How can I access header in a service? - kotlin

Question

I'm trying to handle JWT-authentication in gRPC on my backend. I can extract the JWT in an interceptor but how do I access it in my service? I think it should be done with a CoroutineContextServerInterceptor but this doesn't work:
val jwtKey: Context.Key<String> = Context.key("jwtKey")
fun main() {
ServerBuilder.forPort(8980).intercept(UserInjector).addService(MyService).build().start().awaitTermination()
}
object UserInjector : CoroutineContextServerInterceptor() {
override fun coroutineContext(call: ServerCall<*, *>, headers: Metadata): CoroutineContext {
val jwtString = headers.get(Metadata.Key.of("jwt", Metadata.ASCII_STRING_MARSHALLER))
println("coroutineContext: $jwtString")
return GrpcContextElement(Context.current().withValue(jwtKey, jwtString))
}
}
object MyService : MyServiceGrpcKt.MyServiceCoroutineImplBase() {
override suspend fun testingJWT(request: Test.MyRequest): Test.MyResponse {
println("testingJWT: ${jwtKey.get()}")
return Test.MyResponse.getDefaultInstance()
}
}
Output:
coroutineContext: something
testingJWT: null

Answer 1

I think you'll need to propagate that in its own coroutine context element.
class JwtElement(val jwtString: String) : CoroutineContext.Element {
companion object Key : CoroutineContext.Key<JwtElement>
override val key: CoroutineContext.Key<JwtElement>
get() = Key
}
object UserInjector : CoroutineContextServerInterceptor() {
override fun coroutineContext(call: ServerCall<*, *>, headers: Metadata): CoroutineContext {
val jwtString = headers.get(Metadata.Key.of("jwt", Metadata.ASCII_STRING_MARSHALLER))
println("coroutineContext: $jwtString")
return JwtElement(jwtString)
}
}
object MyService : MyServiceGrpcKt.MyServiceCoroutineImplBase() {
override suspend fun testingJWT(request: Test.MyRequest): Test.MyResponse {
println("testingJWT: ${coroutineContext[JwtElement]}")
return Test.MyResponse.getDefaultInstance()
}
}