Remove duped system in

I had some ideas for allowing the user to get a copy of system in during
a continuous build but I can't really see a good use case now so I'm
going to remove it before 1.3.0.

main/src/main/scala/sbt/internal/Continuous.scala

@@ -79,21 +79,6 @@ private[sbt] object Continuous extends DeprecatedContinuous {
       val inputs: Seq[DynamicInput]
   )
 
-  /**
-   * Provides a copy of System.in that can be scanned independently from System.in itself. This task
-   * will only be valid during a continuous build started via `~` or the `watch` task. The
-   * motivation is that a plugin may want to completely override the parsing of System.in which
-   * is not straightforward since the default implementation is hard-wired to read from and
-   * parse System.in. If an invalid parser is provided by [[Keys.watchInputParser]] and
-   * [[Keys.watchInputStream]] is set to this task, then a custom parser can be provided via
-   * [[Keys.watchInputHandler]] and the default System.in processing will not occur.
-   *
-   * @return the duplicated System.in
-   */
-  def dupedSystemIn: Def.Initialize[Task[InputStream]] = Def.task {
-    Keys.state.value.get(DupedSystemIn).map(_.duped).getOrElse(System.in)
-  }
-
   /**
    * Create a function from InputStream => [[Watch.Action]] from a [[Parser]]. This is intended
    * to be used to set the watchInputHandler setting for a task.
@@ -136,12 +121,6 @@ private[sbt] object Continuous extends DeprecatedContinuous {
       )
     }
 
-  private[this] val DupedSystemIn =
-    AttributeKey[DupedInputStream](
-      "duped-system-in",
-      "Receives a copy of all of the bytes from System.in.",
-      10000
-    )
   private[sbt] val dynamicInputs = taskKey[Option[mutable.Set[DynamicInput]]](
     "The input globs found during task evaluation that are used in watch."
   )
@@ -298,7 +277,6 @@ private[sbt] object Continuous extends DeprecatedContinuous {
       count: Int,
       isCommand: Boolean
   ): State = withCharBufferedStdIn { in =>
-    val duped = new DupedInputStream(in)
     implicit val extracted: Extracted = Project.extract(state)
     val repo = if ("polling" == System.getProperty("sbt.watch.mode")) {
       val service =
@@ -311,7 +289,6 @@ private[sbt] object Continuous extends DeprecatedContinuous {
       val stateWithRepo = state
         .put(Keys.globalFileTreeRepository, repo)
         .put(sbt.nio.Keys.persistentFileAttributeMap, new sbt.nio.Keys.FileAttributeMap)
-        .put(DupedSystemIn, duped)
       setup(stateWithRepo, command) { (commands, s, valid, invalid) =>
         EvaluateTask.withStreams(extracted.structure, s)(_.use(Keys.streams in Global) { streams =>
           implicit val logger: Logger = streams.log

main/src/main/scala/sbt/internal/DupedInputStream.scala

@@ -1,73 +0,0 @@
-/*
- * sbt
- * Copyright 2011 - 2018, Lightbend, Inc.
- * Copyright 2008 - 2010, Mark Harrah
- * Licensed under Apache License 2.0 (see LICENSE)
- */
-
-package sbt.internal
-
-import java.io.{ InputStream, PipedInputStream, PipedOutputStream }
-import java.util.concurrent.LinkedBlockingQueue
-
-import scala.annotation.tailrec
-import scala.collection.JavaConverters._
-
-/**
- * Creates a copy of the provided [[InputStream]] that forwards its contents to an arbitrary
- * number of connected [[InputStream]] instances via pipe.
- * @param in the [[InputStream]] to wrap.
- */
-private[internal] class DupedInputStream(val in: InputStream)
-    extends InputStream
-    with AutoCloseable {
-
-  /**
-   * Returns a copied [[InputStream]] that will receive the same bytes as System.in.
-   * @return
-   */
-  def duped: InputStream = {
-    val pipedOutputStream = new PipedOutputStream()
-    pipes += pipedOutputStream
-    val res = new PollingInputStream(new PipedInputStream(pipedOutputStream))
-    buffer.forEach(pipedOutputStream.write(_))
-    res
-  }
-
-  private[this] val pipes = new java.util.Vector[PipedOutputStream].asScala
-  private[this] val buffer = new LinkedBlockingQueue[Int]
-  private class PollingInputStream(val pipedInputStream: PipedInputStream) extends InputStream {
-    override def available(): Int = {
-      fillBuffer()
-      pipedInputStream.available()
-    }
-    override def read(): Int = {
-      fillBuffer()
-      pipedInputStream.read
-    }
-  }
-  override def available(): Int = {
-    fillBuffer()
-    buffer.size
-  }
-  override def read(): Int = {
-    fillBuffer()
-    buffer.take()
-  }
-
-  private[this] def fillBuffer(): Unit = synchronized {
-    @tailrec
-    def impl(): Unit = in.available match {
-      case i if i > 0 =>
-        val res = in.read()
-        buffer.add(res)
-        pipes.foreach { p =>
-          p.write(res)
-          p.flush()
-        }
-        impl()
-      case _ =>
-    }
-    impl()
-  }
-}