[Android] [R8] IllegalArgumentException thrown when calling a service operation with unused return type

[vjgarciag96]

Steps to reproduce:

1. Define an API service that looks as follows:

@Serializable
data class ResponseBody(
    @SerialName("status") val status: String? = null,
)

interface ExampleApiService {

    @GET("endpoint/v1")
    suspend fun getOperation(): Response<ResponseBody>

    companion object {
        fun create(): ExampleApiService {
            Retrofit.Builder()
                ...
                .addConverterFactory(Json.asConverterFactory("application/json".toMediaType()))
                .build()
                .create(ExampleApiService::class.java)
        }
    }
}

2. Use the API operation but ignore the response body

val apiService  = ExampleApiService.create()

val response = apiService.getOperation()

if (response.isSuccessful) {
    // ignore body
}

3. Run that code in an Android minified app, and the next exception will be thrown:

FATAL EXCEPTION: main
Process: com.vicgarci.repro, PID: 20782
java.lang.IllegalArgumentException: Unable to create converter for class java.lang.Object
  for method ExampleApiService.getOperation
  at retrofit2.Utils.methodError(Utils.java:54)
	at retrofit2.HttpServiceMethod.createResponseConverter(HttpServiceMethod.java:126)
	at retrofit2.HttpServiceMethod.parseAnnotations(HttpServiceMethod.java:85)
	at retrofit2.ServiceMethod.parseAnnotations(ServiceMethod.java:39)
	at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:202)
	at retrofit2.Retrofit$1.invoke(Retrofit.java:160)
	at java.lang.reflect.Proxy.invoke(Proxy.java:1006)
	at $Proxy2.postOperation1(Unknown Source)

...
Caused by: kotlinx.serialization.SerializationException: Serializer for class 'Any' is not found.
Please ensure that class is marked as '@Serializable' and that the serialization compiler plugin is applied.                                                                                                  
  at kotlinx.serialization.internal.PlatformKt.serializerNotRegistered(Platform.kt:31)
	at kotlinx.serialization.SerializersKt__SerializersJvmKt.serializer(SerializersJvm.kt:77)
	at kotlinx.serialization.SerializersKt.serializer(Unknown Source:1)
	at com.jakewharton.retrofit2.converter.kotlinx.serialization.Serializer.serializer(Serializer.kt:21)
	at com.jakewharton.retrofit2.converter.kotlinx.serialization.Factory.responseBodyConverter(Factory.kt:26)
	at retrofit2.Retrofit.nextResponseBodyConverter(Retrofit.java:362)
	at retrofit2.Retrofit.responseBodyConverter(Retrofit.java:345)
	at retrofit2.HttpServiceMethod.createResponseConverter(HttpServiceMethod.java:124)
	... 80 more

Android app project with code to repro the issue: https://github.com/vjgarciag96/retrofit-unused-return-bug-repro

Retrofit version is 2.9.0, but I imagine it can be reproduced in earlier version too.

Cause

I believe the problem is caused by a combination of R8, retrofit, and Json's ConverterFactory in this example:

• R8 detects the ResponseBody type is unused, and so it removes the class and replaces its usages with the Object type.
• When retrofit tries to create the responseConverter for ExampleApiService.getOperation, it delegates to Jsons ConverterFactory, which will provide the appropriate deserializer.
• Jsons ConverterFactory finds that the type it needs to provide a deserializer for is Object (or Any in the Kotlin world) and fails because it does not know how to do that.

Solution

A simple solution would be to remove the unused ResponseBody class, and replace its usage with Unit. This works fine and fixes the issue, although I'm not sure whether you could run into the same problem when building a release build if you only use the result in debug builds.

Issue?

I'm still not sure this is an issue retrofit should try to prevent, but it feels like it is something that apps could run into inadvertently so perhaps it could be a good idea to ship consumer proguard rules in the library that would prevent an scenario like that from happening? I'm keen to hear thoughts and opinions about this, thanks!

[Goooler]

Check #3751 (comment).

[vjgarciag96]

Thanks for the pointer @Goooler! I believe the issue still happens with those rules added. The sample project to repro the problem shared in the issue has the rules that follow:

-keep,allowobfuscation,allowshrinking class kotlin.coroutines.Continuation
-keep,allowobfuscation,allowshrinking interface retrofit2.Call
-keep,allowobfuscation,allowshrinking class retrofit2.Response
# R8 full mode strips generic signatures from return types if not kept.
-if interface * { @retrofit2.http.* public *** *(...); }
-keep,allowoptimization,allowshrinking,allowobfuscation class <3>

https://github.com/vjgarciag96/retrofit-unused-return-bug-repro/blob/main/app/proguard-rules.pro#L26-L31

[JakeWharton]

This is #3588. There is no workaround via rules. You must explicitly keep the concrete types.

I'll move the annotation processor once I'm off of pat leave.

[JakeWharton]

By the way, thanks for the repro! Too few issues have them leading me to guess at the problem, cause, and fix. My responses are terse because I'm on mobile!