gh-113433: Automatically Clean Up Subinterpreters in Py_Finalize() (gh-121060)

This change makes things a little less painful for some users.  It also fixes a failing assert (gh-120765), by making sure all subinterpreters are destroyed before the main interpreter.  As part of that, we make sure Py_Finalize() always runs with the main interpreter active.

Misc/NEWS.d/next/Core and Builtins/2024-06-26-13-42-36.gh-issue-113433.xKAtLB.rst

@@ -0,0 +1,2 @@
+Subinterpreters now get cleaned up automatically during runtime
+finalization.

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

@@ -73,6 +73,7 @@ static PyStatus init_sys_streams(PyThreadState *tstate);
 static PyStatus init_android_streams(PyThreadState *tstate);
 #endif
 static void wait_for_thread_shutdown(PyThreadState *tstate);
+static void finalize_subinterpreters(void);
 static void call_ll_exitfuncs(_PyRuntimeState *runtime);
 
 /* The following places the `_PyRuntime` structure in a location that can be
@@ -1907,20 +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;
@@ -1943,6 +1997,8 @@ Py_FinalizeEx(void)
 
     _PyAtExit_Call(tstate->interp);
 
+    assert(_PyThreadState_GET() == tstate);
+
     /* Copy the core config, PyInterpreterState_Delete() free
        the core config memory */
 #ifdef Py_REF_DEBUG
@@ -2023,6 +2079,9 @@ Py_FinalizeEx(void)
     _PyImport_FiniExternal(tstate->interp);
     finalize_modules(tstate);
 
+    /* Clean up any lingering subinterpreters. */
+    finalize_subinterpreters();
+
     /* Print debug stats if any */
     _PyEval_Fini();
 
@@ -2140,10 +2199,16 @@ Py_FinalizeEx(void)
     return status;
 }
 
+int
+Py_FinalizeEx(void)
+{
+    return _Py_Finalize(&_PyRuntime);
+}
+
 void
 Py_Finalize(void)
 {
-    Py_FinalizeEx();
+    (void)_Py_Finalize(&_PyRuntime);
 }
 
 
@@ -2355,6 +2420,79 @@ _Py_IsInterpreterFinalizing(PyInterpreterState *interp)
     return finalizing != NULL;
 }
 
+static void
+finalize_subinterpreters(void)
+{
+    PyThreadState *final_tstate = _PyThreadState_GET();
+    PyInterpreterState *main_interp = _PyInterpreterState_Main();
+    assert(final_tstate->interp == main_interp);
+    _PyRuntimeState *runtime = main_interp->runtime;
+    struct pyinterpreters *interpreters = &runtime->interpreters;
+
+    /* Get the first interpreter in the list. */
+    HEAD_LOCK(runtime);
+    PyInterpreterState *interp = interpreters->head;
+    if (interp == main_interp) {
+        interp = interp->next;
+    }
+    HEAD_UNLOCK(runtime);
+
+    /* Bail out if there are no subinterpreters left. */
+    if (interp == NULL) {
+        return;
+    }
+
+    /* Warn the user if they forgot to clean up subinterpreters. */
+    (void)PyErr_WarnEx(
+            PyExc_RuntimeWarning,
+            "remaining subinterpreters; "
+            "destroy them with _interpreters.destroy()",
+            0);
+
+    /* Swap out the current tstate, which we know must belong
+       to the main interpreter. */
+    _PyThreadState_Detach(final_tstate);
+
+    /* Clean up all remaining subinterpreters. */
+    while (interp != NULL) {
+        assert(!_PyInterpreterState_IsRunningMain(interp));
+
+        /* Find the tstate to use for fini.  We assume the interpreter
+           will have at most one tstate at this point. */
+        PyThreadState *tstate = interp->threads.head;
+        if (tstate != NULL) {
+            /* Ideally we would be able to use tstate as-is, and rely
+               on it being in a ready state: no exception set, not
+               running anything (tstate->current_frame), matching the
+               current thread ID (tstate->thread_id).  To play it safe,
+               we always delete it and use a fresh tstate instead. */
+            assert(tstate != final_tstate);
+            _PyThreadState_Attach(tstate);
+            PyThreadState_Clear(tstate);
+            _PyThreadState_Detach(tstate);
+            PyThreadState_Delete(tstate);
+        }
+        tstate = _PyThreadState_NewBound(interp, _PyThreadState_WHENCE_FINI);
+
+        /* Destroy the subinterpreter. */
+        _PyThreadState_Attach(tstate);
+        Py_EndInterpreter(tstate);
+        assert(_PyThreadState_GET() == NULL);
+
+        /* Advance to the next interpreter. */
+        HEAD_LOCK(runtime);
+        interp = interpreters->head;
+        if (interp == main_interp) {
+            interp = interp->next;
+        }
+        HEAD_UNLOCK(runtime);
+    }
+
+    /* Switch back to the main interpreter. */
+    _PyThreadState_Attach(final_tstate);
+}
+
+
 /* Add the __main__ module */
 
 static PyStatus
@@ -3216,7 +3354,7 @@ 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;
     }