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 "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

@@ -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)
 
@@ -34,7 +34,12 @@ object IPC {
 
     def createServer(attempts: Int): ServerSocket =
       if (attempts > 0) {
-        try new ServerSocket(nextPort, 1, loopback)
+        val backlog =
+          if (System.getProperty("os.name").toLowerCase.contains("win"))
+            2
+          else
+            1
+        try new ServerSocket(nextPort, backlog, loopback)
         catch { case NonFatal(_) => createServer(attempts - 1) }
       } else sys.error("Could not connect to socket: maximum attempts exceeded")
 

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

@@ -0,0 +1,30 @@
+/*
+ * 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 {
+      val server = IPC.unmanagedServer
+      (1 until 500).foreach { index =>
+        try {
+          IPC.client(server.port)(identity)
+        } catch {
+          case e: Throwable =>
+            val message = s"failure index is $index"
+            info(message)
+            throw e
+        }
+      }
+      server.close()
+    }
+  }
+}