Replicate failing to open port on Windows

.github/workflows/ci.yml

@@ -186,6 +186,7 @@ jobs:
       shell: bash
       run: |
         # test building sbtn on Windows
+        sbt test
         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

@@ -16,7 +16,7 @@ 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)
 

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

@@ -0,0 +1,38 @@
+/*
+ * 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._
+import xsbt.IPC.Server
+
+import java.net.Socket
+
+class IPCSpec extends AnyFlatSpec {
+  "server" should "find free open ports and close them" in {
+    val openedServers = collection.mutable.Set.empty[(Server, Socket)]
+    val portsToTest = 1000
+
+    noException should be thrownBy {
+      (1 to portsToTest).foreach { _ =>
+        val server = IPC.unmanagedServer
+        openedServers.add((server, new Socket(IPC.loopback, server.port)))
+      }
+    }
+
+    openedServers.size shouldBe 1000
+
+    Thread.sleep(10000)
+
+    openedServers.foreach {
+      case (server, socket) =>
+        noException should be thrownBy socket.close()
+        noException should be thrownBy server.close()
+    }
+  }
+}