When a SPA dev server (or prerendering build) takes too long to start up, only fail current request, not future requests. Fixes #1447

Author: SteveSandersonMS

src/Microsoft.AspNetCore.SpaServices.Extensions/AngularCli/AngularCliBuilder.cs

@@ -20,7 +20,6 @@ namespace Microsoft.AspNetCore.SpaServices.AngularCli
     public class AngularCliBuilder : ISpaPrerendererBuilder
     {
         private static TimeSpan RegexMatchTimeout = TimeSpan.FromSeconds(5); // This is a development-time only feature, so a very long timeout is fine
-        private static TimeSpan BuildTimeout = TimeSpan.FromSeconds(50); // Note that the HTTP request itself by default times out after 60s, so you only get useful error information if this is shorter
 
         private readonly string _npmScriptName;
 
@@ -63,8 +62,7 @@ public async Task Build(ISpaBuilder spaBuilder)
                 try
                 {
                     await npmScriptRunner.StdOut.WaitForMatch(
-                        new Regex("Date", RegexOptions.None, RegexMatchTimeout),
-                        BuildTimeout);
+                        new Regex("Date", RegexOptions.None, RegexMatchTimeout));
                 }
                 catch (EndOfStreamException ex)
                 {

src/Microsoft.AspNetCore.SpaServices.Extensions/AngularCli/AngularCliMiddleware.cs

@@ -12,6 +12,7 @@
 using System.Threading.Tasks;
 using System.Threading;
 using System.Net.Http;
+using Microsoft.AspNetCore.SpaServices.Extensions.Util;
 
 namespace Microsoft.AspNetCore.SpaServices.AngularCli
 {
@@ -49,7 +50,15 @@ public static void Attach(
             var targetUriTask = angularCliServerInfoTask.ContinueWith(
                 task => new UriBuilder("http", "localhost", task.Result.Port).Uri);
 
-            SpaProxyingExtensions.UseProxyToSpaDevelopmentServer(spaBuilder, targetUriTask);
+            SpaProxyingExtensions.UseProxyToSpaDevelopmentServer(spaBuilder, () =>
+            {
+                // On each request, we create a separate startup task with its own timeout. That way, even if
+                // the first request times out, subsequent requests could still work.
+                return targetUriTask.WithTimeout(StartupTimeout,
+                    $"The Angular CLI process did not start listening for requests " +
+                    $"within the timeout period of {StartupTimeout.Seconds} seconds. " +
+                    $"Check the log output for error information.");
+            });
         }
 
         private static async Task<AngularCliServerInfo> StartAngularCliServerAsync(
@@ -68,8 +77,7 @@ private static async Task<AngularCliServerInfo> StartAngularCliServerAsync(
                 try
                 {
                     openBrowserLine = await npmScriptRunner.StdOut.WaitForMatch(
-                        new Regex("open your browser on (http\\S+)", RegexOptions.None, RegexMatchTimeout),
-                        StartupTimeout);
+                        new Regex("open your browser on (http\\S+)", RegexOptions.None, RegexMatchTimeout));
                 }
                 catch (EndOfStreamException ex)
                 {
@@ -78,13 +86,6 @@ private static async Task<AngularCliServerInfo> StartAngularCliServerAsync(
                         $"Angular CLI was listening for requests. The error output was: " +
                         $"{stdErrReader.ReadAsString()}", ex);
                 }
-                catch (TaskCanceledException ex)
-                {
-                    throw new InvalidOperationException(
-                        $"The Angular CLI process did not start listening for requests " +
-                        $"within the timeout period of {StartupTimeout.Seconds} seconds. " +
-                        $"Check the log output for error information.", ex);
-                }
             }
 
             var uri = new Uri(openBrowserLine.Groups[1].Value);

src/Microsoft.AspNetCore.SpaServices.Extensions/Prerendering/SpaPrerenderingExtensions.cs

