Fix bug with Unit return values in kotlin codegen (#465)

Author: slinkydeveloper

sdk-api-kotlin-gen/src/main/resources/templates/ServiceDefinitionFactory.hbs

@@ -20,7 +20,7 @@ class {{generatedClassSimpleName}}: dev.restate.sdk.endpoint.definition.ServiceD
                 {{#if isExclusive}}dev.restate.sdk.endpoint.definition.HandlerType.EXCLUSIVE{{else if isWorkflow}}dev.restate.sdk.endpoint.definition.HandlerType.WORKFLOW{{else}}dev.restate.sdk.endpoint.definition.HandlerType.SHARED{{/if}},
                 {{inputSerdeRef}},
                 {{outputSerdeRef}},
-                dev.restate.sdk.kotlin.HandlerRunner.of({{serdeFactoryRef}}, handlerRunnerOptions, bindableService::{{name}})
+                dev.restate.sdk.kotlin.HandlerRunner.{{#if outputEmpty}}ofEmptyReturn{{else}}of{{/if}}({{serdeFactoryRef}}, handlerRunnerOptions, bindableService::{{name}})
                 ){{#if inputAcceptContentType}}.withAcceptContentType("{{inputAcceptContentType}}"){{/if}}{{#unless @last}},{{/unless}}
             {{/handlers}}
             )

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

@@ -33,7 +33,10 @@ internal constructor(
   companion object {
     private val LOG = LogManager.getLogger(HandlerRunner::class.java)
 
-    /** Factory method for [dev.restate.sdk.kotlin.HandlerRunner], used by codegen. */
+    /**
+     * Factory method for [dev.restate.sdk.kotlin.HandlerRunner], used by codegen. Please note this
+     * may be subject to breaking changes.
+     */
     fun <REQ, RES, CTX : Context> of(
         contextSerdeFactory: SerdeFactory,
         options: Options = Options.DEFAULT,
@@ -42,14 +45,53 @@ internal constructor(
       return HandlerRunner(runner, contextSerdeFactory, options)
     }
 
-    /** Factory method for [dev.restate.sdk.kotlin.HandlerRunner], used by codegen. */
+    /**
+     * Factory method for [dev.restate.sdk.kotlin.HandlerRunner], used by codegen. Please note this
+     * may be subject to breaking changes.
+     */
     fun <RES, CTX : Context> of(
         contextSerdeFactory: SerdeFactory,
         options: Options = Options.DEFAULT,
         runner: suspend (CTX) -> RES,
     ): HandlerRunner<Unit, RES, CTX> {
       return HandlerRunner({ ctx: CTX, _: Unit -> runner(ctx) }, contextSerdeFactory, options)
     }
+
+    /**
+     * Factory method for [dev.restate.sdk.kotlin.HandlerRunner], used by codegen. Please note this
+     * may be subject to breaking changes.
+     */
+    fun <REQ, CTX : Context> ofEmptyReturn(
+        contextSerdeFactory: SerdeFactory,
+        options: Options = Options.DEFAULT,
+        runner: suspend (CTX, REQ) -> Unit,
+    ): HandlerRunner<REQ, Unit, CTX> {
+      return HandlerRunner(
+          { ctx: CTX, req: REQ ->
+            runner(ctx, req)
+            Unit
+          },
+          contextSerdeFactory,
+          options)
+    }
+
+    /**
+     * Factory method for [dev.restate.sdk.kotlin.HandlerRunner], used by codegen. Please note this
+     * may be subject to breaking changes.
+     */
+    fun <CTX : Context> ofEmptyReturn(
+        contextSerdeFactory: SerdeFactory,
+        options: Options = Options.DEFAULT,
+        runner: suspend (CTX) -> Unit,
+    ): HandlerRunner<Unit, Unit, CTX> {
+      return HandlerRunner(
+          { ctx: CTX, _: Unit ->
+            runner(ctx)
+            Unit
+          },
+          contextSerdeFactory,
+          options)
+    }
   }
 
   override fun run(

sdk-core/src/test/kotlin/dev/restate/sdk/core/kotlinapi/CodegenTest.kt

@@ -8,6 +8,7 @@
 // https://github.com/restatedev/sdk-java/blob/main/LICENSE
 package dev.restate.sdk.core.kotlinapi
 
+import dev.restate.common.Slice
 import dev.restate.common.Target
 import dev.restate.sdk.annotation.*
 import dev.restate.sdk.core.TestDefinitions
@@ -125,6 +126,13 @@ class CodegenTest : TestDefinitions.TestSuite {
           .returnNull(request) {}
           .await()
     }
+
+    @Exclusive
+    suspend fun badReturnTypeInferred(context: ObjectContext): Unit {
+      CodegenTestCornerCasesClient.fromContext(context, context.key())
+          .send()
+          .badReturnTypeInferred()
+    }
   }
 
   @Workflow
@@ -337,6 +345,19 @@ class CodegenTest : TestDefinitions.TestSuite {
                     null),
                 outputCmd(jsonSerde<String?>(), null),
                 END_MESSAGE),
+        testInvocation({ CornerCases() }, "badReturnTypeInferred")
+            .withInput(startMessage(1, "mykey"), inputCmd())
+            .onlyBidiStream()
+            .expectingOutput(
+                oneWayCallCmd(
+                    1,
+                    Target.virtualObject(
+                        "CodegenTestCornerCases", "mykey", "badReturnTypeInferred"),
+                    null,
+                    null,
+                    Slice.EMPTY),
+                outputCmd(),
+                END_MESSAGE),
     )
   }
 }