FIX Check if future is done before trying to set a result on it

Otherwise, future raises invalid state error.
Question: should we try to avoid getting to the point where we call this in the
first place??

src/core/jsproxy.c

@@ -2637,6 +2637,7 @@ wrap_promise(JsVal promise, JsVal done_callback, PyObject* js2py_converter)
 {
   bool success = false;
   PyObject* loop = NULL;
+  PyObject* helpers = NULL;
   PyObject* set_result = NULL;
   PyObject* set_exception = NULL;
 
@@ -2648,9 +2649,11 @@ wrap_promise(JsVal promise, JsVal done_callback, PyObject* js2py_converter)
   result = _PyObject_CallMethodIdNoArgs(loop, &PyId_create_future);
   FAIL_IF_NULL(result);
 
-  set_result = _PyObject_GetAttrId(result, &PyId_set_result);
+  helpers = PyImport_ImportModule("_pyodide._future_helpers");
+  FAIL_IF_NULL(helpers);
+  set_result = Py_XNewRef(PyTuple_GetItem(helpers, 0));
   FAIL_IF_NULL(set_result);
-  set_exception = _PyObject_GetAttrId(result, &PyId_set_exception);
+  set_exception = Py_XNewRef(PyTuple_GetItem(helpers, 1));
   FAIL_IF_NULL(set_exception);
 
   promise = JsvPromise_Resolve(promise);
@@ -2663,6 +2666,7 @@ wrap_promise(JsVal promise, JsVal done_callback, PyObject* js2py_converter)
   success = true;
 finally:
   Py_CLEAR(loop);
+  Py_CLEAR(helpers);
   Py_CLEAR(set_result);
   Py_CLEAR(set_exception);
   if (!success) {

src/py/_pyodide/_future_helper.py

@@ -0,0 +1,13 @@
+def set_result(fut, val):
+    if fut.done():
+        return
+    fut.set_result(val)
+
+def set_exception(fut, val):
+    if fut.done():
+        return
+    fut.set_result(val)
+
+
+def get_future_resolvers(fut):
+    return (set_result.__get__(fut), set_exception.__get__(fut))