Fix nullability problem wrt return type of kotlin services

sdk-api-kotlin-gen/src/test/kotlin/dev/restate/sdk/kotlin/CodegenTest.kt

@@ -96,6 +96,13 @@ class CodegenTest : TestDefinitions.TestSuite {
       // Just needs to compile
       return CodegenTestCornerCasesClient.fromContext(context, request)._send("my_send").await()
     }
+
+    @Exclusive
+    suspend fun returnNull(context: ObjectContext, request: String?): String? {
+      return CodegenTestCornerCasesClient.fromContext(context, context.key())
+          .returnNull(request)
+          .await()
+    }
   }
 
   @Workflow
@@ -280,6 +287,20 @@ class CodegenTest : TestDefinitions.TestSuite {
                 invokeMessage(
                     Target.service("RawInputOutput", "rawOutputWithCustomCT"), KtSerdes.UNIT, null),
                 outputMessage("{{".toByteArray()),
-                END_MESSAGE))
+                END_MESSAGE),
+        testInvocation({ CornerCases() }, "returnNull")
+            .withInput(
+                startMessage(1, "mykey"),
+                inputMessage(KtSerdes.json<String?>().serialize(null)),
+                completionMessage(1, KtSerdes.json<String?>(), null))
+            .onlyUnbuffered()
+            .expectingOutput(
+                invokeMessage(
+                    Target.virtualObject("CodegenTestCornerCases", "mykey", "returnNull"),
+                    KtSerdes.json<String?>(),
+                    null),
+                outputMessage(KtSerdes.json<String?>(), null),
+                END_MESSAGE),
+    )
   }
 }

sdk-api-kotlin/src/main/kotlin/dev/restate/sdk/kotlin/Awaitables.kt

@@ -57,7 +57,7 @@ internal class SingleAwaitableImpl<T : Any?>(
   }
 }
 
-internal abstract class BaseSingleMappedAwaitableImpl<T : Any, U : Any>(
+internal abstract class BaseSingleMappedAwaitableImpl<T : Any?, U : Any?>(
     private val inner: BaseAwaitableImpl<T>
 ) : BaseAwaitableImpl<U>(inner.syscalls) {
   private var mappedResult: Result<U>? = null
@@ -76,7 +76,7 @@ internal abstract class BaseSingleMappedAwaitableImpl<T : Any, U : Any>(
   }
 }
 
-internal open class SingleSerdeAwaitableImpl<T : Any>
+internal open class SingleSerdeAwaitableImpl<T : Any?>
 internal constructor(
     syscalls: Syscalls,
     deferred: Deferred<ByteBuffer>,

sdk-api-kotlin/src/main/kotlin/dev/restate/sdk/kotlin/ContextImpl.kt

@@ -74,7 +74,7 @@ internal class ContextImpl internal constructor(private val syscalls: Syscalls)
   }
 
   override suspend fun <T : Any> set(key: StateKey<T>, value: T) {
-    val serializedValue = key.serde().serializeWrappingException(syscalls, value)!!
+    val serializedValue = key.serde().serializeWrappingException(syscalls, value)
     return suspendCancellableCoroutine { cont: CancellableContinuation<Unit> ->
       syscalls.set(key.name(), serializedValue, completingUnitContinuation(cont))
     }
@@ -101,7 +101,7 @@ internal class ContextImpl internal constructor(private val syscalls: Syscalls)
     return UnitAwakeableImpl(syscalls, deferred)
   }
 
-  override suspend fun <T : Any, R : Any> callAsync(
+  override suspend fun <T : Any?, R : Any?> callAsync(
       target: Target,
       inputSerde: Serde<T>,
       outputSerde: Serde<R>,
@@ -117,7 +117,7 @@ internal class ContextImpl internal constructor(private val syscalls: Syscalls)
     return SingleSerdeAwaitableImpl(syscalls, deferred, outputSerde)
   }
 
-  override suspend fun <T : Any> send(
+  override suspend fun <T : Any?> send(
       target: Target,
       inputSerde: Serde<T>,
       parameter: T,

sdk-api-kotlin/src/main/kotlin/dev/restate/sdk/kotlin/KtSerdes.kt

@@ -16,6 +16,7 @@ import java.nio.charset.StandardCharsets
 import kotlin.reflect.typeOf
 import kotlinx.serialization.KSerializer
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonNull
 import kotlinx.serialization.serializer
 
 object KtStateKey {
@@ -29,15 +30,15 @@ object KtStateKey {
 object KtDurablePromiseKey {
 
   /** Creates a json [StateKey]. */
-  inline fun <reified T> json(name: String): DurablePromiseKey<T> {
+  inline fun <reified T : Any?> json(name: String): DurablePromiseKey<T> {
     return DurablePromiseKey.of(name, KtSerdes.json())
   }
 }
 
 object KtSerdes {
 
   /** Creates a [Serde] implementation using the `kotlinx.serialization` json module. */
-  inline fun <reified T> json(): Serde<T> {
+  inline fun <reified T : Any?> json(): Serde<T> {
     @Suppress("UNCHECKED_CAST")
     return when (typeOf<T>()) {
       typeOf<Unit>() -> UNIT as Serde<T>
@@ -69,10 +70,13 @@ object KtSerdes {
       }
 
   /** Creates a [Serde] implementation using the `kotlinx.serialization` json module. */
-  fun <T> json(serializer: KSerializer<T>): Serde<T> {
+  fun <T : Any?> json(serializer: KSerializer<T>): Serde<T> {
     return object : Serde<T> {
-      override fun serialize(value: T): ByteArray {
-        return Json.encodeToString(serializer, value!!).encodeToByteArray()
+      override fun serialize(value: T?): ByteArray {
+        if (value == null) {
+          return Json.encodeToString(JsonNull.serializer(), JsonNull).encodeToByteArray()
+        }
+        return Json.encodeToString(serializer, value).encodeToByteArray()
       }
 
       override fun deserialize(value: ByteArray): T {

sdk-api-kotlin/src/main/kotlin/dev/restate/sdk/kotlin/api.kt

@@ -57,7 +57,7 @@ sealed interface Context {
    * @param parameter the invocation request parameter.
    * @return the invocation response.
    */
-  suspend fun <T : Any, R : Any> call(
+  suspend fun <T : Any?, R : Any?> call(
       target: Target,
       inputSerde: Serde<T>,
       outputSerde: Serde<R>,
@@ -75,7 +75,7 @@ sealed interface Context {
    * @param parameter the invocation request parameter.
    * @return an [Awaitable] that wraps the Restate service method result.
    */
-  suspend fun <T : Any, R : Any> callAsync(
+  suspend fun <T : Any?, R : Any?> callAsync(
       target: Target,
       inputSerde: Serde<T>,
       outputSerde: Serde<R>,
@@ -90,7 +90,7 @@ sealed interface Context {
    * @param parameter the invocation request parameter.
    * @param delay time to wait before executing the call
    */
-  suspend fun <T : Any> send(
+  suspend fun <T : Any?> send(
       target: Target,
       inputSerde: Serde<T>,
       parameter: T,