[ws] Any effect you like, as long as it's IO

We seem to make an assumption that we’re running in IO, based on the hardcoded key here.

This can be exposed if your routes are defined in an effect that differs from the effect the server is running in, as via the liftR operation provided by natchez-http4s, which turns an HttpRoutes[Kleisli[F, Span[F], *]] into an HttpRoutes[F] via a pair of natural transformations. A simplified version here exhibits the same problem:

package test

import cats._
import cats.data.Kleisli
import cats.effect._
import org.http4s.HttpRoutes
import org.http4s.blaze.server.BlazeServerBuilder
import org.http4s.dsl.Http4sDsl
import org.http4s.implicits._
import org.http4s.server.websocket.WebSocketBuilder
import org.http4s.websocket.WebSocketFrame.Text
import scala.concurrent.ExecutionContext

object Test extends IOApp.Simple {

  type K[A] = Kleisli[IO,Int,A]

  // Our routes run in Kleisli
  val routes: HttpRoutes[K] = {
    object dsl extends Http4sDsl[K]; import dsl._
    HttpRoutes.of[K] {
      case GET -> Root / "ws" =>
        WebSocketBuilder[K].build(_.map(f => Text(s"You said $f")))
    }
  }

  // We can unlift our routes back to IO
  def unlift(routes: HttpRoutes[K]): HttpRoutes[IO] =
    Kleisli { ioReq =>
      val io2k: IO ~> K = Kleisli.liftK
      val k2io: K ~> IO = Kleisli.applyK(42)
      routes.run(ioReq.mapK(io2k)).mapK(k2io).map(_.mapK(k2io))
    }

  // Main app and server are in IO
  def run: IO[Unit] =
    BlazeServerBuilder[IO](ExecutionContext.global)
      .withHttpApp(unlift(routes).orNotFound)
      .bindHttp(8080, "0.0.0.0")
      .resource
      .use(_ => IO.never)

