[3.13] gh-116510: Fix a Crash Due to Shared Immortal Interned Strings (gh-124865) (gh-125709) (GH-125204)

* gh-116510: Fix a Crash Due to Shared Immortal Interned Strings (gh-124865)

Fix a crash caused by immortal interned strings being shared between
sub-interpreters that use basic single-phase init. In that case, the string
can be used by an interpreter that outlives the interpreter that created and
interned it. For interpreters that share obmalloc state, also share the
interned dict with the main interpreter.

This is an un-revert of gh-124646 that then addresses the Py_TRACE_REFS
failures identified by gh-124785.
(cherry picked from commit f2cb399470)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

* [3.13] gh-125286: Share the Main Refchain With Legacy Interpreters (gh-125709)

They used to be shared, before 3.12.  Returning to sharing them resolves a failure on Py_TRACE_REFS builds.

---------

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Doc/library/sys.rst

@@ -920,6 +920,35 @@ always available.
       It is not guaranteed to exist in all implementations of Python.
 
 
+.. function:: getobjects(limit[, type])
+
+   This function only exists if CPython was built using the
+   specialized configure option :option:`--with-trace-refs`.
+   It is intended only for debugging garbage-collection issues.
+
+   Return a list of up to *limit* dynamically allocated Python objects.
+   If *type* is given, only objects of that exact type (not subtypes)
+   are included.
+
+   Objects from the list are not safe to use.
+   Specifically, the result will include objects from all interpreters that
+   share their object allocator state (that is, ones created with
+   :c:member:`PyInterpreterConfig.use_main_obmalloc` set to 1
+   or using :c:func:`Py_NewInterpreter`, and the
+   :ref:`main interpreter <sub-interpreter-support>`).
+   Mixing objects from different interpreters may lead to crashes
+   or other unexpected behavior.
+
+   .. impl-detail::
+
+      This function should be used for specialized purposes only.
+      It is not guaranteed to exist in all implementations of Python.
+
+   .. versionchanged:: next
+
+      The result may include objects from other interpreters.
+
+
 .. function:: getprofile()
 
    .. index::

Doc/using/configure.rst

@@ -721,7 +721,7 @@ Debug options
    Effects:
 
    * Define the ``Py_TRACE_REFS`` macro.
-   * Add :func:`!sys.getobjects` function.
+   * Add :func:`sys.getobjects` function.
    * Add :envvar:`PYTHONDUMPREFS` environment variable.
 
    The :envvar:`PYTHONDUMPREFS` environment variable can be used to dump

Doc/whatsnew/3.13.rst

@@ -2706,3 +2706,14 @@ Regression Test Changes
   option. If used, it specifies a module that should be imported early
   in the lifecycle of the interpreter, before ``site.py`` is executed.
   (Contributed by Łukasz Langa in :gh:`110769`.)
+
+
+Notable changes in 3.13.1
+=========================
+
+sys
+---
+
+* The previously undocumented special function :func:`sys.getobjects`,
+  which only exists in specialized builds of Python, may now return objects
+  from other interpreters than the one it's called in.

Misc/NEWS.d/next/Core_and_Builtins/2024-09-26-18-21-06.gh-issue-116510.FacUWO.rst

@@ -0,0 +1,5 @@
+Fix a crash caused by immortal interned strings being shared between
+sub-interpreters that use basic single-phase init.  In that case, the string
+can be used by an interpreter that outlives the interpreter that created and
+interned it.  For interpreters that share obmalloc state, also share the
+interned dict with the main interpreter.

Objects/object.c

@@ -169,6 +169,48 @@ _PyDebug_PrintTotalRefs(void) {
 #define REFCHAIN(interp) interp->object_state.refchain
 #define REFCHAIN_VALUE ((void*)(uintptr_t)1)
 
+static inline int
+has_own_refchain(PyInterpreterState *interp)
+{
+    if (interp->feature_flags & Py_RTFLAGS_USE_MAIN_OBMALLOC) {
+        return (_Py_IsMainInterpreter(interp)
+            || _PyInterpreterState_Main() == NULL);
+    }
+    return 1;
+}
+
+static int
+refchain_init(PyInterpreterState *interp)
+{
+    if (!has_own_refchain(interp)) {
+        // Legacy subinterpreters share a refchain with the main interpreter.
+        REFCHAIN(interp) = REFCHAIN(_PyInterpreterState_Main());
+        return 0;
+    }
+    _Py_hashtable_allocator_t alloc = {
+        // Don't use default PyMem_Malloc() and PyMem_Free() which
+        // require the caller to hold the GIL.
+        .malloc = PyMem_RawMalloc,
+        .free = PyMem_RawFree,
+    };
+    REFCHAIN(interp) = _Py_hashtable_new_full(
+        _Py_hashtable_hash_ptr, _Py_hashtable_compare_direct,
+        NULL, NULL, &alloc);
+    if (REFCHAIN(interp) == NULL) {
+        return -1;
+    }
+    return 0;
+}
+
+static void
+refchain_fini(PyInterpreterState *interp)
+{
+    if (has_own_refchain(interp) && REFCHAIN(interp) != NULL) {
+        _Py_hashtable_destroy(REFCHAIN(interp));
+    }
+    REFCHAIN(interp) = NULL;
+}
+
 bool
 _PyRefchain_IsTraced(PyInterpreterState *interp, PyObject *obj)
 {
@@ -2171,16 +2213,7 @@ PyStatus
 _PyObject_InitState(PyInterpreterState *interp)
 {
 #ifdef Py_TRACE_REFS
-    _Py_hashtable_allocator_t alloc = {
-        // Don't use default PyMem_Malloc() and PyMem_Free() which
-        // require the caller to hold the GIL.
-        .malloc = PyMem_RawMalloc,
-        .free = PyMem_RawFree,
-    };
-    REFCHAIN(interp) = _Py_hashtable_new_full(
-        _Py_hashtable_hash_ptr, _Py_hashtable_compare_direct,
-        NULL, NULL, &alloc);
-    if (REFCHAIN(interp) == NULL) {
+    if (refchain_init(interp) < 0) {
         return _PyStatus_NO_MEMORY();
     }
 #endif
@@ -2191,8 +2224,7 @@ void
 _PyObject_FiniState(PyInterpreterState *interp)
 {
 #ifdef Py_TRACE_REFS
-    _Py_hashtable_destroy(REFCHAIN(interp));
-    REFCHAIN(interp) = NULL;
+    refchain_fini(interp);
 #endif
 }
 

