gh-120838: Make Sure Py_Finalize() Always Runs Against the Main Interpreter

Misc/NEWS.d/next/Core and Builtins/2024-06-26-14-09-31.gh-issue-120838.nFeTL9.rst

@@ -0,0 +1,2 @@
+:c:func:`Py_Finalize()` and :c:func:`Py_FinalizeEx()` now always run with
+the main interpreter active.

Python/pylifecycle.c

@@ -1908,20 +1908,73 @@ finalize_interp_delete(PyInterpreterState *interp)
 }
 
 
-int
-Py_FinalizeEx(void)
+/* Conceptually, there isn't a good reason for Py_Finalize()
+   to be called in any other thread than the one where Py_Initialize()
+   was called.  Consequently, it would make sense to fail if the thread
+   or thread state (or interpreter) don't match.  However, such
+   constraints have never been enforced, and, as unlikely as it may be,
+   there may be users relying on the unconstrained behavior.  Thus,
+   we do our best here to accommodate that possibility. */
+
+static PyThreadState *
+resolve_final_tstate(_PyRuntimeState *runtime)
+{
+    PyThreadState *main_tstate = runtime->main_tstate;
+    assert(main_tstate != NULL);
+    assert(main_tstate->thread_id == runtime->main_thread);
+    PyInterpreterState *main_interp = _PyInterpreterState_Main();
+    assert(main_tstate->interp == main_interp);
+
+    PyThreadState *tstate = _PyThreadState_GET();
+    if (_Py_IsMainThread()) {
+        if (tstate != main_tstate) {
+            /* This implies that Py_Finalize() was called while
+               a non-main interpreter was active or while the main
+               tstate was temporarily swapped out with another.
+               Neither case should be allowed, but, until we get around
+               to fixing that (and Py_Exit()), we're letting it go. */
+            (void)PyThreadState_Swap(main_tstate);
+        }
+    }
+    else {
+        /* This is another unfortunate case where Py_Finalize() was
+           called when it shouldn't have been.  We can't simply switch
+           over to the main thread.  At the least, however, we can make
+           sure the main interpreter is active. */
+        if (!_Py_IsMainInterpreter(tstate->interp)) {
+            /* We don't go to the trouble of updating runtime->main_tstate
+               since it will be dead soon anyway. */
+            main_tstate =
+                _PyThreadState_New(main_interp, _PyThreadState_WHENCE_FINI);
+            if (main_tstate != NULL) {
+                _PyThreadState_Bind(main_tstate);
+                (void)PyThreadState_Swap(main_tstate);
+            }
+            else {
+                /* Fall back to the current tstate.  It's better than nothing. */
+                main_tstate = tstate;
+            }
+        }
+    }
+    assert(main_tstate != NULL);
+
+    /* We might want to warn if main_tstate->current_frame != NULL. */
+
+    return main_tstate;
+}
+
+static int
+_Py_Finalize(_PyRuntimeState *runtime)
 {
     int status = 0;
 
-    _PyRuntimeState *runtime = &_PyRuntime;
+    /* Bail out early if already finalized (or never initialized). */
     if (!runtime->initialized) {
         return status;
     }
 
-    /* Get current thread state and interpreter pointer */
-    PyThreadState *tstate = _PyThreadState_GET();
-    // XXX assert(_Py_IsMainInterpreter(tstate->interp));
-    // XXX assert(_Py_IsMainThread());
+    /* Get final thread state pointer. */
+    PyThreadState *tstate = resolve_final_tstate(runtime);
 
     // Block some operations.
     tstate->interp->finalizing = 1;
@@ -2141,10 +2194,16 @@ Py_FinalizeEx(void)
     return status;
 }
 
+int
+Py_FinalizeEx(void)
+{
+    return _Py_Finalize(&_PyRuntime);
+}
+
 void
 Py_Finalize(void)
 {
-    Py_FinalizeEx();
+    (void)_Py_Finalize(&_PyRuntime);
 }
 
 
@@ -3217,7 +3276,7 @@ Py_Exit(int sts)
     if (tstate != NULL && _PyThreadState_IsRunningMain(tstate)) {
         _PyInterpreterState_SetNotRunningMain(tstate->interp);
     }
-    if (Py_FinalizeEx() < 0) {
+    if (_Py_Finalize(&_PyRuntime) < 0) {
         sts = 120;
     }