feat: add support for unbound event stream payloads

.changes/704b9a6e-d859-4706-b39f-91052b375834.json

@@ -0,0 +1,5 @@
+{
+    "id": "704b9a6e-d859-4706-b39f-91052b375834",
+    "type": "misc",
+    "description": "Add unbound event stream payload deserialization path"
+}

codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/eventstream/EventStreamParserGenerator.kt

@@ -150,8 +150,14 @@ class EventStreamParserGenerator(
             } else {
                 if (unbound.isNotEmpty()) {
                     // all remaining members are bound to payload (but not explicitly bound via @eventPayload)
-                    // FIXME - this would require us to generate a "partial" deserializer like we do for where the builder is passed in
-                    TODO("support for unbound event stream members is not implemented")
+                    // generate a payload deserializer specific to the unbound members (note this will be a deserializer
+                    // for the overall event shape but only payload members will be considered for deserialization),
+                    // and then assign each deserialized payload member to the current builder instance
+                    val payloadDeserializeFn = sdg.payloadDeserializer(ctx, member, unbound)
+                    writer.write("val tmp = #T(message.payload)", payloadDeserializeFn)
+                    unbound.forEach {
+                        writer.write("eb.#1L = tmp.#1L", it.defaultName())
+                    }
                 }
             }
 

tests/codegen/event-stream/event-stream-model-template.smithy

@@ -64,6 +64,11 @@ structure MessageWithNoHeaderPayloadTraits {
     someString: String,
 }
 
+structure MessageWithUnboundPayloadTraits {
+    @eventHeader header: String,
+    unboundString: String,
+}
+
 @streaming
 union TestStream {
     MessageWithBlob: MessageWithBlob,
@@ -73,5 +78,6 @@ union TestStream {
     MessageWithHeaders: MessageWithHeaders,
     MessageWithHeaderAndPayload: MessageWithHeaderAndPayload,
     MessageWithNoHeaderPayloadTraits: MessageWithNoHeaderPayloadTraits,
+    MessageWithUnboundPayloadTraits: MessageWithUnboundPayloadTraits,
     SomeError: SomeError,
 }

tests/codegen/event-stream/src/test/kotlin/EventStreamTests.kt

@@ -237,4 +237,30 @@ class EventStreamTests {
         assertIs<TestStream.MessageWithNoHeaderPayloadTraits>(deserialized)
         assertEquals(event, deserialized)
     }
+
+    @Test
+    fun testSerializeMessageWithUnboundPayload() = runTest {
+        val event = TestStream.MessageWithUnboundPayloadTraits(
+            MessageWithUnboundPayloadTraits {
+                header = "a korf is a tiger"
+                unboundString = "a flix is comb"
+            },
+        )
+
+        val message = serializedMessage(event)
+
+        val headers = message.headers.associate { it.name to it.value }
+        assertEquals("event", headers[":message-type"]?.expectString())
+        assertEquals("MessageWithUnboundPayloadTraits", headers[":event-type"]?.expectString())
+        assertEquals("application/json", headers[":content-type"]?.expectString())
+
+        assertEquals("a korf is a tiger", headers["header"]?.expectString())
+
+        val expectedBody = """{"unboundString":"a flix is comb"}"""
+        assertJsonStringsEqual(expectedBody, message.payload.decodeToString())
+
+        val deserialized = deserializedEvent(message)
+        assertIs<TestStream.MessageWithUnboundPayloadTraits>(deserialized)
+        assertEquals(event, deserialized)
+    }
 }