Add State extension

Fixes https://github.com/sbt/sbt/issues/3112

This unpacks Extracted as State's extension methods.

In addition this provides a way of responding via LSP.

build.sbt

@@ -675,6 +675,7 @@ lazy val protocolProj = (project in file("protocol"))
       exclude[DirectMissingMethodProblem]("sbt.protocol.SettingQuerySuccess.copy$default$*"),
       // ignore missing methods in sbt.internal
       exclude[DirectMissingMethodProblem]("sbt.internal.*"),
+      exclude[MissingTypesProblem]("sbt.internal.protocol.JsonRpcResponseError"),
     )
   )
 

main/src/main/scala/sbt/BuildSyntax.scala

@@ -22,5 +22,8 @@ private[sbt] trait BuildSyntax {
   def dependsOn(deps: ClasspathDep[ProjectReference]*): DslEntry = DslEntry.DslDependsOn(deps)
   // avoid conflict with `sbt.Keys.aggregate`
   def aggregateProjects(refs: ProjectReference*): DslEntry = DslEntry.DslAggregate(refs)
+
+  implicit def sbtStateToUpperStateOps(s: State): UpperStateOps =
+    new UpperStateOps.UpperStateOpsImpl(s)
 }
 private[sbt] object BuildSyntax extends BuildSyntax

main/src/main/scala/sbt/MainLoop.scala

@@ -13,6 +13,7 @@ import java.util.Properties
 import jline.TerminalFactory
 import sbt.internal.{ Aggregation, ShutdownHooks }
 import sbt.internal.langserver.ErrorCodes
+import sbt.internal.protocol.JsonRpcResponseError
 import sbt.internal.util.complete.Parser
 import sbt.internal.util.{ ErrorHandling, GlobalLogBacking }
 import sbt.io.{ IO, Using }
@@ -230,6 +231,9 @@ object MainLoop {
           }
       }
     } catch {
+      case err: JsonRpcResponseError =>
+        StandardMain.exchange.respondError(err, exec.execId, channelName.map(CommandSource(_)))
+        throw err
       case err: Throwable =>
         val errorEvent = ExecStatusEvent(
           "Error",
@@ -237,9 +241,10 @@ object MainLoop {
           exec.execId,
           Vector(),
           ExitCode(ErrorCodes.UnknownError),
+          Option(err.getMessage),
         )
         import sbt.protocol.codec.JsonProtocol._
-        StandardMain.exchange publishEvent errorEvent
+        StandardMain.exchange.publishEvent(errorEvent)
         throw err
     }
   }

