improvement: Send error back to client to show

Previously, it would be very difficult to show a proper error message to the user. Especially in VS Code a generic message was show, which was very confusing for users.

Now, we send the error message in the response so that the client can handle it as they want.

I wonder if we should keep displaying the message, but not sure if it's not better to just change the clients here to show the error.

metals/src/main/scala/scala/meta/internal/metals/DebugSession.scala

@@ -1,3 +1,19 @@
 package scala.meta.internal.metals
 
-final case class DebugSession(name: String, uri: String)
+import javax.annotation.Nullable
+
+final case class DebugSession(
+    @Nullable name: String,
+    @Nullable uri: String,
+    @Nullable error: String,
+)
+
+object DebugSession {
+
+  def success(name: String, uri: String): DebugSession = {
+    DebugSession(name, uri, null)
+  }
+  def failure(message: String): DebugSession = {
+    DebugSession(null, null, message)
+  }
+}

metals/src/main/scala/scala/meta/internal/metals/MetalsLspService.scala

@@ -1908,6 +1908,7 @@ class MetalsLspService(
     debugProvider
       .debugDiscovery(params)
       .flatMap(debugProvider.asSession)
+      .recover(t => DebugSession.failure(t.getMessage()))
       .asJavaObject
 
   def findBuildTargetByDisplayName(target: String): Option[b.BuildTarget] =

metals/src/main/scala/scala/meta/internal/metals/WorkspaceLspService.scala

@@ -23,8 +23,6 @@ import scala.meta.internal.metals.WindowStateDidChangeParams
 import scala.meta.internal.metals.clients.language.ConfiguredLanguageClient
 import scala.meta.internal.metals.clients.language.MetalsLanguageClient
 import scala.meta.internal.metals.config.StatusBarState
-import scala.meta.internal.metals.debug.BuildTargetNotFoundException
-import scala.meta.internal.metals.debug.BuildTargetUndefinedException
 import scala.meta.internal.metals.debug.DebugProvider
 import scala.meta.internal.metals.doctor.DoctorVisibilityDidChangeParams
 import scala.meta.internal.metals.doctor.HeadDoctor
@@ -639,11 +637,6 @@ class WorkspaceLspService(
       folderServices.foreach(_.pause())
     }
 
-  private def displayNotFound(objectName: String, id: String) =
-    languageClient.showMessage(
-      Messages.errorMessageParams(s"$objectName not found: $id")
-    )
-
   private def onFirstSatifying[T, R](mapTo: MetalsLspService => Future[T])(
       satisfies: T => Boolean,
       exec: (MetalsLspService, T) => Future[R],
@@ -854,8 +847,9 @@ class WorkspaceLspService(
           .getResultFromSearches(
             folderServices.map(_.mainClassSearch(params))
           ) {
-            displayNotFound("Main class", params.mainClass)
-            Future.failed(new ju.NoSuchElementException(params.mainClass))
+            Future.successful(
+              DebugSession.failure(s"Main class not found: ${params.mainClass}")
+            )
           }
           .asJavaObject
 
@@ -868,8 +862,11 @@ class WorkspaceLspService(
           (service, someTarget) =>
             service.startTestSuite(someTarget.get, params),
           () => {
-            displayNotFound("Build target", params.target.toString())
-            Future.failed(BuildTargetNotFoundException(params.target.getUri))
+            Future.successful(
+              DebugSession.failure(
+                s"Build target not found: ${params.target.toString()}"
+              )
+            )
           },
         ).asJavaObject
       case ServerCommands.ResolveAndStartTestSuite(params)
@@ -878,8 +875,9 @@ class WorkspaceLspService(
           .getResultFromSearches(
             folderServices.map(_.testClassSearch(params))
           ) {
-            displayNotFound("Test class", params.testClass)
-            Future.failed(new ju.NoSuchElementException(params.testClass))
+            Future.successful(
+              DebugSession.failure(s"Test class: ${params.testClass}")
+            )
           }
           .asJavaObject
       case ServerCommands.StartAttach(params) if params.hostName != null =>
@@ -892,8 +890,9 @@ class WorkspaceLspService(
           (service, someTarget) =>
             service.createDebugSession(someTarget.get.getId()),
           () => {
-            displayNotFound("Build target", params.buildTarget)
-            Future.failed(BuildTargetUndefinedException())
+            Future.successful(
+              DebugSession.failure(s"Build target: ${params.buildTarget}")
+            )
           },
         ).asJavaObject
       case ServerCommands.DiscoverAndRun(params) =>

metals/src/main/scala/scala/meta/internal/metals/debug/DebugProvider.scala

@@ -404,7 +404,7 @@ class DebugProvider(
       )
     } yield {
       statusBar.addMessage("Started debug server!")
-      DebugSession(server.sessionName, server.uri.toString)
+      DebugSession.success(server.sessionName, server.uri.toString)
     }
   }
 

tests/unit/src/main/scala/tests/TestingServer.scala

@@ -666,7 +666,7 @@ final case class TestingServer(
     params.setDataKind(kind)
     params.setData(parameter.toJson)
     executeCommandUnsafe(ServerCommands.StartDebugAdapter.id, Seq(params))
-      .collect { case DebugSession(_, uri) =>
+      .collect { case DebugSession(_, uri, _) =>
         scribe.info(s"Starting debug session for $uri")
         TestDebugger(URI.create(uri), stoppageHandler)
       }
@@ -708,7 +708,7 @@ final case class TestingServer(
   ): Future[TestDebugger] = {
     assertSystemExit(params)
     executeCommandUnsafe(ServerCommands.StartDebugAdapter.id, Seq(params))
-      .collect { case DebugSession(_, uri) =>
+      .collect { case DebugSession(_, uri, _) =>
         TestDebugger(URI.create(uri), stoppageHandler)
       }
   }