Cannot serialize OpenApi instance

[lenguyenthanh]

More specifically this test failed:

"Serialize MediaType success" {
    val mediaType = MediaType(
      schema = Referenced.Other(
        value = Schema(
        )
      )
    )
    val str = json.encodeToString(mediaType)
  }

With stacktrace:

lass arrow.endpoint.docs.openapi.Schema cannot be cast to class kotlin.Unit (arrow.endpoint.docs.openapi.Schema and kotlin.Unit are in unnamed module of loader 'app')
java.lang.ClassCastException: class arrow.endpoint.docs.openapi.Schema cannot be cast to class kotlin.Unit (arrow.endpoint.docs.openapi.Schema and kotlin.Unit are in unnamed module of loader 'app')
  at kotlinx.serialization.internal.UnitSerializer.serialize(Primitives.kt:79)
  at kotlinx.serialization.json.internal.StreamingJsonEncoder.encodeSerializableValue(StreamingJsonEncoder.kt:219)
  at arrow.endpoint.docs.openapi.ReferencedSerializer.serialize(Serializers.kt:125)
  at arrow.endpoint.docs.openapi.ReferencedSerializer.serialize(Serializers.kt:108)
  at kotlinx.serialization.json.internal.StreamingJsonEncoder.encodeSerializableValue(StreamingJsonEncoder.kt:219)
  at kotlinx.serialization.encoding.Encoder$DefaultImpls.encodeNullableSerializableValue(Encoding.kt:302)
  at kotlinx.serialization.encoding.AbstractEncoder.encodeNullableSerializableValue(AbstractEncoder.kt:18)
  at kotlinx.serialization.encoding.AbstractEncoder.encodeNullableSerializableElement(AbstractEncoder.kt:90)
  at kotlinx.serialization.json.internal.StreamingJsonEncoder.encodeNullableSerializableElement(StreamingJsonEncoder.kt:157)
  at arrow.endpoint.docs.openapi.MediaType.write$Self(OpenAPI.kt:404)
  at arrow.endpoint.docs.openapi.MediaType$$serializer.serialize(OpenAPI.kt:404)
  at arrow.endpoint.docs.openapi.MediaType$$serializer.serialize(OpenAPI.kt:404)
  at kotlinx.serialization.json.internal.StreamingJsonEncoder.encodeSerializableValue(StreamingJsonEncoder.kt:219)
  at kotlinx.serialization.json.Json.encodeToString(Json.kt:85)
  at arrow.endpoint.docs.openapi.ResponseSerializerTest$1$4.invokeSuspend(ResponseSerializerTest.kt:78)
  at arrow.endpoint.docs.openapi.ResponseSerializerTest$1$4.invoke(ResponseSerializerTest.kt)
  at arrow.endpoint.docs.openapi.ResponseSerializerTest$1$4.invoke(ResponseSerializerTest.kt)
  at io.kotest.core.spec.style.scopes.StringSpecRootScope$invoke$1.invokeSuspend(StringSpecRootScope.kt:58)
  at io.kotest.core.spec.style.scopes.StringSpecRootScope$invoke$1.invoke(StringSpecRootScope.kt)
  at io.kotest.core.internal.ExecutionsKt$executeWithBehaviours$2$1.invokeSuspend(executions.kt:13)
  at io.kotest.core.internal.ExecutionsKt$executeWithBehaviours$2$1.invoke(executions.kt)
  at io.kotest.core.internal.ExecutionsKt.wrapTestWithGlobalAssert(executions.kt:39)
  at io.kotest.core.internal.ExecutionsKt$executeWithBehaviours$2.invokeSuspend(executions.kt:12)
  at io.kotest.core.internal.ExecutionsKt$executeWithBehaviours$2.invoke(executions.kt)
  at io.kotest.core.internal.ExecutionsKt$wrapTestWithAssertionModeCheck$2.invokeSuspend(executions.kt:25)
  at io.kotest.core.internal.ExecutionsKt$wrapTestWithAssertionModeCheck$2.invoke(executions.kt)
  at io.kotest.core.internal.AssertionsCheckKt.executeWithAssertionsCheck(assertionsCheck.kt:25)
  at io.kotest.core.internal.ExecutionsKt.wrapTestWithAssertionModeCheck(executions.kt:24)
  at io.kotest.core.internal.ExecutionsKt.executeWithBehaviours(executions.kt:11)
  at io.kotest.core.internal.TestCaseExecutor$executeInScope$2.invokeSuspend(TestCaseExecutor.kt:266)
  at io.kotest.core.internal.TestCaseExecutor$executeInScope$2.invoke(TestCaseExecutor.kt)
  at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn(Undispatched.kt:91)
  at kotlinx.coroutines.CoroutineScopeKt.coroutineScope(CoroutineScope.kt:194)
  at io.kotest.core.internal.TestCaseExecutor.executeInScope(TestCaseExecutor.kt:260)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1$1$3$1$1.invokeSuspend(TestCaseExecutor.kt:232)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1$1$3$1$1.invoke(TestCaseExecutor.kt)
  at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturnIgnoreTimeout(Undispatched.kt:102)
  at kotlinx.coroutines.TimeoutKt.setupTimeout(Timeout.kt:148)