main/src/main/scala/sbt/UpperStateOps.scala

@@ -0,0 +1,142 @@
+/*
+ * sbt
+ * Copyright 2011 - 2018, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+
+package sbt
+
+import sjsonnew.JsonFormat
+import Def.Setting
+import sbt.internal.{ BuildStructure, LoadedBuildUnit, SessionSettings }
+
+/**
+ * Extends State with setting-level knowledge.
+ */
+trait UpperStateOps extends Any {
+
+  /**
+   * ProjectRef to the current project of the state session that can be change using
+   * `project` commmand.
+   */
+  def currentRef: ProjectRef
+
+  /**
+   * Current project of the state session that can be change using `project` commmand.
+   */
+  def currentProject: ResolvedProject
+
+  private[sbt] def structure: BuildStructure
+
+  private[sbt] def session: SessionSettings
+
+  private[sbt] def currentUnit: LoadedBuildUnit
+
+  /**
+   * Gets the value assigned to `key` in the computed settings map.
+   * If the project axis is not explicitly specified, it is resolved to be the current project according to the extracted `session`.
+   * Other axes are resolved to be `Zero` if they are not specified.
+   */
+  def setting[A](key: SettingKey[A]): A
+
+  /**
+   * Gets the value assigned to `key` in the computed settings map.
+   * If the project axis is not explicitly specified, it is resolved to be the current project according to the extracted `session`.
+   * Other axes are resolved to be `Zero` if they are not specified.
+   */
+  def taskValue[A](key: TaskKey[A]): Task[A]
+
+  /**
+   * Runs the task specified by `key` and returns the transformed State and the resulting value of the task.
+   * If the project axis is not defined for the key, it is resolved to be the current project.
+   * Other axes are resolved to `Zero` if unspecified.
+   *
+   * This method requests execution of only the given task and does not aggregate execution.
+   * See `runAggregated` for that.
+   *
+   * To avoid race conditions, this should NOT be called from a task.
+   */
+  def unsafeRunTask[A](key: TaskKey[A]): (State, A)
+
+  /**
+   * Runs the input task specified by `key`, using the `input` as the input to it, and returns the transformed State
+   * and the resulting value of the input task.
+   *
+   * If the project axis is not defined for the key, it is resolved to be the current project.
+   * Other axes are resolved to `Zero` if unspecified.
+   *
+   * This method requests execution of only the given task and does not aggregate execution.
+   * To avoid race conditions, this should NOT be called from a task.
+   */
+  def unsafeRunInputTask[A](key: InputKey[A], input: String): (State, A)
+
+  /**
+   * Runs the tasks selected by aggregating `key` and returns the transformed State.
+   * If the project axis is not defined for the key, it is resolved to be the current project.
+   * The project axis is what determines where aggregation starts, so ensure this is set to what you want.
+   * Other axes are resolved to `Zero` if unspecified.
+   *
+   * To avoid race conditions, this should NOT be called from a task.
+   */
+  def unsafeRunAggregated[A](key: TaskKey[A]): State
+
+  /** Appends the given settings to all the build state settings, including session settings. */
+  def appendWithSession(settings: Seq[Setting[_]]): State
+
+  /**
+   * Appends the given settings to the original build state settings, discarding any settings
+   * appended to the session in the process.
+   */
+  def appendWithoutSession(settings: Seq[Setting[_]], state: State): State
+
+  def respondEvent[A: JsonFormat](event: A): Unit
+  def respondError(code: Long, message: String): Unit
+  def notifyEvent[A: JsonFormat](method: String, params: A): Unit
+}
+
+object UpperStateOps {
+  lazy val exchange = StandardMain.exchange
+
+  implicit class UpperStateOpsImpl(val s: State) extends AnyVal with UpperStateOps {
+    def extract: Extracted = Project.extract(s)
+
+    def currentRef = extract.currentRef
+
+    def currentProject: ResolvedProject = extract.currentProject
+
+    private[sbt] def structure: BuildStructure = Project.structure(s)
+
+    private[sbt] def currentUnit: LoadedBuildUnit = extract.currentUnit
+
+    private[sbt] def session: SessionSettings = Project.session(s)
+
+    def setting[A](key: SettingKey[A]): A = extract.get(key)
+
+    def taskValue[A](key: TaskKey[A]): Task[A] = extract.get(key)
+
+    def unsafeRunTask[A](key: TaskKey[A]): (State, A) = extract.runTask(key, s)
+
+    def unsafeRunInputTask[A](key: InputKey[A], input: String): (State, A) =
+      extract.runInputTask(key, input, s)
+
+    def unsafeRunAggregated[A](key: TaskKey[A]): State =
+      extract.runAggregated(key, s)
+
+    def appendWithSession(settings: Seq[Setting[_]]): State =
+      extract.appendWithSession(settings, s)
+
+    def appendWithoutSession(settings: Seq[Setting[_]], state: State): State =
+      extract.appendWithoutSession(settings, s)
+
+    def respondEvent[A: JsonFormat](event: A): Unit = {
+      exchange.respondEvent(event, s.currentCommand.flatMap(_.execId), s.source)
+    }
+    def respondError(code: Long, message: String): Unit = {
+      exchange.respondError(code, message, s.currentCommand.flatMap(_.execId), s.source)
+    }
+    def notifyEvent[A: JsonFormat](method: String, params: A): Unit = {
+      exchange.notifyEvent(method, params)
+    }
+  }
+}

main/src/main/scala/sbt/internal/CommandExchange.scala

@@ -15,6 +15,7 @@ import java.util.concurrent.atomic._
 
 import sbt.BasicKeys._
 import sbt.nio.Watch.NullLogger
+import sbt.internal.protocol.JsonRpcResponseError
 import sbt.internal.langserver.{ LogMessageParams, MessageType }
 import sbt.internal.server._
 import sbt.internal.util.codec.JValueFormats
@@ -52,6 +53,17 @@ private[sbt] final class CommandExchange {
   private lazy val jsonFormat = new sjsonnew.BasicJsonProtocol with JValueFormats {}
 
