Use better method to find free port

.github/workflows/ci.yml

@@ -186,6 +186,7 @@ jobs:
  shell: bash
  run: |
  # test building sbtn on Windows
+ sbt "commandProj/testOnly xsbt.IPCSpec"
  sbt "-Dsbt.io.virtual=false" nativeImage
  # test launcher script
  echo build using JDK 8, test using JDK 8, on Windows

main-command/src/main/scala/xsbt/IPC.scala

@@ -14,9 +14,7 @@ import scala.annotation.tailrec
 import scala.util.control.NonFatal
 
 object IPC {
- private val portMin = 1025
- private val portMax = 65536
- private val loopback = InetAddress.getByName(null)
+ private[xsbt] val loopback = InetAddress.getByName(null)
 
  def client[T](port: Int)(f: IPC => T): T = ipc(new Socket(loopback, port))(f)
 
@@ -29,12 +27,10 @@ object IPC {
  def unmanagedServer: Server = new Server(makeServer)
 
  def makeServer: ServerSocket = {
- val random = new java.util.Random
- def nextPort = random.nextInt(portMax - portMin + 1) + portMin
 
  def createServer(attempts: Int): ServerSocket =
  if (attempts > 0) {
- try new ServerSocket(nextPort, 1, loopback)
+ try new ServerSocket(0, 1, loopback)
  catch { case NonFatal(_) => createServer(attempts - 1) }
  } else sys.error("Could not connect to socket: maximum attempts exceeded")
 
@@ -63,18 +59,18 @@ object IPC {
  finally s.close()
 
  final class Server private[IPC] (s: ServerSocket) {
- def port = s.getLocalPort
- def close() = s.close()
+ def port: Int = s.getLocalPort
+ def close(): Unit = s.close()
  def isClosed: Boolean = s.isClosed
  def connection[T](f: IPC => T): T = IPC.ipc(s.accept())(f)
  }
 }
 
 final class IPC private (s: Socket) {
- def port = s.getLocalPort
+ def port: Int = s.getLocalPort
  private val in = new BufferedReader(new InputStreamReader(s.getInputStream))
  private val out = new BufferedWriter(new OutputStreamWriter(s.getOutputStream))
 
- def send(s: String) = { out.write(s); out.newLine(); out.flush() }
+ def send(s: String): Unit = { out.write(s); out.newLine(); out.flush() }
  def receive: String = in.readLine()
 }

main-command/src/test/scala/xsbt/IPCSpec.scala

@@ -0,0 +1,20 @@
+/*
+ * sbt
+ * Copyright 2011 - 2018, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+package xsbt
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers._
+
+class IPCSpec extends AnyFlatSpec {
+ "server" should "find free open ports and close them" in {
+ noException should be thrownBy {
+ (1 until 100).foreach { _ =>
+ IPC.unmanagedServer.close()
+ }
+ }
+ }
+}