  at kotlinx.coroutines.TimeoutKt.withTimeout(Timeout.kt:44)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1$1$3$1.invokeSuspend(TestCaseExecutor.kt:231)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1$1$3$1.invoke(TestCaseExecutor.kt)
  at io.kotest.engine.ExecutorExecutionContext$executeWithTimeoutInterruption$3.invokeSuspend(ExecutorExecutionContext.kt:77)
  at io.kotest.engine.ExecutorExecutionContext$executeWithTimeoutInterruption$3.invoke(ExecutorExecutionContext.kt)
  at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn(Undispatched.kt:91)
  at kotlinx.coroutines.BuildersKt__Builders_commonKt.withContext(Builders.common.kt:165)
  at kotlinx.coroutines.BuildersKt.withContext(Unknown Source)
  at io.kotest.engine.ExecutorExecutionContext.executeWithTimeoutInterruption(ExecutorExecutionContext.kt:75)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1$1$3.invokeSuspend(TestCaseExecutor.kt:230)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1$1$3.invoke(TestCaseExecutor.kt)
  at io.kotest.mpp.ReplayKt.replay(replay.kt:18)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1$1.invokeSuspend(TestCaseExecutor.kt:225)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1$1.invoke(TestCaseExecutor.kt)
  at io.kotest.engine.ExecutorExecutionContext$executeWithTimeoutInterruption$3.invokeSuspend(ExecutorExecutionContext.kt:77)
  at io.kotest.engine.ExecutorExecutionContext$executeWithTimeoutInterruption$3.invoke(ExecutorExecutionContext.kt)
  at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn(Undispatched.kt:91)
  at kotlinx.coroutines.BuildersKt__Builders_commonKt.withContext(Builders.common.kt:165)
  at kotlinx.coroutines.BuildersKt.withContext(Unknown Source)
  at io.kotest.engine.ExecutorExecutionContext.executeWithTimeoutInterruption(ExecutorExecutionContext.kt:75)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1.invokeSuspend(TestCaseExecutor.kt:218)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2$1.invoke(TestCaseExecutor.kt)
  at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturnIgnoreTimeout(Undispatched.kt:102)
  at kotlinx.coroutines.TimeoutKt.setupTimeout(Timeout.kt:148)
  at kotlinx.coroutines.TimeoutKt.withTimeout(Timeout.kt:44)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2.invokeSuspend(TestCaseExecutor.kt:217)
  at io.kotest.core.internal.TestCaseExecutor$executeAndWait$2.invoke(TestCaseExecutor.kt)
  at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn(Undispatched.kt:91)
  at kotlinx.coroutines.SupervisorKt.supervisorScope(Supervisor.kt:57)
  at io.kotest.core.internal.TestCaseExecutor.executeAndWait(TestCaseExecutor.kt:213)
  at io.kotest.core.internal.TestCaseExecutor.invokeTestCase(TestCaseExecutor.kt:182)
  at io.kotest.core.internal.TestCaseExecutor.executeActiveTest(TestCaseExecutor.kt:151)
  at io.kotest.core.internal.TestCaseExecutor$intercept$innerExecute$1$1.invokeSuspend(TestCaseExecutor.kt:89)
  at io.kotest.core.internal.TestCaseExecutor$intercept$innerExecute$1$1.invoke(TestCaseExecutor.kt)
  at io.kotest.core.internal.TestCaseExecutor.executeIfActive(TestCaseExecutor.kt:115)
  at io.kotest.core.internal.TestCaseExecutor$intercept$innerExecute$1.invokeSuspend(TestCaseExecutor.kt:89)
  at io.kotest.core.internal.TestCaseExecutor$intercept$innerExecute$1.invoke(TestCaseExecutor.kt)
  at io.kotest.core.internal.TestCaseExecutor.intercept(TestCaseExecutor.kt:103)
  at io.kotest.core.internal.TestCaseExecutor.execute(TestCaseExecutor.kt:69)
  at io.kotest.engine.spec.runners.SingleInstanceSpecRunner.runTest(SingleInstanceSpecRunner.kt:106)
  at io.kotest.engine.spec.runners.SingleInstanceSpecRunner$execute$2$invokeSuspend$$inlined$invoke$lambda$2.invokeSuspend(SingleInstanceSpecRunner.kt:56)
  at io.kotest.engine.spec.runners.SingleInstanceSpecRunner$execute$2$invokeSuspend$$inlined$invoke$lambda$2.invoke(SingleInstanceSpecRunner.kt)
  at io.kotest.engine.launchers.SequentialTestLauncher$launch$$inlined$forEach$lambda$1$1.invokeSuspend(SequentialTestLauncher.kt:22)
  at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
  at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
  at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
  at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
  at java.base/java.lang.Thread.run(Thread.java:832)