   def channels: List[CommandChannel] = channelBuffer.toList
+  private[this] def removeChannels(toDel: List[CommandChannel]): Unit = {
+    toDel match {
+      case Nil => // do nothing
+      case xs =>
+        channelBufferLock.synchronized {
+          channelBuffer --= xs
+          ()
+        }
+    }
+  }
+
   def subscribe(c: CommandChannel): Unit = channelBufferLock.synchronized {
     channelBuffer.append(c)
     c.register(commandChannelQueue)
@@ -181,6 +193,69 @@ private[sbt] final class CommandExchange {
     server = None
   }
 
+  // This is an interface to directly respond events.
+  private[sbt] def respondError(
+      code: Long,
+      message: String,
+      execId: Option[String],
+      source: Option[CommandSource]
+  ): Unit = {
+    val toDel: ListBuffer[CommandChannel] = ListBuffer.empty
+    channels.foreach {
+      case _: ConsoleChannel =>
+      case c: NetworkChannel =>
+        try {
+          // broadcast to all network channels
+          c.respondError(code, message, execId, source)
+        } catch {
+          case _: IOException =>
+            toDel += c
+        }
+    }
+    removeChannels(toDel.toList)
+  }
+
+  private[sbt] def respondError(
+      err: JsonRpcResponseError,
+      execId: Option[String],
+      source: Option[CommandSource]
+  ): Unit = {
+    val toDel: ListBuffer[CommandChannel] = ListBuffer.empty
+    channels.foreach {
+      case _: ConsoleChannel =>
+      case c: NetworkChannel =>
+        try {
+          // broadcast to all network channels
+          c.respondError(err, execId, source)
+        } catch {
+          case _: IOException =>
+            toDel += c
+        }
+    }
+    removeChannels(toDel.toList)
+  }
+
+  // This is an interface to directly respond events.
+  private[sbt] def respondEvent[A: JsonFormat](
+      event: A,
+      execId: Option[String],
+      source: Option[CommandSource]
+  ): Unit = {
+    val toDel: ListBuffer[CommandChannel] = ListBuffer.empty
+    channels.foreach {
+      case _: ConsoleChannel =>
+      case c: NetworkChannel =>
+        try {
+          // broadcast to all network channels
+          c.respondEvent(event, execId, source)
+        } catch {
+          case _: IOException =>
+            toDel += c
+        }
+    }
+    removeChannels(toDel.toList)
+  }
+
   // This is an interface to directly notify events.
   private[sbt] def notifyEvent[A: JsonFormat](method: String, params: A): Unit = {
     val toDel: ListBuffer[CommandChannel] = ListBuffer.empty
@@ -195,14 +270,7 @@ private[sbt] final class CommandExchange {
             toDel += c
         }
     }
-    toDel.toList match {
+    removeChannels(toDel.toList)
-      case Nil => // do nothing
-      case xs =>
-        channelBufferLock.synchronized {
-          channelBuffer --= xs
-          ()
-        }
-    }
   }
 
   private def tryTo(x: => Unit, c: CommandChannel, toDel: ListBuffer[CommandChannel]): Unit =
@@ -248,14 +316,7 @@ private[sbt] final class CommandExchange {
             tryTo(c.publishEvent(event), c, toDel)
         }
     }
-    toDel.toList match {
+    removeChannels(toDel.toList)
-      case Nil => // do nothing
-      case xs =>
-        channelBufferLock.synchronized {
-          channelBuffer --= xs
-          ()
-        }
-    }
   }
 
   private[sbt] def toLogMessageParams(event: StringEvent): LogMessageParams = {
@@ -290,14 +351,7 @@ private[sbt] final class CommandExchange {
             toDel += c
         }
     }
-    toDel.toList match {
+    removeChannels(toDel.toList)
-      case Nil => // do nothing
-      case xs =>
-        channelBufferLock.synchronized {
-          channelBuffer --= xs
-          ()
-        }
-    }
   }
 
