Hey guys, I'm struggling with a weird issue in ktor using protobuf content negotiation. It seems tha...

# ktor

[Alexander Städing]

Hey guys, I'm struggling with a weird issue in ktor using protobuf content negotiation. It seems that the application/protobuf Content-Type header is being set on the client but not received on the server. When I set the Content-Type header to something else (e.g. text) it works. Any ideas? Here is the code:

client.post("<http://localhost:8080/launch>") {
            contentType(ContentType.Application.ProtoBuf)
            setBody(launch)
            println("Sent content type: ${contentType()}")
        }

This prints:

Sent content type: application/protobuf
Response: HttpResponse[<http://localhost:8080/launch>, 415 Unsupported Media Type]

But the server receives no Content-Type header:

[24-Mar-29 11:37:47] [INFO] - Autoreload is disabled because the development mode is off.
[24-Mar-29 11:37:47] [INFO] - Application started in 0.034 seconds.
[24-Mar-29 11:37:47] [INFO] - Responding at <http://0.0.0.0:8080>
[24-Mar-29 11:37:53] [WARN] - Content-Type: null

val logger by inject<Logger>()
        post("/launch") {
            val contentType = call.request.header(HttpHeaders.ContentType)
            logger.warn("Content-Type: $contentType")

If I use, contentType(ContentType.Text.Plain , the server prints:

[24-Mar-29 11:39:24] [WARN] - Content-Type: text/plain; charset=UTF-8

P.S. I am using .proto files which are converted to Java with the protobuf gradle plugin. I wrote my own ContentConverter and register it like this:

val client = HttpClient {
        install(ContentNegotiation) {
            register(ContentType.Application.ProtoBuf, ProtobufContentConverter())
        }
    }

[Aleksei Tirman [JB]]

Can you please check if the Content-Type header is actually sent through the network?

[Alexander Städing]

How should I check that?

[Aleksei Tirman [JB]]

You can use a network traffic analyzing tool, for example, https://www.wireshark.org/

[Alexander Städing]

It doesn't seem to get sent.

[Alexander Städing]

I tried again with plain text Content-Type and it is visible in wireshark

[Alexander Städing]

It seems the header is being prevented from being sent when I use the setBody method with the protobuf object. If I use setBody("foo") the protobuf content type header is sent.

[Alexander Städing]

This is the serialization method. The writeTo method is called:

override suspend fun serializeNullable(contentType: ContentType, charset: Charset, typeInfo: TypeInfo, value: Any?): OutgoingContent? {
        if (value !is GeneratedMessage) return null
        return object : OutgoingContent.WriteChannelContent() {
            override suspend fun writeTo(channel: ByteWriteChannel) = value.writeTo(channel.toOutputStream())
        }
    }

[Aleksei Tirman [JB]]

Can you share the definition of the ProtobufContentConverter ?

[Alexander Städing]

I have a feeling I implemented serializeNullable incorrectly, and that I have to somehow write the ContentType again. Here is the full code:

https://gist.github.com/alexstaeding/27b7bc51d8ed4edcb1170d4016e4e50f

[Alexander Städing]

However, I can't find any documentation that mentions how to deal with the ContentType in this method

[Aleksei Tirman [JB]]

You need to override the contentType property of the OutgoingContent.WriteChannelContent object in the serializeNullable method.

[Alexander Städing]

Ah yes, that makes sense. Thanks for taking the time to help, it works now 👍