Make progress an object

This commit reworks TaskProgress so that it is a singleton object. By
using a singleton, we ensure that there is at most one progress thread
running at a time. With multiple threads, there can be flickering in the
progress reports.

This fixes https://github.com/sbt/sbt/issues/5547. There also was a bug
that the reference to the progress thread was not reset when the thread
itself exited. As a result, it was possible for progress reporting to
stop while tasks were still running. This seemed to primarily happen in
multi-project builds. It should be fixed by this change.

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

@@ -255,7 +255,7 @@ object EvaluateTask {
           extracted,
           structure
         )
-        val reporters = maker.map(_.progress) ++ Some(new TaskProgress) ++
+        val reporters = maker.map(_.progress) ++ Some(TaskProgress) ++
           (if (SysProp.taskTimings)
              new TaskTimings(reportOnShutdown = false, state.globalLogging.full) :: Nil
            else Nil)

main/src/main/scala/sbt/internal/TaskProgress.scala

@@ -16,14 +16,14 @@ import sbt.internal.util._
 import scala.annotation.tailrec
 import scala.concurrent.duration._
 
+object TaskProgress extends TaskProgress
+
 /**
  * implements task progress display on the shell.
  */
-private[sbt] final class TaskProgress
+private[sbt] class TaskProgress private ()
     extends AbstractTaskExecuteProgress
     with ExecuteProgress[Task] {
-  @deprecated("Use the no argument constructor.", "1.4.0")
-  def this(log: ManagedLogger) = this()
   private[this] val lastTaskCount = new AtomicInteger(0)
   private[this] val currentProgressThread = new AtomicReference[Option[ProgressThread]](None)
   private[this] val sleepDuration = SysProp.supershellSleep.millis
@@ -38,6 +38,13 @@ private[sbt] final class TaskProgress
     private[this] def doReport(): Unit = { hasReported.set(true); report() }
     setDaemon(true)
     start()
+    private def resetThread(): Unit =
+      currentProgressThread.synchronized {
+        currentProgressThread.getAndSet(None) match {
+          case Some(t) if t != this => currentProgressThread.set(Some(t))
+          case _                    =>
+        }
+      }
     @tailrec override def run(): Unit = {
       if (!isClosed.get() && (!hasReported.get || active.nonEmpty)) {
         try {
@@ -61,6 +68,8 @@ private[sbt] final class TaskProgress
 
         }
         run()
+      } else {
+        resetThread()
       }
     }
 
@@ -71,7 +80,7 @@ private[sbt] final class TaskProgress
       interrupt()
       report()
       appendProgress(ProgressEvent("Info", Vector(), None, None, None))
-      ()
+      resetThread()
     }
   }