[3.13] gh-126986: Stop Using _PyInterpreterState_FailIfNotRunning() (gh-127112)

This is a pseudo-backport of d6b3e78 (gh-126988).  In that change for 3.14+, we dropped _PyInterpreterState_FailIfNotRunning() and added _PyErr_SetInterpreterAlreadyRunning().  Here, we replace usage of _PyInterpreterState_FailIfNotRunning() with the inlined equivalent of _PyErr_SetInterpreterAlreadyRunning(), without adding that function.  That way we avoid changing the 3.13 ABI.

Author: ericsnowcurrently

Python/crossinterp.c

@@ -985,8 +985,8 @@ _PyXI_ApplyErrorCode(_PyXI_errcode code, PyInterpreterState *interp)
         break;
     case _PyXI_ERR_ALREADY_RUNNING:
         assert(interp != NULL);
-        assert(_PyInterpreterState_IsRunningMain(interp));
-        _PyInterpreterState_FailIfRunningMain(interp);
+        // In 3.14+ we use _PyErr_SetInterpreterAlreadyRunning().
+        PyErr_SetString(PyExc_InterpreterError, "interpreter already running");
         break;
     case _PyXI_ERR_MAIN_NS_FAILURE:
         PyErr_SetString(PyExc_InterpreterError,

Python/pystate.c

@@ -1053,7 +1053,9 @@ get_main_thread(PyInterpreterState *interp)
 int
 _PyInterpreterState_SetRunningMain(PyInterpreterState *interp)
 {
-    if (_PyInterpreterState_FailIfRunningMain(interp) < 0) {
+    if (get_main_thread(interp) != NULL) {
+        // In 3.14+ we use _PyErr_SetInterpreterAlreadyRunning().
+        PyErr_SetString(PyExc_InterpreterError, "interpreter already running");
         return -1;
     }
     PyThreadState *tstate = current_fast_get();
@@ -1099,6 +1101,7 @@ _PyThreadState_IsRunningMain(PyThreadState *tstate)
     return get_main_thread(interp) == tstate;
 }
 
+// This has been removed in 3.14.
 int
 _PyInterpreterState_FailIfRunningMain(PyInterpreterState *interp)
 {