fix(python): returning Promise<void> breaks runtime (#3916)

The `EndResponse` struct did not supply a default value for the `result` property, making it required (despite it is typed `Any`, which `None` is valid for). Fix the schema to represent as `Optional[Any] = None` and add a compliance test to validate this scenario.

Note for readers here - async support is very much experimental and only partially imported in most jsii front-end languages. Caution is still advised if you are going to expose async APIs.



---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

gh-pages/content/specification/6-compliance-report.md

@@ -5,7 +5,7 @@
 This section details the current state of each language binding with respect to our standard compliance suite.
 
 
-| number | test                                                                                                                                                             | java (98.36%) | golang (79.51%)                              | Dotnet | Python |
+| number | test                                                                                                                                                             | java (97.56%) | golang (78.86%)                              | Dotnet | Python |
 | ------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------- | -------------------------------------------- | ------ | ------ |
 | 1      | asyncOverrides_overrideCallsSuper                                                                                                                                | 🟢            | [🔴](https://github.com/aws/jsii/issues/2670) | ⭕      | ⭕      |
 | 2      | [arrayReturnedByMethodCanBeRead]("Array created in the kernel can be queried for its elements")                                                                  | 🟢            | 🟢                                           | ⭕      | ⭕      |
@@ -129,3 +129,4 @@ This section details the current state of each language binding with respect to
 | 120    | [downcasting]("Ensures unsafe-cast features work as expected")                                                                                                   | ⭕             | 🟢                                           | ⭕      | ⭕      |
 | 121    | [strippedDeprecatedMemberCanBeReceived]("Ensures --strip-deprecated does not cause odd runtime errors")                                                          | 🟢            | 🟢                                           | ⭕      | ⭕      |
 | 122    | [exceptionMessage]("Verifies that custom exception names are correctly forwarded")                                                                               | 🟢            | 🟢                                           | ⭕      | ⭕      |
+| 123    | [voidReturningAsync]("Verifies that returning Promise<void> is correctly handled")                                                                               | ⭕             | ⭕                                            | ⭕      | ⭕      |

packages/@jsii/python-runtime/src/jsii/_kernel/types.py

@@ -185,7 +185,7 @@ class EndRequest:
 @attr.s(auto_attribs=True, frozen=True, slots=True)
 class EndResponse:
 
-    result: Any
+    result: Optional[Any] = None
 
 
 @attr.s(auto_attribs=True, frozen=True, slots=True)

packages/@jsii/python-runtime/tests/test_compliance.py

@@ -76,6 +76,7 @@
     StructParameterType,
     AnonymousImplementationProvider,
     UpcasingReflectable,
+    PromiseNothing,
 )
 from jsii_calc.cdk16625 import Cdk16625
 from jsii_calc.cdk22369 import AcceptsPath
@@ -1354,3 +1355,11 @@ def test_stripped_deprecated_member_can_be_received():
 def test_exception_message():
     with pytest.raises(RuntimeError, match="Cannot find asset"):
         AcceptsPath(source_path="A Bad Path")
+
+
+def test_void_returning_async():
+    """Verifies it's okay to return a Promise<void>."""
+
+    assert PromiseNothing().instance_promise_it() is None
+    ## TODO: This is currently broken as code-gen is incorrect for static async.
+    # assert PromiseNothing.promise_it() is None

packages/jsii-calc/lib/compliance.ts

@@ -3131,3 +3131,13 @@ export interface ParamShadowsBuiltinsProps {
   readonly booleanProperty: boolean;
   readonly structProperty: StructA;
 }
+
+export class PromiseNothing {
+  public static async promiseIt(): Promise<void> {
+    return Promise.resolve();
+  }
+
+  public async instancePromiseIt(): Promise<void> {
+    return PromiseNothing.promiseIt();
+  }
+}

packages/jsii-calc/test/assembly.jsii

@@ -11823,6 +11823,50 @@
       ],
       "symbolId": "lib/calculator:Power"
     },
+    "jsii-calc.PromiseNothing": {
+      "assembly": "jsii-calc",
+      "docs": {
+        "stability": "stable"
+      },
+      "fqn": "jsii-calc.PromiseNothing",
+      "initializer": {
+        "docs": {
+          "stability": "stable"
+        }
+      },
+      "kind": "class",
+      "locationInModule": {
+        "filename": "lib/compliance.ts",
+        "line": 3135
+      },
+      "methods": [
+        {
+          "async": true,
+          "docs": {
+            "stability": "stable"
+          },
+          "locationInModule": {
+            "filename": "lib/compliance.ts",
+            "line": 3136
+          },
+          "name": "promiseIt",
+          "static": true
+        },
+        {
+          "async": true,
+          "docs": {
+            "stability": "stable"
+          },
+          "locationInModule": {
+            "filename": "lib/compliance.ts",
+            "line": 3140
+          },
+          "name": "instancePromiseIt"
+        }
+      ],
+      "name": "PromiseNothing",
+      "symbolId": "lib/compliance:PromiseNothing"
+    },
     "jsii-calc.PropertyNamedProperty": {
       "assembly": "jsii-calc",
       "docs": {
@@ -18799,5 +18843,5 @@
     }
   },
   "version": "3.20.120",
-  "fingerprint": "kmlc5is+t/xffykk7b4m8jurrxFDkj9pjv2cW/V1J50="
+  "fingerprint": "EH7xszNdCh9PCFUZ8Foi7g2CPhdrKeZm8CQaUCNv4GQ="
 }