   // fanout publishEvent
@@ -328,13 +382,6 @@ private[sbt] final class CommandExchange {
         }
     }
 
-    toDel.toList match {
+    removeChannels(toDel.toList)
-      case Nil => // do nothing
-      case xs =>
-        channelBufferLock.synchronized {
-          channelBuffer --= xs
-          ()
-        }
-    }
   }
 }

main/src/main/scala/sbt/internal/server/LanguageServerProtocol.scala

@@ -181,6 +181,15 @@ private[sbt] trait LanguageServerProtocol extends CommandChannel { self =>
   ): Unit =
     jsonRpcRespondErrorImpl(execId, code, message, Option(Converter.toJson[A](data).get))
 
+  private[sbt] def jsonRpcRespondError(
+      execId: Option[String],
+      err: JsonRpcResponseError
+  ): Unit = {
+    val m = JsonRpcResponseMessage("2.0", execId, None, Option(err))
+    val bytes = Serialization.serializeResponseMessage(m)
+    publishBytes(bytes)
+  }
+
   private[this] def jsonRpcRespondErrorImpl(
       execId: Option[String],
       code: Long,

main/src/main/scala/sbt/internal/server/NetworkChannel.scala

@@ -19,7 +19,11 @@ import sbt.internal.langserver.{ ErrorCodes, CancelRequestParams }
 import sbt.internal.util.{ ObjectEvent, StringEvent }
 import sbt.internal.util.complete.Parser
 import sbt.internal.util.codec.JValueFormats
-import sbt.internal.protocol.{ JsonRpcRequestMessage, JsonRpcNotificationMessage }
+import sbt.internal.protocol.{
+  JsonRpcResponseError,
+  JsonRpcRequestMessage,
+  JsonRpcNotificationMessage
+}
 import sbt.util.Logger
 import scala.util.Try
 import scala.util.control.NonFatal
@@ -241,6 +245,25 @@ final class NetworkChannel(
     }
   }
 
+  private[sbt] def respondError(
+      err: JsonRpcResponseError,
+      execId: Option[String],
+      source: Option[CommandSource]
+  ): Unit = jsonRpcRespondError(execId, err)
+
+  private[sbt] def respondError(
+      code: Long,
+      message: String,
+      execId: Option[String],
+      source: Option[CommandSource]
+  ): Unit = jsonRpcRespondError(execId, code, message)
+
+  private[sbt] def respondEvent[A: JsonFormat](
+      event: A,
+      execId: Option[String],
+      source: Option[CommandSource]
+  ): Unit = jsonRpcRespond(event, execId)
+
   private[sbt] def notifyEvent[A: JsonFormat](method: String, params: A): Unit = {
     if (isLanguageServerProtocol) {
       jsonRpcNotify(method, params)
@@ -255,9 +278,10 @@ final class NetworkChannel(
         case entry: StringEvent => logMessage(entry.level, entry.message)
         case entry: ExecStatusEvent =>
           entry.exitCode match {
-            case None           => jsonRpcRespond(event, entry.execId)
+            case None    => jsonRpcRespond(event, entry.execId)
-            case Some(0)        => jsonRpcRespond(event, entry.execId)
+            case Some(0) => jsonRpcRespond(event, entry.execId)
-            case Some(exitCode) => jsonRpcRespondError(entry.execId, exitCode, "")
+            case Some(exitCode) =>
+              jsonRpcRespondError(entry.execId, exitCode, entry.message.getOrElse(""))
           }
         case _ => jsonRpcRespond(event, execId)
       }

protocol/src/main/contraband-scala/sbt/internal/protocol/JsonRpcResponseError.scala

@@ -13,7 +13,7 @@ package sbt.internal.protocol
 final class JsonRpcResponseError private (
   val code: Long,
   val message: String,
-  val data: Option[sjsonnew.shaded.scalajson.ast.unsafe.JValue]) extends Serializable {
+  val data: Option[sjsonnew.shaded.scalajson.ast.unsafe.JValue]) extends RuntimeException(message) with Serializable {
   
   
   
