Don't use exception for reloading

I completely forgot about the StateTransform class which allows a task
to modify the state through its return value.

main-command/src/main/scala/sbt/MainControl.scala

@@ -21,8 +21,6 @@ final case class Reboot(
   def arguments = argsList.toArray
 }
 
-private[sbt] case object Reload extends Exception(null, null, false, false)
-
 final case class ApplicationID(
     groupID: String,
     name: String,

main/src/main/scala/sbt/EvaluateTask.scala

@@ -346,7 +346,7 @@ object EvaluateTask {
       ExceptionCategory(ex) match {
         case AlreadyHandled => ()
         case m: MessageOnly => if (msg.isEmpty) log.error(m.message)
-        case f: Full        => if (f.exception != Reload) log.trace(f.exception)
+        case f: Full        => log.trace(f.exception)
       }
     }
 
@@ -354,7 +354,7 @@ object EvaluateTask {
       val msgString = (msg.toList ++ ex.toList.map(ErrorHandling.reducedToString)).mkString("\n\t")
       val log = getStreams(key, streams).log
       val display = contextDisplay(state, ConsoleAppender.formatEnabledInEnv)
-      if (!ex.contains(Reload)) log.error("(" + display.show(key) + ") " + msgString)
+      log.error("(" + display.show(key) + ") " + msgString)
     }
   }
 

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

@@ -144,9 +144,6 @@ object MainLoop {
         case Right(s)                  => s
         case Left(t: xsbti.FullReload) => throw t
         case Left(t: RebootCurrent)    => throw t
-        case Left(Reload) =>
-          val remaining = state.currentCommand.toList ::: state.remainingCommands
-          state.copy(remainingCommands = Exec("reload", None, None) :: remaining)
         case Left(t)                   => state.handleError(t)
       }
     } catch {

main/src/main/scala/sbt/internal/Aggregation.scala

@@ -9,17 +9,14 @@ package sbt
 package internal
 
 import java.text.DateFormat
-import java.util.{ Collections, IdentityHashMap }
 
-import Def.ScopedKey
-import Keys.{ showSuccess, showTiming, timingFormat }
+import sbt.Def.ScopedKey
+import sbt.Keys.{ showSuccess, showTiming, timingFormat }
 import sbt.internal.util.complete.Parser
-import sbt.internal.util.{ AttributeKey, Dag, HList, Settings, Util }
+import sbt.internal.util.complete.Parser.{ failure, seq, success }
+import sbt.internal.util._
+import sbt.std.Transform.DummyTaskMap
 import sbt.util.{ Logger, Show }
-import Parser.{ failure, seq, success }
-import std.Transform.DummyTaskMap
-
-import scala.annotation.tailrec
 
 sealed trait Aggregation
 object Aggregation {
@@ -115,25 +112,7 @@ object Aggregation {
   )(implicit display: Show[ScopedKey[_]]): State = {
     val complete = timedRun[T](s, ts, extra)
     showRun(complete, show)
-    /*
-     * In the first implementation, we tried to use Set[Incomplete] for visited. It had very poor
-     * performance because hashCode can be expensive on Incomplete -- especially when the
-     * Incomplete has many instances in the causes field.
-     */
-    lazy val visited = Collections
-      .newSetFromMap[Incomplete](new IdentityHashMap[Incomplete, java.lang.Boolean])
-    @tailrec def findReload(incomplete: Incomplete, remaining: List[Incomplete]): Boolean = {
-      visited.add(incomplete)
-      incomplete.directCause.contains(Reload) || ((remaining ::: incomplete.causes.toList)
-        .filterNot(visited.contains) match {
-        case Nil       => false
-        case h :: tail => findReload(h, tail.filterNot(visited.contains))
-      })
-    }
     complete.results match {
-      case Inc(i) if findReload(i, i.causes.toList) =>
-        val remaining = s.currentCommand.toList ::: s.remainingCommands
-        complete.state.copy(remainingCommands = Exec("reload", None, None) :: remaining)
       case Inc(i)   => complete.state.handleError(i)
       case Value(_) => complete.state
     }

main/src/main/scala/sbt/internal/nio/CheckBuildSources.scala

@@ -15,12 +15,12 @@ import sbt.nio.Keys._
 import sbt.nio.file.{ ChangedFiles, Glob, RecursiveGlob }
 
 private[sbt] object CheckBuildSources {
-  private[sbt] def needReloadImpl: Def.Initialize[Task[Unit]] = Def.task {
+  private[sbt] def needReloadImpl: Def.Initialize[Task[StateTransform]] = Def.task {
     val logger = streams.value.log
-    val checkMetaBuildParam = state.value.get(hasCheckedMetaBuild)
-    val firstTime = checkMetaBuildParam.fold(true)(_.get == false)
+    val st: State = state.value
+    val firstTime = st.get(hasCheckedMetaBuild).fold(true)(_.compareAndSet(false, true))
     (onChangedBuildSource in Scope.Global).value match {
-      case IgnoreSourceChanges => ()
+      case IgnoreSourceChanges => new StateTransform(st)
       case o =>
         logger.debug("Checking for meta build source updates")
         (changedInputFiles in checkBuildSources).value match {
@@ -37,18 +37,20 @@ private[sbt] object CheckBuildSources {
             val prefix = rawPrefix.linesIterator.filterNot(_.trim.isEmpty).mkString("\n")
             if (o == ReloadOnSourceChanges) {
               logger.info(s"$prefix\nReloading sbt...")
-              throw Reload
+              val remaining =
+                Exec("reload", None, None) :: st.currentCommand.toList ::: st.remainingCommands
+              new StateTransform(st.copy(currentCommand = None, remainingCommands = remaining))
             } else {
               val tail = "Apply these changes by running `reload`.\nAutomatically reload the " +
                 "build when source changes are detected by setting " +
                 "`Global / onChangedBuildSource := ReloadOnSourceChanges`.\nDisable this " +
                 "warning by setting `Global / onChangedBuildSource := IgnoreSourceChanges`."
               logger.warn(s"$prefix\n$tail")
+              new StateTransform(st)
             }
-          case _ => ()
+          case _ => new StateTransform(st)
         }
     }
-    checkMetaBuildParam.foreach(_.set(true))
   }
   private[sbt] def buildSourceFileInputs: Def.Initialize[Seq[Glob]] = Def.setting {
     if (onChangedBuildSource.value != IgnoreSourceChanges) {

main/src/main/scala/sbt/nio/Keys.scala

@@ -50,7 +50,7 @@ object Keys {
     taskKey[FileTreeView.Nio[FileAttributes]]("A view of the local file system tree")
 
   val checkBuildSources =
-    taskKey[Unit]("Check if any meta build sources have changed").withRank(DSetting)
+    taskKey[StateTransform]("Check if any meta build sources have changed").withRank(DSetting)
 
   // watch related settings
   val watchAntiEntropyRetentionPeriod = settingKey[FiniteDuration](