It failed when serializing Referenced type. But the stand alone test for Referenced works fine:

  "Referenced.Other serialize" {
    val schema = Referenced.Other(
      Schema()
    ) as Referenced<Schema>
    val str = json.encodeToString(schema)
  }

[nomisRev]

The whole serialization should get hidden behind the API and KotlinX Serialization should not be exposed.
Since the produced json cannot be decoded back to OpenAPI due to how the OpenAPI Spec works.

[lenguyenthanh]

Because of this failure the current example doesn't work. Do you have any idea how to fix this @nomisRev? I think it is related to a kotlinx serialization bug, but I couldn't find it now.

And I totally agree with you point about hiding this from public api.

[nomisRev]

It seems to be picking up Seriliazer for Unit for the Encoder for Schema... Strange.

What is the use-case for converting MediaType to JSON?

Would we be able to close this issue if we make KotlinX Serialization internal and only expose OpenAPI#toJson

[lenguyenthanh]

Actually OpenAPI#toJson leads to this issue anyway. That's why I discovered this bug. You can see the error by running the current example.

[nomisRev]

Do you have a snippet that reproduces this with OpenAPI#toJson?

I tried to take a quick look to try and figure out what's going wrong. Seems that something is going wrong when resolving the generic serializer in KotlinX Serialization.

So not sure what is wrong yet, perhaps we have to explicitly register some of the polymorphic serializers.

[nomisRev]

@lenguyenthanh if you have any meaningful tests (even failing ones) or code, a PR is always welcome for it :)

[lenguyenthanh]

@nomisRev yeah, it seems there is a bug in Kotlinx Serialization like you said earlier: It seems to be picking up Seriliazer for Unit for the Encoder for Schema... Strange.

I have some failed tests for it, will create a PR soon.