Objects/unicodeobject.c

@@ -277,14 +277,38 @@ hashtable_unicode_compare(const void *key1, const void *key2)
     }
 }
 
+/* Return true if this interpreter should share the main interpreter's
+   intern_dict.  That's important for interpreters which load basic
+   single-phase init extension modules (m_size == -1).  There could be interned
+   immortal strings that are shared between interpreters, due to the
+   PyDict_Update(mdict, m_copy) call in import_find_extension().
+
+   It's not safe to deallocate those strings until all interpreters that
+   potentially use them are freed.  By storing them in the main interpreter, we
+   ensure they get freed after all other interpreters are freed.
+*/
+static bool
+has_shared_intern_dict(PyInterpreterState *interp)
+{
+    PyInterpreterState *main_interp = _PyInterpreterState_Main();
+    return interp != main_interp  && interp->feature_flags & Py_RTFLAGS_USE_MAIN_OBMALLOC;
+}
+
 static int
 init_interned_dict(PyInterpreterState *interp)
 {
     assert(get_interned_dict(interp) == NULL);
-    PyObject *interned = interned = PyDict_New();
+    PyObject *interned;
+    if (has_shared_intern_dict(interp)) {
+        interned = get_interned_dict(_PyInterpreterState_Main());
+        Py_INCREF(interned);
+    }
+    else {
+        interned = PyDict_New();
         if (interned == NULL) {
             return -1;
         }
+    }
     _Py_INTERP_CACHED_OBJECT(interp, interned_strings) = interned;
     return 0;
 }
@@ -294,7 +318,10 @@ clear_interned_dict(PyInterpreterState *interp)
 {
     PyObject *interned = get_interned_dict(interp);
     if (interned != NULL) {
+        if (!has_shared_intern_dict(interp)) {
+            // only clear if the dict belongs to this interpreter
             PyDict_Clear(interned);
+        }
         Py_DECREF(interned);
         _Py_INTERP_CACHED_OBJECT(interp, interned_strings) = NULL;
     }
@@ -15306,6 +15333,13 @@ _PyUnicode_ClearInterned(PyInterpreterState *interp)
     }
     assert(PyDict_CheckExact(interned));
 
+    if (has_shared_intern_dict(interp)) {
+        // the dict doesn't belong to this interpreter, skip the debug
+        // checks on it and just clear the pointer to it
+        clear_interned_dict(interp);
+        return;
+    }
+
 #ifdef INTERNED_STATS
     fprintf(stderr, "releasing %zd interned strings\n",
             PyDict_GET_SIZE(interned));
@@ -15827,8 +15861,10 @@ _PyUnicode_Fini(PyInterpreterState *interp)
 {
     struct _Py_unicode_state *state = &interp->unicode;
 
+    if (!has_shared_intern_dict(interp)) {
         // _PyUnicode_ClearInterned() must be called before _PyUnicode_Fini()
         assert(get_interned_dict(interp) == NULL);
+    }
 
     _PyUnicode_FiniEncodings(&state->fs_codec);
 

Python/pylifecycle.c

@@ -670,6 +670,13 @@ pycore_create_interpreter(_PyRuntimeState *runtime,
         return status;
     }
 
+    // This could be done in init_interpreter() (in pystate.c) if it
+    // didn't depend on interp->feature_flags being set already.
+    status = _PyObject_InitState(interp);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
+
     // initialize the interp->obmalloc state.  This must be done after
     // the settings are loaded (so that feature_flags are set) but before
     // any calls are made to obmalloc functions.
@@ -2290,6 +2297,13 @@ new_interpreter(PyThreadState **tstate_p,
         goto error;
     }
 
+    // This could be done in init_interpreter() (in pystate.c) if it
+    // didn't depend on interp->feature_flags being set already.
+    status = _PyObject_InitState(interp);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
+
     // initialize the interp->obmalloc state.  This must be done after
     // the settings are loaded (so that feature_flags are set) but before
     // any calls are made to obmalloc functions.

Python/pystate.c

@@ -632,10 +632,8 @@ init_interpreter(PyInterpreterState *interp,
     assert(next != NULL || (interp == runtime->interpreters.main));
     interp->next = next;
 
-    PyStatus status = _PyObject_InitState(interp);
-    if (_PyStatus_EXCEPTION(status)) {
-        return status;
-    }
+    // We would call _PyObject_InitState() at this point
+    // if interp->feature_flags were alredy set.
 
     _PyEval_InitState(interp);
     _PyGC_InitState(&interp->gc);