Allow suspend calls inside ktor rpc builder

[lsafer-meemer]

Is your feature request related to a problem? Please describe.
When registering rpc services, I can't make suspend calls.

Describe the solution you'd like
The code for registering rpc services is executing per connection thus KrpcRoute implements WebSocketSession. Why not just the block suspendable?

I traced the code and it appears that there is no issues with making it suspend other than changing the signature:

// ...
@KtorDsl
public fun Route.rpc(builder: KrpcRoute.() -> Unit) {
    createRpcServer(builder)
}

private fun Route.createRpcServer(rpcRouteBuilder: KrpcRoute.() -> Unit) {
    // ...

    webSocket {
        val rpcRoute = KrpcRoute(this).apply(rpcRouteBuilder)

        // ...
    }
}

full code here

Describe alternatives you've considered

1. Copy-paste with signature change -> not effective
2. Using call attributes -> too confusing and error prune

Additional context
I need it to be suspend to decode the access token which needs the keyset of the service which might not be in the cache.

[Mr3zee]

Hey! Can use please provide a snippet of how you want to use it?

[lsafer-meemer]

Ok, I just copy-pasted from my codebase:

This is how I am doing it right now:

    routing {
        route("/krpc") {
            val userSubjectPrincipalKey = AttributeKey<UserSubjectPrincipal>("customKrpcUserSubjectPrincipal")
            val userEmployeePrincipalKey = AttributeKey<UserEmployeePrincipal>("customKrpcUserEmployeePrincipal")

            install(createApplicationPlugin("customRpcPlugin") {
                onCall { call ->
                    call.userSubjectPrincipalNoValidation() // <-- this suspends
                        ?.let { call.attributes.put(userSubjectPrincipalKey, it) }
                    call.userEmployeePrincipalNoValidation() // <-- this suspends
                        ?.let { call.attributes.put(userEmployeePrincipalKey, it) }
                }
            })

            rpc {
                val userSubjectPrincipal = call.attributes.getOrNull(userSubjectPrincipalKey)
                val userEmployeePrincipal = call.attributes.getOrNull(userEmployeePrincipalKey)

                registerService<OAuthRPC> {
                    OAuthRPCImpl(local, userSubjectPrincipal)
                }
                // ...
            }
        }
    }

As you can see, this becomes very long and confusing the more

Here is what I want to be able to do:

    routing {
        rpc("krpc") {
            val userSubjectPrincipal = call.userSubjectPrincipalNoValidation() // <-- this suspends
            val userEmployeePrincipal = call.userEmployeePrincipalNoValidation() // <-- this suspends

            registerService<OAuthRPC> {
                OAuthRPCImpl(local, userSubjectPrincipal)
            }
            // ...
        }
    }

[lsafer-meemer]

Or even better (if possible), make the lambda passed to registerService be suspend! This makes the principals evaluated on demand.

[Mr3zee]

Ok, makes sense, I'll do that, but this

Or even better (if possible), make the lambda passed to registerService be suspend! This makes the principals evaluated on demand.

Can't do, at least for now, but I'll consider it a little later