Capture errors thrown inside `bgRun`

This captures non-fatal errors in `bgRun` background thread, so if needed we can re-throw it for `run`.

main/src/main/scala/sbt/BackgroundJobService.scala

@@ -5,6 +5,7 @@ import sbt.util.Logger
 import Def.{ ScopedKey, Classpath }
 import sbt.internal.util.complete._
 import java.io.File
+import scala.util.Try
 
 abstract class BackgroundJobService extends Closeable {
 
@@ -24,6 +25,12 @@ abstract class BackgroundJobService extends Closeable {
   def shutdown(): Unit
   def jobs: Vector[JobHandle]
   def stop(job: JobHandle): Unit
+
+  def waitForTry(job: JobHandle): Try[Unit] = {
+    // This implementation is provided only for backward compatibility.
+    Try(waitFor(job))
+  }
+
   def waitFor(job: JobHandle): Unit
 
   /** Copies classpath to temporary directories. */

main/src/main/scala/sbt/Defaults.scala

@@ -1168,14 +1168,14 @@ object Defaults extends BuildCommon {
     Def.inputTask {
       val handle = bgRunMain.evaluated
       val service = bgJobService.value
-      service.waitFor(handle)
+      service.waitForTry(handle).get
     }
   // run calls bgRun in the background and waits for the result.
   def foregroundRunTask: Initialize[InputTask[Unit]] =
     Def.inputTask {
       val handle = bgRun.evaluated
       val service = bgJobService.value
-      service.waitFor(handle)
+      service.waitForTry(handle).get
     }
   def runMainTask(classpath: Initialize[Task[Classpath]],
                   scalaRun: Initialize[Task[ScalaRun]]): Initialize[InputTask[Unit]] = {

main/src/main/scala/sbt/internal/DefaultBackgroundJobService.scala

@@ -5,6 +5,7 @@ import java.util.concurrent.atomic.AtomicLong
 import java.io.Closeable
 import Def.{ ScopedKey, Setting, Classpath }
 import scala.concurrent.ExecutionContext
+import scala.util.Try
 import Scope.GlobalScope
 import java.io.File
 import sbt.io.{ IO, Hash }
@@ -18,6 +19,13 @@ import sbt.internal.util.{ Attributed, ManagedLogger }
 private[sbt] abstract class BackgroundJob {
   def humanReadableName: String
   def awaitTermination(): Unit
+
+  /** This waits till the job ends, and returns inner error via `Try`. */
+  def awaitTerminationTry(): Try[Unit] = {
+    // This implementation is provided only for backward compatibility.
+    Try(awaitTermination())
+  }
+
   def shutdown(): Unit
   // this should be true on construction and stay true until
   // the job is complete
@@ -132,15 +140,27 @@ private[sbt] abstract class AbstractBackgroundJobService extends BackgroundJobSe
 
   private def withHandle(job: JobHandle)(f: ThreadJobHandle => Unit): Unit = job match {
     case handle: ThreadJobHandle @unchecked => f(handle)
-    case dead: DeadHandle @unchecked        => () // nothing to stop or wait for
+    case _: DeadHandle @unchecked           => () // nothing to stop or wait for
     case other =>
       sys.error(
         s"BackgroundJobHandle does not originate with the current BackgroundJobService: $other")
   }
 
+  private def withHandleTry(job: JobHandle)(f: ThreadJobHandle => Try[Unit]): Try[Unit] =
+    job match {
+      case handle: ThreadJobHandle @unchecked => f(handle)
+      case _: DeadHandle @unchecked           => Try(()) // nothing to stop or wait for
+      case other =>
+        Try(sys.error(
+          s"BackgroundJobHandle does not originate with the current BackgroundJobService: $other"))
+    }
+
   override def stop(job: JobHandle): Unit =
     withHandle(job)(_.job.shutdown())
 
+  override def waitForTry(job: JobHandle): Try[Unit] =
+    withHandleTry(job)(_.job.awaitTerminationTry())
+
   override def waitFor(job: JobHandle): Unit =
     withHandle(job)(_.job.awaitTermination())
 
@@ -212,6 +232,9 @@ private[sbt] class BackgroundThreadPool extends java.io.Closeable {
     @volatile
     private var status: Status = Waiting
 
+    // This is used to capture exceptions that are caught in this background job.
+    private var exitTry: Option[Try[Unit]] = None
+
     // double-finally for extra paranoia that we will finishedLatch.countDown
     override def run() =
       try {
@@ -226,7 +249,11 @@ private[sbt] class BackgroundThreadPool extends java.io.Closeable {
               throw new RuntimeException("Impossible status of bg thread")
           }
         }
-        try { if (go) body() } finally cleanup()
+        try {
+          if (go) {
+            exitTry = Option(Try(body()))
+          }
+        } finally cleanup()
       } finally finishedLatch.countDown()
 
     private class StopListener(val callback: () => Unit, val executionContext: ExecutionContext)
@@ -269,6 +296,12 @@ private[sbt] class BackgroundThreadPool extends java.io.Closeable {
         result
       }
     override def awaitTermination(): Unit = finishedLatch.await()
+
+    override def awaitTerminationTry(): Try[Unit] = {
+      awaitTermination()
+      exitTry.getOrElse(Try(()))
+    }
+
     override def humanReadableName: String = taskName
     override def isRunning(): Boolean =
       status match {