`call.receiveOrNull<T>()` isn't working as e...

# ktor

[nilTheDev]

call.receiveOrNull<T>() isn't working as expected. This is how you can reproduce,

1. Install the ContentNegotiation plugin for automatic deserialization of json content. (I am using kotlinx.serialization)
2. Define a model for the expected data
3. Send request with missing json field
4. Instead of returning null, it's throwing the exception and causing a 500 internal server error
Code,

fun Application.module() {
    install(ContentNegotiation) {
        json(
            Json { ignoreUnknownKeys = true }
        )
    }

    routing {
        post("/") {
            val user = call.receiveOrNull<User>()
                ?: return@post call.respondText(
                    text = "invalid request",
                    status = HttpStatusCode.BadRequest
                )
            call.respondText(text = "valid request", status = HttpStatusCode.Accepted)
        }
    }
}

Model,

@Serializable
data class User(
    @SerialName("username")
    val userName: String,
    val password: String
)

Sent request,

POST / HTTP/1.1
Accept: application/json, */*;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Content-Length: 24
Content-Type: application/json
Host: localhost:8080
User-Agent: HTTPie/2.5.0

{
    "username": "nilanjan"
}

Received response,

HTTP/1.1 500 Internal Server Error
Connection: keep-alive
Content-Length: 0

Exception thrown in the server,

kotlinx.serialization.MissingFieldException: Field 'password' is required for type with serial name 'com.jetbrains.handson.httpapi.User'

As a workaround I had to define my own receiveOrNull extension function.

suspend inline fun <reified T : Any> ApplicationCall.receiveOrNull(): T? {
    return try {
        receive<T>()
    } catch (cause: Exception) {
        null
    }
}

The actual receiveOrNull function in the io.ktor.request package looks like that,

public suspend fun <T : Any> ApplicationCall.receiveOrNull(type: KType): T? {
    return try {
        receive<T>(type)
    } catch (cause: ContentTransformationException) {
        application.log.debug("Conversion failed, null returned", cause)
        null
    }
}

Apparently, it's only catching the ContentTransformationException nothing else.

This issue in YourTrack is similar to my issue,

https://youtrack.jetbrains.com/issue/KTOR-545

So, my questions are,
1. Is this how the receiveOrNull function supposed to work? Or it is a bug? Do I have to report it?
2. If it's not a bug then why it is designed this way? Isn't it a good design to catch all kind of errors related to deserialization in the receiveOrNull function rather than causing an internal server error?

[andylamax]

I have no answer, but just wanted to say. Nice catch and proper presentation of the problem. This should be an example of asking a question, or bug reporting

[enleur]

expectSuccess = false

https://ktor.io/docs/response-validation.html#custom

[nilTheDev]

@enleur Didn't understand what you wanted to say. The link you provided points to the response validation section of the ktor client. How is it relevant?

[enleur]

response validation enabled by default. everything not 2xx goes with exception. in your case it responded with 500 status, so I guess it was a response validation exception

[nilTheDev]

But I am not even using ktor client. It's about ktor server. It's more about request validation rather than response validation.

[enleur]

I see now. Yep, sorry

[Aleksei Tirman [JB]]

@nilTheDev the bug you found KTOR-545 is exactly the same problem you described. The inline documentation for this method isn't clear because it's meant to return null only if a suitable converter isn't found so the transformation isn't applied. The server responds with the 500 status code because it's the default behavior for any uncaught exceptions and it's overridable by the StatusPages plugin.