Use different ports in ServerFixture for testing (#1112)

test/Common/ServerFixture.cs

@@ -3,6 +3,8 @@
 
 using System;
 using System.IO;
+using System.Net;
+using System.Net.Sockets;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.Extensions.DependencyInjection;
@@ -21,7 +23,7 @@ namespace Microsoft.AspNetCore.SignalR.Tests.Common
         private IApplicationLifetime _lifetime;
         private readonly IDisposable _logToken;
 
-        public string BaseUrl => "http://localhost:3000";
+        public string BaseUrl { get; private set; }
 
         public string WebSocketsUrl => BaseUrl.Replace("http", "ws");
 
@@ -30,6 +32,7 @@ namespace Microsoft.AspNetCore.SignalR.Tests.Common
             var testLog = AssemblyTestLog.ForAssembly(typeof(ServerFixture<TStartup>).Assembly);
             _logToken = testLog.StartTestLog(null, $"{nameof(ServerFixture<TStartup>)}_{typeof(TStartup).Name}" , out _loggerFactory, "ServerFixture");
             _logger = _loggerFactory.CreateLogger<ServerFixture<TStartup>>();
+            BaseUrl = "http://localhost:" + GetNextPort();
 
             StartServer();
         }
@@ -90,5 +93,20 @@ namespace Microsoft.AspNetCore.SignalR.Tests.Common
                 return _loggerFactory.CreateLogger(categoryName);
             }
         }
+
+        // Copied from https://github.com/aspnet/KestrelHttpServer/blob/47f1db20e063c2da75d9d89653fad4eafe24446c/test/Microsoft.AspNetCore.Server.Kestrel.FunctionalTests/AddressRegistrationTests.cs#L508
+        private static int GetNextPort()
+        {
+            using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
+            {
+                // Let the OS assign the next available port. Unless we cycle through all ports
+                // on a test run, the OS will always increment the port number when making these calls.
+                // This prevents races in parallel test runs where a test is already bound to
+                // a given port, and a new test is able to bind to the same port due to port
+                // reuse being enabled by default by the OS.
+                socket.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+                return ((IPEndPoint)socket.LocalEndPoint).Port;
+            }
+        }
     }
 }