@@ -44,7 +44,7 @@ final class JsonRpcResponseError private (
   }
 }
 object JsonRpcResponseError {
-  
+  def apply(code: Long, message: String): JsonRpcResponseError = new JsonRpcResponseError(code, message, None)
   def apply(code: Long, message: String, data: Option[sjsonnew.shaded.scalajson.ast.unsafe.JValue]): JsonRpcResponseError = new JsonRpcResponseError(code, message, data)
   def apply(code: Long, message: String, data: sjsonnew.shaded.scalajson.ast.unsafe.JValue): JsonRpcResponseError = new JsonRpcResponseError(code, message, Option(data))
 }

protocol/src/main/contraband-scala/sbt/protocol/ExecStatusEvent.scala

@@ -10,22 +10,24 @@ final class ExecStatusEvent private (
   val channelName: Option[String],
   val execId: Option[String],
   val commandQueue: Vector[String],
-  val exitCode: Option[Long]) extends sbt.protocol.EventMessage() with Serializable {
+  val exitCode: Option[Long],
+  val message: Option[String]) extends sbt.protocol.EventMessage() with Serializable {
   
-  private def this(status: String, channelName: Option[String], execId: Option[String], commandQueue: Vector[String]) = this(status, channelName, execId, commandQueue, None)
+  private def this(status: String, channelName: Option[String], execId: Option[String], commandQueue: Vector[String]) = this(status, channelName, execId, commandQueue, None, None)
+  private def this(status: String, channelName: Option[String], execId: Option[String], commandQueue: Vector[String], exitCode: Option[Long]) = this(status, channelName, execId, commandQueue, exitCode, None)
   
   override def equals(o: Any): Boolean = o match {
-    case x: ExecStatusEvent => (this.status == x.status) && (this.channelName == x.channelName) && (this.execId == x.execId) && (this.commandQueue == x.commandQueue) && (this.exitCode == x.exitCode)
+    case x: ExecStatusEvent => (this.status == x.status) && (this.channelName == x.channelName) && (this.execId == x.execId) && (this.commandQueue == x.commandQueue) && (this.exitCode == x.exitCode) && (this.message == x.message)
     case _ => false
   }
   override def hashCode: Int = {
-    37 * (37 * (37 * (37 * (37 * (37 * (17 + "sbt.protocol.ExecStatusEvent".##) + status.##) + channelName.##) + execId.##) + commandQueue.##) + exitCode.##)
+    37 * (37 * (37 * (37 * (37 * (37 * (37 * (17 + "sbt.protocol.ExecStatusEvent".##) + status.##) + channelName.##) + execId.##) + commandQueue.##) + exitCode.##) + message.##)
   }
   override def toString: String = {
-    "ExecStatusEvent(" + status + ", " + channelName + ", " + execId + ", " + commandQueue + ", " + exitCode + ")"
+    "ExecStatusEvent(" + status + ", " + channelName + ", " + execId + ", " + commandQueue + ", " + exitCode + ", " + message + ")"
   }
-  private[this] def copy(status: String = status, channelName: Option[String] = channelName, execId: Option[String] = execId, commandQueue: Vector[String] = commandQueue, exitCode: Option[Long] = exitCode): ExecStatusEvent = {
+  private[this] def copy(status: String = status, channelName: Option[String] = channelName, execId: Option[String] = execId, commandQueue: Vector[String] = commandQueue, exitCode: Option[Long] = exitCode, message: Option[String] = message): ExecStatusEvent = {
-    new ExecStatusEvent(status, channelName, execId, commandQueue, exitCode)
+    new ExecStatusEvent(status, channelName, execId, commandQueue, exitCode, message)
   }
   def withStatus(status: String): ExecStatusEvent = {
     copy(status = status)
@@ -51,6 +53,12 @@ final class ExecStatusEvent private (
   def withExitCode(exitCode: Long): ExecStatusEvent = {
     copy(exitCode = Option(exitCode))
   }
+  def withMessage(message: Option[String]): ExecStatusEvent = {
+    copy(message = message)
+  }
+  def withMessage(message: String): ExecStatusEvent = {
+    copy(message = Option(message))
+  }
 }
 object ExecStatusEvent {
   
