVAILABLE
 
 #endif
 #endif // Py_TESTCAPI_PARTS_H