gh-146102: Fix use-after-free and exception leak in crossinterp subsystem

Author: kairosci

Python/crossinterp.c

@@ -491,24 +491,24 @@ _get_xidata(PyThreadState *tstate,
             Py_DECREF(obj);
             return -1;
         }
-        // Fall back to obj
-        Py_DECREF(obj);
         if (!_PyErr_Occurred(tstate)) {
             _set_xid_lookup_failure(tstate, obj, NULL, NULL);
         }
+        Py_DECREF(obj);
         return -1;
     }
     int res = getdata.basic != NULL
         ? getdata.basic(tstate, obj, xidata)
         : getdata.fallback(tstate, obj, fallback, xidata);
-    Py_DECREF(obj);
     if (res != 0) {
         PyObject *cause = _PyErr_GetRaisedException(tstate);
         assert(cause != NULL);
         _set_xid_lookup_failure(tstate, obj, NULL, cause);
         Py_XDECREF(cause);
+        Py_DECREF(obj);
         return -1;
     }
+    Py_DECREF(obj);
 
     // Fill in the blanks and validate the result.
     _PyXIData_INTERPID(xidata) = PyInterpreterState_GetID(interp);

Python/crossinterp_data_lookup.h

@@ -100,6 +100,7 @@ _PyXI_UnwrapNotShareableError(PyThreadState * tstate, _PyXI_failure *failure)
     if (failure != NULL) {
         _PyXI_errcode code = _PyXI_ERR_NOT_SHAREABLE;
         if (_PyXI_InitFailure(failure, code, exc) < 0) {
+            _PyErr_SetRaisedException(tstate, exc);
             return -1;
         }
     }

Python/crossinterp_exceptions.h

@@ -83,6 +83,7 @@ _ensure_notshareableerror(PyThreadState *tstate,
             // A NotShareableError instance is already set.
             assert(cause == NULL);
             _PyErr_SetRaisedException(tstate, ctx);
+            return;
         }
     }
     else {