Change local RPC port selection logic (#1800)

Current RPC port selection logic favors ephemeral ports. This leads to
issues with the web apps sandbox.

Instead, we now use randomized port selection from an approved range of
ports.

Author: Connor McMahon

release_notes.md

@@ -1,5 +1,6 @@
 ## New Features
 
 ## Bug fixes
+- Fix issue with local RPC endpoint used by non-.NET languages on Windows apps (#1800)
 
 ## Breaking Changes

src/WebJobs.Extensions.DurableTask/LocalHttpListener.cs

@@ -22,9 +22,18 @@ internal class LocalHttpListener : IDisposable
     {
         private const int DefaultPort = 17071;
 
+        // Pick a large, fixed range of ports that are going to be valid in all environment.
+        // Avoiding ports below 1024 as those are blocked by app service sandbox.
+        // Ephemeral ports for most OS start well above 32768. See https://www.ncftp.com/ncftpd/doc/misc/ephemeral_ports.html
+        private const int MinPort = 30000;
+        private const int MaxPort = 31000;
+
         private readonly Func<HttpRequestMessage, Task<HttpResponseMessage>> handler;
         private readonly EndToEndTraceHelper traceHelper;
         private readonly DurableTaskOptions durableTaskOptions;
+        private readonly Random portGenerator;
+        private readonly HashSet<int> attemptedPorts;
+
         private IWebHost localWebHost;
 
         public LocalHttpListener(
@@ -39,6 +48,8 @@ public LocalHttpListener(
             // Set to a non null value
             this.InternalRpcUri = new Uri($"http://uninitialized");
             this.localWebHost = new NoOpWebHost();
+            this.portGenerator = new Random();
+            this.attemptedPorts = new HashSet<int>();
         }
 
         public Uri InternalRpcUri { get; private set; }
@@ -59,10 +70,12 @@ public async Task StartAsync()
             int numAttempts = 1;
             do
             {
-                int availablePort = this.GetAvailablePort();
+                int listeningPort = numAttempts == 1
+                    ? DefaultPort
+                    : this.GetRandomPort();
                 try
                 {
-                    this.InternalRpcUri = new Uri($"http://127.0.0.1:{availablePort}/durabletask/");
+                    this.InternalRpcUri = new Uri($"http://127.0.0.1:{listeningPort}/durabletask/");
                     var listenUri = new Uri(this.InternalRpcUri.GetLeftPart(UriPartial.Authority));
                     this.localWebHost = new WebHostBuilder()
                         .UseKestrel()
@@ -80,11 +93,9 @@ public async Task StartAsync()
                         this.durableTaskOptions.HubName,
                         functionName: string.Empty,
                         instanceId: string.Empty,
-                        message: $"Failed to open local socket {availablePort}. This was attempt #{numAttempts} to open a local port.");
+                        message: $"Failed to open local port {listeningPort}. This was attempt #{numAttempts} to open a local port.");
+                    this.attemptedPorts.Add(listeningPort);
                     numAttempts++;
-                    var random = new Random();
-                    var millisecondsToWait = (int)Math.Round(random.NextDouble() * 1000);
-                    await Task.Delay(millisecondsToWait);
                 }
             }
             while (numAttempts <= maxAttempts);
@@ -99,6 +110,20 @@ public async Task StartAsync()
 #endif
         }
 
+        private int GetRandomPort()
+        {
+            // Get a random port that has not already been attempted so we don't waste time trying
+            // to listen to a port we know is not free.
+            int randomPort;
+            do
+            {
+                randomPort = this.portGenerator.Next(MinPort, MaxPort);
+            }
+            while (this.attemptedPorts.Contains(randomPort));
+
+            return randomPort;
+        }
+
         public async Task StopAsync()
         {
 #if !FUNCTIONS_V1
@@ -110,29 +135,6 @@ public async Task StopAsync()
             this.IsListening = false;
         }
 
-        private int GetAvailablePort()
-        {
-            // If we are able to successfully start a listener looking on the default port without
-            // an exception, we can use the default port. Otherwise, let the TcpListener class decide for us.
-            try
-            {
-                var listener = new TcpListener(IPAddress.Loopback, DefaultPort);
-                listener.Start();
-                listener.Stop();
-                return DefaultPort;
-            }
-            catch (SocketException)
-            {
-                // Following guidance of this stack overflow answer
-                // to find available port: https://stackoverflow.com/a/150974/9035640
-                var listener = new TcpListener(IPAddress.Loopback, 0);
-                listener.Start();
-                int availablePort = ((IPEndPoint)listener.LocalEndpoint).Port;
-                listener.Stop();
-                return availablePort;
-            }
-        }
-
         private async Task HandleRequestAsync(HttpContext context)
         {
             try