@@ -6,6 +6,7 @@
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.NodeServices;
 using Microsoft.AspNetCore.SpaServices;
+using Microsoft.AspNetCore.SpaServices.Extensions.Util;
 using Microsoft.AspNetCore.SpaServices.Prerendering;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Net.Http.Headers;
@@ -23,6 +24,8 @@ namespace Microsoft.AspNetCore.Builder
     /// </summary>
     public static class SpaPrerenderingExtensions
     {
+        private static TimeSpan BuildTimeout = TimeSpan.FromSeconds(50); // Note that the HTTP request itself by default times out after 60s, so you only get useful error information if this is shorter
+
         /// <summary>
         /// Enables server-side prerendering middleware for a Single Page Application.
         /// </summary>
@@ -85,9 +88,15 @@ public static void UseSpaPrerendering(
                 }
 
                 // If we're building on demand, wait for that to finish, or raise any build errors
-                if (buildOnDemandTask != null)
+                if (buildOnDemandTask != null && !buildOnDemandTask.IsCompleted)
                 {
-                    await buildOnDemandTask;
+                    // For better debuggability, create a per-request timeout that makes it clear if the
+                    // prerendering builder took too long for this request, but without aborting the
+                    // underlying build task so that subsequent requests could still work.
+                    await buildOnDemandTask.WithTimeout(BuildTimeout,
+                        $"The prerendering build process did not complete within the " +
+                        $"timeout period of {BuildTimeout.Seconds} seconds. " +
+                        $"Check the log output for error information.");
                 }
 
                 // It's no good if we try to return a 304. We need to capture the actual

src/Microsoft.AspNetCore.SpaServices.Extensions/Proxying/SpaProxyingExtensions.cs

@@ -45,7 +45,7 @@ public static void UseProxyToSpaDevelopmentServer(
         {
             UseProxyToSpaDevelopmentServer(
                 spaBuilder,
-                Task.FromResult(baseUri));
+                () => Task.FromResult(baseUri));
         }
 
         /// <summary>
@@ -54,10 +54,10 @@ public static void UseProxyToSpaDevelopmentServer(
         /// development. Do not enable this middleware in production applications.
         /// </summary>
         /// <param name="spaBuilder">The <see cref="ISpaBuilder"/>.</param>
-        /// <param name="baseUriTask">A <see cref="Task"/> that resolves with the target base URI to which requests should be proxied.</param>
+        /// <param name="baseUriTaskFactory">A callback that will be invoked on each request to supply a <see cref="Task"/> that resolves with the target base URI to which requests should be proxied.</param>
         public static void UseProxyToSpaDevelopmentServer(
             this ISpaBuilder spaBuilder,
-            Task<Uri> baseUriTask)
+            Func<Task<Uri>> baseUriTaskFactory)
         {
             var applicationBuilder = spaBuilder.ApplicationBuilder;
             var applicationStoppingToken = GetStoppingToken(applicationBuilder);
@@ -72,11 +72,11 @@ public static void UseProxyToSpaDevelopmentServer(
             var neverTimeOutHttpClient =
                 SpaProxy.CreateHttpClientForProxy(Timeout.InfiniteTimeSpan);
 
-            // Proxy all requests into the Angular CLI server
+            // Proxy all requests to the SPA development server
             applicationBuilder.Use(async (context, next) =>
             {
                 var didProxyRequest = await SpaProxy.PerformProxyRequest(
-                    context, neverTimeOutHttpClient, baseUriTask, applicationStoppingToken,
+                    context, neverTimeOutHttpClient, baseUriTaskFactory(), applicationStoppingToken,
                     proxy404s: true);
             });
         }

src/Microsoft.AspNetCore.SpaServices.Extensions/ReactDevelopmentServer/ReactDevelopmentServerMiddleware.cs

@@ -11,6 +11,7 @@
 using System.Collections.Generic;
 using System.Text.RegularExpressions;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.SpaServices.Extensions.Util;
 
 namespace Microsoft.AspNetCore.SpaServices.ReactDevelopmentServer
 {
@@ -48,7 +49,15 @@ public static void Attach(
             var targetUriTask = portTask.ContinueWith(
                 task => new UriBuilder("http", "localhost", task.Result).Uri);
 
