tests: Check that server accepts large number of connected clients

jvm/src/test/scala/cs214/webapp/AppSuite.scala

@@ -169,3 +169,48 @@ class WebSocketSuite extends PingPongSuite:
           ws.sendText(app.wire.eventFormat.encode("b").toString)
           val r0b = decodeActions(ws.receiveText())
           assertEquals(r0b, Seq(Action.Render("b")))
+
+  test("ws: Webserver survives many connections to a single app"):
+    withServer: server ?=>
+      def userId(n: Int) = f"p$n"
+      def withJavaWss[Lst <: JWebSocket.Listener]
+                     (instanceId: String, nUsers: Int, listener: String => Lst)
+                     (body: Seq[(JWebSocket, Lst)] => Unit) =
+        def loop(i: Int, wss: Seq[(JWebSocket, Lst)]): Unit =
+          if i >= nUsers then
+            body(wss)
+            wss.foreach: (ws, lst) =>
+              ws.sendClose(1000, "bye")
+              ws.abort()
+          else
+            val uId = userId(i)
+            val lst = listener(uId)
+            val ws = withJavaWs(instanceId, uId, lst)(ws => ws)
+            loop(i + 1, (ws, lst) +: wss)
+        loop(0, Seq())
+
+      val nUsers = 1000
+      val inst = createInstance((0 to nUsers).map(userId))
+
+      class QueuedListener extends JWebSocket.Listener:
+        var partials: String = ""
+        val queue = java.util.concurrent.LinkedTransferQueue[String]()
+
+        def receiveText() =
+          queue.take()
+
+        override def onText(webSocket: JWebSocket, data: CharSequence, last: Boolean)
+            : java.util.concurrent.CompletionStage[?] =
+          webSocket.synchronized:
+            partials += data.toString
+            if last then
+              queue.add(partials)
+              partials = ""
+            super.onText(webSocket, data, last)
+
+      withJavaWss(inst.instanceId, nUsers, _ => QueuedListener()): wss =>
+        wss.head._1.sendText(app.wire.eventFormat.encode("a").toString, true)
+        val rng = scala.util.Random(12)
+        rng.shuffle(wss).foreach: (ws, lst) =>
+          assertEquals(decodeActions(lst.receiveText()), Seq(Action.Render("hello")))
+          assertEquals(decodeActions(lst.receiveText()), Seq(Action.Render("a")))