[BUG] Cannot use SttpClientInterpreter with SttpBackendStub

[chessman]

Tapir version: 1.11.5

Scala version: 3.5.1

Describe the bug

Documentation mentions that a stub server can be used with an endpoint interpreted as a client:

You can define the sttp requests by hand, to see how an arbitrary request will be handled by your server endpoints. Or, you can interpret an endpoint as a client, to test both how the client & server interpreters interact with your endpoints.

I tried to adapt the example but it throws an error:

❯ scala-cli test tapir.scala
MySpec:
- should work *** FAILED ***
  java.lang.RuntimeException: Endpoint GET /api {header Authorization} -> {body as text/plain (UTF-8)}/{body as text/plain (UTF-8)} returned error: , for security inputs: password, inputs: ().
  at sttp.tapir.client.sttp.SttpClientInterpreter.toSecureRequestThrowErrors$$anonfun$1$$anonfun$1$$anonfun$2(SttpClientInterpreter.scala:203)
  at sttp.client3.ResponseAs.map$$anonfun$1(ResponseAs.scala:30)
  at sttp.client3.MappedResponseAs.mapWithMetadata$$anonfun$1(ResponseAs.scala:92)
  at sttp.client3.testing.SttpBackendStub$.tryAdjustResponseBody$$anonfun$2$$anonfun$2$$anonfun$1(SttpBackendStub.scala:236)
  at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:687)
  at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:467)
  at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1395)
  at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
  at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
  at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)

How to reproduce?

//> using scala 3.5.1
//> using dep com.softwaremill.sttp.tapir::tapir-sttp-client:1.11.5
//> using dep com.softwaremill.sttp.tapir::tapir-sttp-stub-server:1.11.5
//> using dep org.scalatest::scalatest:3.2.19

import org.scalatest.flatspec.AsyncFlatSpec
import org.scalatest.matchers.should.Matchers
import sttp.client3.*
import sttp.client3.testing.SttpBackendStub
import sttp.tapir.server.stub.TapirStubInterpreter
import sttp.tapir.client.sttp.SttpClientInterpreter

import sttp.tapir.*
import sttp.tapir.server.ServerEndpoint
import scala.concurrent.Future

class MySpec extends AsyncFlatSpec with Matchers:

  val someEndpoint: Endpoint[String, Unit, String, String, Any] = endpoint.get
    .in("api")
    .securityIn(auth.bearer[String]())
    .out(stringBody)
    .errorOut(stringBody)
    
  val someServerEndpoint: ServerEndpoint[Any, Future] = someEndpoint
    .serverSecurityLogic(token =>
      Future.successful {
        if (token == "password") Right("user123") else Left("unauthorized")
      }
    )
    .serverLogic(user => _ => Future.successful(Right(s"hello $user"))) 

  it should "work" in {
    // given
    val backendStub: SttpBackend[Future, Any] = TapirStubInterpreter(SttpBackendStub.asynchronousFuture)
      .whenServerEndpoint(someServerEndpoint)
      .thenRunLogic()
      .backend()
      
    val client = SttpClientInterpreter().toSecureClientThrowErrors(
      someEndpoint,
      Some(uri"http://localhost/api"),
      backendStub
    )

    val response = client("password")(())
      
    // then
    response.map(_ shouldBe "hello user123")  
  }

[adamw]

The reason for this is that the server is returning a 404 for the request. This in turn is caused by the URI that is being constructed. You provide http://localhost/api as the base URI; to that URI, the client appends the path of the endpoint that should be called, so you end up with http://localhost/api/api. And the server doesn't know how to handle this request.

But definitely the error reporting here should be better, I'll see if there are some places for improvement :)

[adamw]

With #4085, the exception message will be a bit more helpful:

Exception in thread "main" java.lang.RuntimeException: Endpoint GET /api {header Authorization} -> {body as text/plain (UTF-8)}/{body as text/plain (UTF-8)} returned error: , for security inputs: password, inputs: (). Request: GET http://localhost/api/api.