Better error message if socket is taken

We need to communicate the error states in the thread, so I added a `Future[Unit]` called `ready`.

If something goes wrong during the startup, like if the port is already taken, this can be used to communicate back to the main thread, and display the error accordingly.

main-command/src/main/scala/sbt/internal/server/Server.scala

@@ -8,9 +8,13 @@ package server
 import java.net.{ SocketTimeoutException, InetAddress, ServerSocket, Socket }
 import java.util.concurrent.atomic.AtomicBoolean
 import sbt.util.Logger
+import sbt.internal.util.ErrorHandling
+import scala.concurrent.{ Future, Promise }
+import scala.util.{ Try, Success, Failure }
 
 private[sbt] sealed trait ServerInstance {
   def shutdown(): Unit
+  def ready: Future[Unit]
 }
 
 private[sbt] object Server {
@@ -20,23 +24,31 @@ private[sbt] object Server {
 
       // val lock = new AnyRef {}
       // val clients: mutable.ListBuffer[ClientConnection] = mutable.ListBuffer.empty
-      val running = new AtomicBoolean(true)
+      val running = new AtomicBoolean(false)
+      val p: Promise[Unit] = Promise[Unit]()
+      val ready: Future[Unit] = p.future
 
       val serverThread = new Thread("sbt-socket-server") {
-
         override def run(): Unit = {
-          val serverSocket = new ServerSocket(port, 50, InetAddress.getByName(host))
-          serverSocket.setSoTimeout(5000)
-
-          log.info(s"sbt server started at $host:$port")
-          while (running.get()) {
-            try {
-              val socket = serverSocket.accept()
-              onIncomingSocket(socket)
-            } catch {
-              case _: SocketTimeoutException => // its ok
+          Try {
+            ErrorHandling.translate(s"server failed to start on $host:$port. ") {
+              new ServerSocket(port, 50, InetAddress.getByName(host))
             }
-
+          } match {
+            case Failure(e) => p.failure(e)
+            case Success(serverSocket) =>
+              serverSocket.setSoTimeout(5000)
+              log.info(s"sbt server started at $host:$port")
+              running.set(true)
+              p.success(())
+              while (running.get()) {
+                try {
+                  val socket = serverSocket.accept()
+                  onIncomingSocket(socket)
+                } catch {
+                  case _: SocketTimeoutException => // its ok
+                }
+              }
           }
         }
       }

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

@@ -12,6 +12,9 @@ import scala.annotation.tailrec
 import BasicKeys.serverPort
 import java.net.Socket
 import sjsonnew.JsonFormat
+import scala.concurrent.Await
+import scala.concurrent.duration.Duration
+import scala.util.{ Success, Failure }
 
 /**
  * The command exchange merges multiple command channels (e.g. network and console),
@@ -81,7 +84,15 @@ private[sbt] final class CommandExchange {
       server match {
         case Some(x) => // do nothing
         case _ =>
-          server = Some(Server.start("127.0.0.1", port, onIncomingSocket, s.log))
+          val x = Server.start("127.0.0.1", port, onIncomingSocket, s.log)
+          Await.ready(x.ready, Duration("10s"))
+          x.ready.value match {
+            case Some(Success(_)) =>
+            case Some(Failure(e)) =>
+              s.log.error(e.toString)
+            case None    => // this won't happen because we awaited
+          }
+          server = Some(x)
       }
       s
     }