@@ -58,4 +66,6 @@ object ExecStatusEvent {
   def apply(status: String, channelName: String, execId: String, commandQueue: Vector[String]): ExecStatusEvent = new ExecStatusEvent(status, Option(channelName), Option(execId), commandQueue)
   def apply(status: String, channelName: Option[String], execId: Option[String], commandQueue: Vector[String], exitCode: Option[Long]): ExecStatusEvent = new ExecStatusEvent(status, channelName, execId, commandQueue, exitCode)
   def apply(status: String, channelName: String, execId: String, commandQueue: Vector[String], exitCode: Long): ExecStatusEvent = new ExecStatusEvent(status, Option(channelName), Option(execId), commandQueue, Option(exitCode))
+  def apply(status: String, channelName: Option[String], execId: Option[String], commandQueue: Vector[String], exitCode: Option[Long], message: Option[String]): ExecStatusEvent = new ExecStatusEvent(status, channelName, execId, commandQueue, exitCode, message)
+  def apply(status: String, channelName: String, execId: String, commandQueue: Vector[String], exitCode: Long, message: String): ExecStatusEvent = new ExecStatusEvent(status, Option(channelName), Option(execId), commandQueue, Option(exitCode), Option(message))
 }

protocol/src/main/contraband-scala/sbt/protocol/codec/ExecStatusEventFormats.scala

@@ -16,8 +16,9 @@ implicit lazy val ExecStatusEventFormat: JsonFormat[sbt.protocol.ExecStatusEvent
       val execId = unbuilder.readField[Option[String]]("execId")
       val commandQueue = unbuilder.readField[Vector[String]]("commandQueue")
       val exitCode = unbuilder.readField[Option[Long]]("exitCode")
+      val message = unbuilder.readField[Option[String]]("message")
       unbuilder.endObject()
-      sbt.protocol.ExecStatusEvent(status, channelName, execId, commandQueue, exitCode)
+      sbt.protocol.ExecStatusEvent(status, channelName, execId, commandQueue, exitCode, message)
       case None =>
       deserializationError("Expected JsObject but found None")
     }
@@ -29,6 +30,7 @@ implicit lazy val ExecStatusEventFormat: JsonFormat[sbt.protocol.ExecStatusEvent
     builder.addField("execId", obj.execId)
     builder.addField("commandQueue", obj.commandQueue)
     builder.addField("exitCode", obj.exitCode)
+    builder.addField("message", obj.message)
     builder.endObject()
   }
 }

protocol/src/main/contraband/jsonrpc.contra

@@ -56,7 +56,12 @@ type JsonRpcResponseError
   ## information about the error. Can be omitted.
   data: sjsonnew.shaded.scalajson.ast.unsafe.JValue
 
+  #xinterface RuntimeException(message)
+
   #xtostring s"""JsonRpcResponseError($code, $message, ${sbt.protocol.Serialization.compactPrintJsonOpt(data)})"""
+
+  #xcompanion def apply(code: Long, message: String): JsonRpcResponseError = new JsonRpcResponseError(code, message, None)
+
 }
 
 type JsonRpcNotificationMessage implements JsonRpcMessage

protocol/src/main/contraband/server.contra

@@ -47,6 +47,7 @@ type ExecStatusEvent implements EventMessage {
   execId: String
   commandQueue: [String]
   exitCode: Long @since("1.1.2")
+  message: String @since("1.4.0")
 }
 
