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.

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

@@ -2491,6 +2491,42 @@ _Py_ResurrectReference(PyObject *op)
 
 
 #ifdef Py_TRACE_REFS
+/* Make sure the ref is associated with the right interpreter.
+ * This only needs special attention for heap-allocated objects
+ * that have been immortalized, and only when the object might
+ * outlive the interpreter where it was created.  That means the
+ * object was necessarily created using a global allocator
+ * (i.e. from the main interpreter).  Thus in that specific case
+ * we move the object over to the main interpreter's refchain.
+ *
+ * This was added for the sake of the immortal interned strings,
+ * where legacy subinterpreters share the main interpreter's
+ * interned dict (and allocator), and therefore the strings can
+ * outlive the subinterpreter.
+ *
+ * It may make sense to fold this into _Py_SetImmortalUntracked(),
+ * but that requires further investigation.  In the meantime, it is
+ * up to the caller to know if this is needed.  There should be
+ * very few cases.
+ */
+void
+_Py_NormalizeImmortalReference(PyObject *op)
+{
+    assert(_Py_IsImmortal(op));
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (!_PyRefchain_IsTraced(interp, op)) {
+        return;
+    }
+    PyInterpreterState *main_interp = _PyInterpreterState_Main();
+    if (interp != main_interp
+           && interp->feature_flags & Py_RTFLAGS_USE_MAIN_OBMALLOC)
+    {
+        assert(!_PyRefchain_IsTraced(main_interp, op));
+        _PyRefchain_Remove(interp, op);
+        _PyRefchain_Trace(main_interp, op);
+    }
+}
+
 void
 _Py_ForgetReference(PyObject *op)
 {

Objects/unicodeobject.c

@@ -281,13 +281,37 @@ 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();
-    if (interned == NULL) {
-        return -1;
+    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;
@@ -298,7 +322,10 @@ clear_interned_dict(PyInterpreterState *interp)
 {
     PyObject *interned = get_interned_dict(interp);
     if (interned != NULL) {
-        PyDict_Clear(interned);
+        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;
     }
@@ -15401,6 +15428,10 @@ _PyUnicode_InternStatic(PyInterpreterState *interp, PyObject **p)
     assert(*p);
 }
 
+#ifdef Py_TRACE_REFS
+extern void _Py_NormalizeImmortalReference(PyObject *);
+#endif
+
 static void
 immortalize_interned(PyObject *s)
 {
@@ -15416,6 +15447,10 @@ immortalize_interned(PyObject *s)
 #endif
     _PyUnicode_STATE(s).interned = SSTATE_INTERNED_IMMORTAL;
     _Py_SetImmortal(s);
+#ifdef Py_TRACE_REFS
+    /* Make sure the ref is associated with the right interpreter. */
+    _Py_NormalizeImmortalReference(s);
+#endif
 }
 
 static /* non-null */ PyObject*
@@ -15609,6 +15644,13 @@ _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));
@@ -16117,8 +16159,10 @@ _PyUnicode_Fini(PyInterpreterState *interp)
 {
     struct _Py_unicode_state *state = &interp->unicode;
 
-    // _PyUnicode_ClearInterned() must be called before _PyUnicode_Fini()
-    assert(get_interned_dict(interp) == NULL);
+    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);