[Host] Allow disabling reloadOnChange for Host's CreateDefaultBuilder (…

…#2940)

* Add environment var to allow disabling live reload in default builder

* Switch to using hostingContext, rename flag to fit nomenclature.

* Change flag to memory data source. Change content root default instead of runtime default.

* Update config key to be hierarchical. Change await on positive case to be longer and cancel using the reload token.

* Uncomment a development test case.

* Apply suggestions from code review

Co-authored-by: Chris Ross <Tratcher@Outlook.com>

src/Hosting/Hosting/src/Host.cs

@@ -71,9 +71,11 @@ public static IHostBuilder CreateDefaultBuilder(string[] args)
             builder.ConfigureAppConfiguration((hostingContext, config) =>
             {
                 var env = hostingContext.HostingEnvironment;
+
+                var reloadOnChange = hostingContext.Configuration.GetValue("hostBuilder:reloadConfigOnChange", defaultValue: true);
 
-                config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
-                      .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: true);
+                config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: reloadOnChange)
+                      .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: reloadOnChange);
 
                 if (env.IsDevelopment() && !string.IsNullOrEmpty(env.ApplicationName))
                 {

src/Hosting/Hosting/test/HostTests.cs

@@ -8,6 +8,8 @@
 using System.Diagnostics.Tracing;
 using System.IO;
 using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
@@ -82,6 +84,81 @@ public void CreateDefaultBuilder_EnablesValidateOnBuild()
             Assert.Throws<AggregateException>(() => hostBuilder.Build());
         }
 
+        [Fact]
+        public async Task CreateDefaultBuilder_ConfigJsonDoesNotReload()
+        {
+            var reloadFlagConfig = new Dictionary<string, string>() {{ "hostbuilder:reloadConfigOnChange", "false" }};
+            var appSettingsPath = Path.Combine(Path.GetTempPath(), "appsettings.json");
+
+            string SaveRandomConfig()
+            {
+                var newMessage = $"Hello ASP.NET Core: {Guid.NewGuid():N}";
+                File.WriteAllText(appSettingsPath, $"{{ \"Hello\": \"{newMessage}\" }}");
+                return newMessage;
+            }
+
+            var dynamicConfigMessage1 = SaveRandomConfig();
+
+            var host = Host.CreateDefaultBuilder()
+                .UseContentRoot(Path.GetDirectoryName(appSettingsPath))
+                .ConfigureHostConfiguration(builder =>
+                {
+                    builder.AddInMemoryCollection(reloadFlagConfig);
+                })
+                .Build();
+
+            var config = host.Services.GetRequiredService<IConfiguration>();
+
+            Assert.Equal(dynamicConfigMessage1, config["Hello"]);
+
+            var dynamicConfigMessage2 = SaveRandomConfig();
+            await Task.Delay(1000); // Give reload time to fire if it's going to.
+            Assert.NotEqual(dynamicConfigMessage1, dynamicConfigMessage2); // Messages are different.
+            Assert.Equal(dynamicConfigMessage1, config["Hello"]); // Config did not reload
+        }
+
+        [Fact]
+        public async Task CreateDefaultBuilder_ConfigJsonDoesReload()
+        {
+            var reloadFlagConfig = new Dictionary<string, string>() { { "hostbuilder:reloadConfigOnChange", "true" } };
+            var appSettingsPath = Path.Combine(Path.GetTempPath(), "appsettings.json");
+
+            string SaveRandomConfig()
+            {
+                var newMessage = $"Hello ASP.NET Core: {Guid.NewGuid():N}";
+                File.WriteAllText(appSettingsPath, $"{{ \"Hello\": \"{newMessage}\" }}");
+                return newMessage;
+            }
+
+            var dynamicConfigMessage1 = SaveRandomConfig();
+
+            var host = Host.CreateDefaultBuilder()
+                .UseContentRoot(Path.GetDirectoryName(appSettingsPath))
+                .ConfigureHostConfiguration(builder =>
+                {
+                    builder.AddInMemoryCollection(reloadFlagConfig);
+                })
+                .Build();
+
+            var config = host.Services.GetRequiredService<IConfiguration>();
+
+            Assert.Equal(dynamicConfigMessage1, config["Hello"]);
+
+            var dynamicConfigMessage2 = SaveRandomConfig();
+
+            var configReloadedCancelTokenSource = new CancellationTokenSource();
+            var configReloadedCancelToken = configReloadedCancelTokenSource.Token;
+
+            config.GetReloadToken().RegisterChangeCallback(o =>
+            {
+                configReloadedCancelTokenSource.Cancel();
+            }, null);
+            // Wait for up to 10 seconds, if config reloads at any time, cancel the wait.
+            await Task.WhenAny(Task.Delay(10000, configReloadedCancelToken)); // Task.WhenAny ignores the task throwing on cancellation.
+            Assert.NotEqual(dynamicConfigMessage1, dynamicConfigMessage2); // Messages are different.
+            Assert.Equal(dynamicConfigMessage2, config["Hello"]); // Config DID reload from disk
+        }
+
         internal class ServiceA { }
 
         internal class ServiceB