 interface SettingQueryResponse implements EventMessage {}

server-test/src/server-test/response/build.sbt

@@ -0,0 +1,59 @@
+import sbt.internal.server.{ ServerHandler, ServerIntent }
+
+ThisBuild / scalaVersion := "2.12.10"
+
+Global / serverLog / logLevel := Level.Debug
+// custom handler
+Global / serverHandlers += ServerHandler({ callback =>
+  import callback._
+  import sjsonnew.BasicJsonProtocol._
+  import sbt.internal.protocol.JsonRpcRequestMessage
+  ServerIntent(
+    {
+      case r: JsonRpcRequestMessage if r.method == "foo/export" =>
+        appendExec(Exec("fooExport", Some(r.id), Some(CommandSource(callback.name))))
+        ()
+      case r: JsonRpcRequestMessage if r.method == "foo/fail" =>
+        appendExec(Exec("fooFail", Some(r.id), Some(CommandSource(callback.name))))
+        ()
+      case r: JsonRpcRequestMessage if r.method == "foo/customfail" =>
+        appendExec(Exec("fooCustomFail", Some(r.id), Some(CommandSource(callback.name))))
+        ()
+      case r: JsonRpcRequestMessage if r.method == "foo/notification" =>
+        appendExec(Exec("fooNotification", Some(r.id), Some(CommandSource(callback.name))))
+        ()
+      case r: JsonRpcRequestMessage if r.method == "foo/rootClasspath" =>
+        appendExec(Exec("fooClasspath", Some(r.id), Some(CommandSource(callback.name))))
+        ()
+    },
+    PartialFunction.empty
+  )
+})
+
+lazy val fooClasspath = taskKey[Unit]("")
+lazy val root = (project in file("."))
+  .settings(
+    name := "response",
+    commands += Command.command("fooExport") { s0: State =>
+      val (s1, cp) = s0.unsafeRunTask(Compile / fullClasspath)
+      s0.respondEvent(cp.map(_.data))
+      s1
+    },
+    commands += Command.command("fooFail") { s0: State =>
+      sys.error("fail message")
+    },
+    commands += Command.command("fooCustomFail") { s0: State =>
+      import sbt.internal.protocol.JsonRpcResponseError
+      throw JsonRpcResponseError(500, "some error")
+    },
+    commands += Command.command("fooNotification") { s0: State =>
+      import CacheImplicits._
+      s0.notifyEvent("foo/something", "something")
+      s0
+    },
+    fooClasspath := {
+      val s = state.value
+      val cp = (Compile / fullClasspath).value
+      s.respondEvent(cp.map(_.data))
+    },
+  )

server-test/src/test/scala/testpkg/ResponseTest.scala

@@ -0,0 +1,67 @@
+/*
+ * sbt
+ * Copyright 2011 - 2018, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+
+package testpkg
+
+import scala.concurrent.duration._
+
+// starts svr using server-test/response and perform custom server tests
+object ResponseTest extends AbstractServerTest {
+  override val testDirectory: String = "response"
+
+  test("response from a command") { _ =>
+    svr.sendJsonRpc(
+      """{ "jsonrpc": "2.0", "id": "10", "method": "foo/export", "params": {} }"""
+    )
+    assert(svr.waitForString(10.seconds) { s =>
+      println(s)
+      (s contains """"id":"10"""") &&
+      (s contains "scala-library.jar")
+    })
+  }
+
+  test("response from a task") { _ =>
+    svr.sendJsonRpc(
+      """{ "jsonrpc": "2.0", "id": "11", "method": "foo/rootClasspath", "params": {} }"""
+    )
+    assert(svr.waitForString(10.seconds) { s =>
+      println(s)
+      (s contains """"id":"11"""") &&
+      (s contains "scala-library.jar")
+    })
+  }
+
+  test("a command failure") { _ =>
+    svr.sendJsonRpc(
+      """{ "jsonrpc": "2.0", "id": "12", "method": "foo/fail", "params": {} }"""
+    )
+    assert(svr.waitForString(10.seconds) { s =>
+      println(s)
+      (s contains """"error":{"code":-33000,"message":"fail message"""")
+    })
+  }
+
+  test("a command failure with custom code") { _ =>
+    svr.sendJsonRpc(
+      """{ "jsonrpc": "2.0", "id": "13", "method": "foo/customfail", "params": {} }"""
+    )
+    assert(svr.waitForString(10.seconds) { s =>
+      println(s)
+      (s contains """"error":{"code":500,"message":"some error"""")
+    })
+  }
+
+  test("a command with a notification") { _ =>
+    svr.sendJsonRpc(
+      """{ "jsonrpc": "2.0", "id": "14", "method": "foo/notification", "params": {} }"""
+    )
+    assert(svr.waitForString(10.seconds) { s =>
+      println(s)
+      (s contains """{"jsonrpc":"2.0","method":"foo/something","params":"something"}""")
+    })
+  }
+}