[WIP] bpo-38865: Py_FinalizeEx() cannot be called in a subinterpreter

Doc/c-api/init.rst

@@ -290,6 +290,8 @@ Initializing and finalizing the interpreter
    developer might want to free all memory allocated by Python before exiting from
    the application.
 
+   This function cannot be called in a subinterpreter.
+
    **Bugs and caveats:** The destruction of modules and objects in modules is done
    in random order; this may cause destructors (:meth:`__del__` methods) to fail
    when they depend on other objects (even functions) or modules.  Dynamically
@@ -305,6 +307,9 @@ Initializing and finalizing the interpreter
 
    .. versionadded:: 3.6
 
+   .. versionchanged:: 3.9
+      This function cannot be called in a subinterpreter anymore.
+
 .. c:function:: void Py_Finalize()
 
    This is a backwards-compatible version of :c:func:`Py_FinalizeEx` that

Doc/whatsnew/3.9.rst

@@ -514,6 +514,9 @@ Build and C API Changes
 
   Extension modules without module state (``m_size <= 0``) are not affected.
 
+* :c:func:`Py_FinalizeEx` cannot be called in a subinterpreter anymore.
+  (Contributed by Victor Stinner in :issue:`38865`.)
+
 
 Deprecated
 ==========

Misc/NEWS.d/next/C API/2020-03-18-22-52-27.bpo-38865.KeRYAu.rst

@@ -0,0 +1 @@
+:c:func:`Py_FinalizeEx` cannot be called in a subinterpreter anymore.

Programs/_testembed.c

@@ -1165,7 +1165,7 @@ static volatile int _audit_subinterpreter_interpreter_count = 0;
 
 static int _audit_subinterpreter_hook(const char *event, PyObject *args, void *userdata)
 {
-    printf("%s\n", event);
+    fprintf(stderr, "audit event: %s\n", event);
     if (strcmp(event, "cpython.PyInterpreterState_New") == 0) {
         _audit_subinterpreter_interpreter_count += 1;
     }
@@ -1178,17 +1178,30 @@ static int test_audit_subinterpreter(void)
     PySys_AddAuditHook(_audit_subinterpreter_hook, NULL);
     _testembed_Py_Initialize();
 
-    Py_NewInterpreter();
-    Py_NewInterpreter();
-    Py_NewInterpreter();
+    PyThreadState *mainstate = PyThreadState_Get();
+
+    for (int i=0; i<3; i++) {
+        PyThreadState *substate = Py_NewInterpreter();
+        if (substate == NULL) {
+            fprintf(stderr, "Py_NewInterpreter() failed\n");
+            return 1;
+        }
+        Py_EndInterpreter(substate);
+
+        /* Restore tstate after each Py_EndInterpreter() call. Otherwise the
+           current Python thread state is NULL and so PySys_Audit() does
+           nothing. */
+        PyThreadState_Swap(mainstate);
+    }
 
     Py_Finalize();
 
-    switch (_audit_subinterpreter_interpreter_count) {
-        case 3: return 0;
-        case 0: return -1;
-        default: return _audit_subinterpreter_interpreter_count;
+    printf("audit subinterpreter count: %i\n",
+           _audit_subinterpreter_interpreter_count);
+    if (_audit_subinterpreter_interpreter_count != 3) {
+        return 2;
     }
+    return 0;
 }
 
 typedef struct {

Python/pylifecycle.c

@@ -1340,6 +1340,10 @@ Py_FinalizeEx(void)
     PyThreadState *tstate = _PyRuntimeState_GetThreadState(runtime);
     PyInterpreterState *interp = tstate->interp;
 
+    if (!_Py_IsMainInterpreter(tstate)) {
+        Py_FatalError("Py_FinalizeEx cannot be called in a subinterpreter");
+    }
+
     // Wrap up existing "threading"-module-created, non-daemon threads.
     wait_for_thread_shutdown(tstate);