-            SpaProxyingExtensions.UseProxyToSpaDevelopmentServer(spaBuilder, targetUriTask);
+            SpaProxyingExtensions.UseProxyToSpaDevelopmentServer(spaBuilder, () =>
+            {
+                // On each request, we create a separate startup task with its own timeout. That way, even if
+                // the first request times out, subsequent requests could still work.
+                return targetUriTask.WithTimeout(StartupTimeout,
+                    $"The create-react-app server did not start listening for requests " +
+                    $"within the timeout period of {StartupTimeout.Seconds} seconds. " +
+                    $"Check the log output for error information.");
+            });
         }
 
         private static async Task<int> StartCreateReactAppServerAsync(
@@ -75,8 +84,7 @@ private static async Task<int> StartCreateReactAppServerAsync(
                     // no compiler warnings. So instead of waiting for that, consider it ready as soon
                     // as it starts listening for requests.
                     await npmScriptRunner.StdOut.WaitForMatch(
-                        new Regex("Starting the development server", RegexOptions.None, RegexMatchTimeout),
-                        StartupTimeout);
+                        new Regex("Starting the development server", RegexOptions.None, RegexMatchTimeout));
                 }
                 catch (EndOfStreamException ex)
                 {
@@ -85,13 +93,6 @@ await npmScriptRunner.StdOut.WaitForMatch(
                         $"create-react-app server was listening for requests. The error output was: " +
                         $"{stdErrReader.ReadAsString()}", ex);
                 }
-                catch (TaskCanceledException ex)
-                {
-                    throw new InvalidOperationException(
-                        $"The create-react-app server did not start listening for requests " +
-                        $"within the timeout period of {StartupTimeout.Seconds} seconds. " +
-                        $"Check the log output for error information.", ex);
-                }
             }
 
             return portNumber;

src/Microsoft.AspNetCore.SpaServices.Extensions/Util/EventedStreamReader.cs

@@ -34,7 +34,7 @@ public EventedStreamReader(StreamReader streamReader)
             Task.Factory.StartNew(Run);
         }
 
-        public Task<Match> WaitForMatch(Regex regex, TimeSpan timeout = default)
+        public Task<Match> WaitForMatch(Regex regex)
         {
             var tcs = new TaskCompletionSource<Match>();
             var completionLock = new object();
@@ -72,15 +72,6 @@ void ResolveIfStillPending(Action applyResolution)
             OnReceivedLine += onReceivedLineHandler;
             OnStreamClosed += onStreamClosedHandler;
 
-            if (timeout != default)
-            {
-                var timeoutToken = new CancellationTokenSource(timeout);
-                timeoutToken.Token.Register(() =>
-                {
-                    ResolveIfStillPending(() => tcs.SetCanceled());
-                });
-            }
-
             return tcs.Task;
         }
 

src/Microsoft.AspNetCore.SpaServices.Extensions/Util/TaskTimeoutExtensions.cs

@@ -0,0 +1,35 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.SpaServices.Extensions.Util
+{
+    internal static class TaskTimeoutExtensions
+    {
+        public static async Task WithTimeout(this Task task, TimeSpan timeoutDelay, string message)
+        {
+            if (task == await Task.WhenAny(task, Task.Delay(timeoutDelay)))
+            {
+                task.Wait(); // Allow any errors to propagate
+            }
+            else
+            {
+                throw new TimeoutException(message);
+            }
+        }
+
+        public static async Task<T> WithTimeout<T>(this Task<T> task, TimeSpan timeoutDelay, string message)
+        {
+            if (task == await Task.WhenAny(task, Task.Delay(timeoutDelay)))
+            {
+                return task.Result;
+            }
+            else
+            {
+                throw new TimeoutException(message);
+            }
+        }
+    }
+}