Cross-point from the <#C0A974TJ9|ktor> channel (<https://kotlinlang.slack.com/archives/C0A974TJ9/p15...

# coroutines

[rahulrav]

Cross-point from the #ktor channel (https://kotlinlang.slack.com/archives/C0A974TJ9/p1584027549237200)

[Evan R.]

Why are you creating an entire new thread pool here? You should just be able to write:

Untitled

[Evan R.]

If you really need to use a different scope you can use withContext() { }

[Evan R.]

e.g. withContext(<http://Dispatchers.IO|Dispatchers.IO>) { }

[rahulrav]

I tried that too before. When I try:

val oauthScope = "<scope>"
        val accessToken = AppIdentityServiceFactory.getAppIdentityService().getAccessToken(listOf(oauthScope))
        call.respondText {
            accessToken.accessToken
        }

I get this:

SEVERE: Something bad happened
com.google.apphosting.api.ApiProxy$CallNotFoundException: Can't make API call memcache.Get in a thread that is neither the original request thread nor a thread created by ThreadManager
        at com.google.apphosting.api.ApiProxy$CallNotFoundException.foreignThread(ApiProxy.java:800)
        at com.google.apphosting.api.ApiProxy$1.get(ApiProxy.java:179)
        at com.google.apphosting.api.ApiProxy$1.get(ApiProxy.java:176)
        at com.google.appengine.api.utils.FutureWrapper.get(FutureWrapper.java:89)
        at com.google.appengine.api.memcache.MemcacheServiceImpl.quietGet(MemcacheServiceImpl.java:26)
        at com.google.appengine.api.memcache.MemcacheServiceImpl.get(MemcacheServiceImpl.java:49)
        at com.google.appengine.api.appidentity.AppIdentityServiceImpl.getAccessToken(AppIdentityServiceImpl.java:288)
        at androidx.snapshots.RequestHandlersKt$main$3.invokeSuspend(RequestHandlers.kt:30)

[rahulrav]

My guess is that the request dispatch is actually happening on a Thread which was not created by ThreadManager at all

[Evan R.]

It’s possible that because coroutines may switch threads during execution you may hit a thread switch while the Ktor pipeline is running… I’m not really sure though

[rahulrav]

https://github.com/ktorio/ktor/issues/1486
I also found this, but I am not sure what this solution actually entails.

[Evan R.]

I’m not super familiar with app engine but I’ll give it a shot. So what that answer seems to be doing is creating a subclass of ServletApplicationEngine which I assume is the thing accepting app engine requests? The subclass then creates a single-threaded coroutine dispatcher to prevent thread switches so when you attempt to use anything that isn’t thread safe you should withContext() { } to that dispatcher

[rahulrav]

Let me give that a shot