Hello all! I'm looking for some guidance on approaches to unit testing Ktor client plugins. We use Hilt for the DI layer and I'm having trouble reasoning about the best way to wrap the Ktor plugin for testing. For instance, take the following OkHttp interceptor:

/** Adds the package name as headers for every request. */
class BuildMetadataMiddleware @Inject constructor(private val packageInfoProvider: PackageInfoProvider) : Interceptor {
  companion object {
    const val ClientNameHeader = "X-Client-Name"
  }

  override fun intercept(chain: Interceptor.Chain): Response = chain.proceed(
    chain.request().newBuilder()
      .addHeader(ClientNameHeader, packageInfoProvider.packageName)
      .build()
  )
}

I can then create a test client and assert on the

intercept

method with a fake

PackageInfoProvider

. With Ktor, it doesn't immediately seem that you want to subclass the

ClientPlugin

interface but rather provide a plugin config class, which to my understanding requires a default value. I have two questions: • How can I implement this if I do not want a default value (always have it be provided by the injected dependency) • How can a plugin fail safely? Would I need to create custom exceptions and handle them or is there an API like

request.cancel(reason)

? This is what the above impl could look like in Ktor:

class BuildMetadataConfig {
  val packageName: String? = null
}

val buildMetadataPlugin = createClientPlugin("BuildMetadata", ::BuildMetadataConfig) {
  val packageName = pluginConfig.packageName

  onRequest { request, _ ->
    request.header("X-Client-Name", packageName)
  }
}

So do you want to test that the client with the installed plugin sends the

X-Client-Name

header on each request?

Yes, but also that it can pull the value of the header dynamically from a dependency on every request.

Do you mean that the header value is configurable?

In this case just the header value, which is provided by a

PackageInfoProvider

interface that gets injected where the client is created

You can test the plugin using the MockEngine (https://ktor.io/docs/http-client-testing.html).

Okay, for the other part of my question, if a client's plugin fails, is it recommended to throw an exception and catch it where the request was made or is there a different recommended approach to cancelling/failing a request?

What kind of failure are you talking about?

Let's say for instance I have a plugin that tries to encode some data in the request body as JSON but the encoding itself fails. Or, it tries to read a header value (like in the example above) from a cache/db but it is not there. I want to short-circuit the request and be able to catch the failure. In OkHttp the way to do that is by throwing an exception - wondering if Ktor has a different approach?

In case of failure, you can throw an exception, and so the execution context of the request will be canceled with the reason.