}

Using websocat to connect and send a message yields

$ websocat ws://localhost:8080/ws
hi

websocat: WebSocketError: I/O failure
websocat: error running
$

And over on the console we see

[INFO] org.http4s.blaze.channel.nio1.NIO1SocketServerGroup - Service bound to address /0:0:0:0:0:0:0:0:8080
[INFO] org.http4s.blaze.server.BlazeServerBuilder -
  _   _   _        _ _
 | |_| |_| |_ _ __| | | ___
 | ' \  _|  _| '_ \_  _(_-<
 |_||_\__|\__| .__/ |_|/__/
             |_|
[INFO] org.http4s.blaze.server.BlazeServerBuilder - http4s v0.15.1 on blaze v0.15.1 started at http://[::]:8080/
[INFO] org.http4s.blaze.channel.nio1.NIO1SocketServerGroup - Accepted connection from /0:0:0:0:0:0:0:1:55117
[ERROR] org.http4s.blaze.server.WSFrameAggregator - Exception caught when attempting inbound command
java.lang.ClassCastException: class cats.data.Kleisli cannot be cast to class cats.effect.IO (cats.data.Kleisli and cats.effect.IO are in unnamed module of loader 'app')
	at cats.effect.IO$$anon$2.attempt(IO.scala:1268)
	at cats.syntax.ApplicativeErrorOps$.attempt$extension(applicativeError.scala:52)
	at org.http4s.blazecore.websocket.Http4sWSStage.stageStartup(Http4sWSStage.scala:174)
	at org.http4s.blaze.pipeline.Stage.inboundCommand(Stages.scala:85)
	at org.http4s.blaze.pipeline.Stage.inboundCommand$(Stages.scala:83)
	at org.http4s.blazecore.websocket.Http4sWSStage.inboundCommand(Http4sWSStage.scala:44)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand(Stages.scala:268)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand$(Stages.scala:264)
	at org.http4s.blaze.server.WSFrameAggregator.sendInboundCommand(WSFrameAggregator.scala:32)
	at org.http4s.blaze.pipeline.Head.inboundCommand(Stages.scala:282)
	at org.http4s.blaze.pipeline.Head.inboundCommand$(Stages.scala:280)
	at org.http4s.blaze.server.WSFrameAggregator.inboundCommand(WSFrameAggregator.scala:32)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand(Stages.scala:268)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand$(Stages.scala:264)
	at org.http4s.blaze.server.WebSocketDecoder.sendInboundCommand(WebSocketDecoder.scala:25)
	at org.http4s.blaze.pipeline.Head.inboundCommand(Stages.scala:282)
	at org.http4s.blaze.pipeline.Head.inboundCommand$(Stages.scala:280)
	at org.http4s.blaze.server.WebSocketDecoder.inboundCommand(WebSocketDecoder.scala:25)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand(Stages.scala:268)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand$(Stages.scala:264)
	at org.http4s.blazecore.IdleTimeoutStage.sendInboundCommand(IdleTimeoutStage.scala:29)
	at org.http4s.blaze.pipeline.Tail.replaceTail(Stages.scala:193)
	at org.http4s.blaze.pipeline.Tail.replaceTail$(Stages.scala:172)
	at org.http4s.blaze.server.Http1ServerStage.replaceTail(Http1ServerStage.scala:86)
	at org.http4s.blaze.server.WebSocketSupport.$anonfun$renderResponse$7(WebSocketSupport.scala:109)
	at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:448)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1426)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:183)
[ERROR] org.http4s.blaze.channel.nio1.NIO1HeadStage - Abnormal NIO1HeadStage termination
java.lang.ClassCastException: class cats.data.Kleisli cannot be cast to class cats.effect.IO (cats.data.Kleisli and cats.effect.IO are in unnamed module of loader 'app')
	at cats.effect.IO$$anon$2.attempt(IO.scala:1268)
	at cats.syntax.ApplicativeErrorOps$.attempt$extension(applicativeError.scala:52)
	at org.http4s.blazecore.websocket.Http4sWSStage.stageStartup(Http4sWSStage.scala:174)
	at org.http4s.blaze.pipeline.Stage.inboundCommand(Stages.scala:85)
	at org.http4s.blaze.pipeline.Stage.inboundCommand$(Stages.scala:83)
	at org.http4s.blazecore.websocket.Http4sWSStage.inboundCommand(Http4sWSStage.scala:44)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand(Stages.scala:268)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand$(Stages.scala:264)
	at org.http4s.blaze.server.WSFrameAggregator.sendInboundCommand(WSFrameAggregator.scala:32)
	at org.http4s.blaze.pipeline.Head.inboundCommand(Stages.scala:282)
	at org.http4s.blaze.pipeline.Head.inboundCommand$(Stages.scala:280)
	at org.http4s.blaze.server.WSFrameAggregator.inboundCommand(WSFrameAggregator.scala:32)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand(Stages.scala:268)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand$(Stages.scala:264)
	at org.http4s.blaze.server.WebSocketDecoder.sendInboundCommand(WebSocketDecoder.scala:25)
	at org.http4s.blaze.pipeline.Head.inboundCommand(Stages.scala:282)
	at org.http4s.blaze.pipeline.Head.inboundCommand$(Stages.scala:280)
	at org.http4s.blaze.server.WebSocketDecoder.inboundCommand(WebSocketDecoder.scala:25)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand(Stages.scala:268)
	at org.http4s.blaze.pipeline.Head.sendInboundCommand$(Stages.scala:264)
	at org.http4s.blazecore.IdleTimeoutStage.sendInboundCommand(IdleTimeoutStage.scala:29)
	at org.http4s.blaze.pipeline.Tail.replaceTail(Stages.scala:193)
	at org.http4s.blaze.pipeline.Tail.replaceTail$(Stages.scala:172)
	at org.http4s.blaze.server.Http1ServerStage.replaceTail(Http1ServerStage.scala:86)
	at org.http4s.blaze.server.WebSocketSupport.$anonfun$renderResponse$7(WebSocketSupport.scala:109)
	at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:448)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1426)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:183)

Thanks @ChristopherDavenport for helping me track this down.

[RaasAhsan]

I’ll start a github discussion sometime in the next few days. I’ve started work on the websocket client for ember and I think it’ll be worth looking at this holistically

[tpolecat]