Avoid building the configuration sources twice (#36570)

- When making the internal host builder, we copy the sources from the ConfigurationManager to the internal IConfigurationBuilder and the HostBuilder creates. This results in building configuration twice which can be problematic for performance reasons. Instead, we copy over the already built configuration providers in a custom IConfigurationSource implementation. This change does means the looking at the list of sources will not match the original list of sources though.
- Added a test

Author: davidfowl

src/DefaultBuilder/DefaultBuilder.slnf

@@ -11,6 +11,7 @@
       "src\\Hosting\\Hosting\\src\\Microsoft.AspNetCore.Hosting.csproj",
       "src\\Hosting\\Server.Abstractions\\src\\Microsoft.AspNetCore.Hosting.Server.Abstractions.csproj",
       "src\\Hosting\\Server.IntegrationTesting\\src\\Microsoft.AspNetCore.Server.IntegrationTesting.csproj",
+      "src\\Hosting\\TestHost\\src\\Microsoft.AspNetCore.TestHost.csproj",
       "src\\Http\\Authentication.Abstractions\\src\\Microsoft.AspNetCore.Authentication.Abstractions.csproj",
       "src\\Http\\Authentication.Core\\src\\Microsoft.AspNetCore.Authentication.Core.csproj",
       "src\\Http\\Headers\\src\\Microsoft.Net.Http.Headers.csproj",
@@ -35,7 +36,8 @@
       "src\\Servers\\Kestrel\\Core\\src\\Microsoft.AspNetCore.Server.Kestrel.Core.csproj",
       "src\\Servers\\Kestrel\\Kestrel\\src\\Microsoft.AspNetCore.Server.Kestrel.csproj",
       "src\\Servers\\Kestrel\\Transport.Quic\\src\\Microsoft.AspNetCore.Server.Kestrel.Transport.Quic.csproj",
-      "src\\Servers\\Kestrel\\Transport.Sockets\\src\\Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.csproj"
+      "src\\Servers\\Kestrel\\Transport.Sockets\\src\\Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.csproj",
+      "src\\Testing\\src\\Microsoft.AspNetCore.Testing.csproj"
     ]
   }
 }

src/DefaultBuilder/src/WebApplicationBuilder.cs

@@ -146,14 +146,13 @@ public WebApplication Build()
                 builder.AddInMemoryCollection(_hostConfigurationValues);
             });
 
-            // Wire up the application configuration by copying the sources over to final configuration builder.
-            // this will also contain host configuration since it's chained (unless the sources were cleared)
-            // but it can't affect the hosting configuration at this point so it's harmless.
+            // Wire up the application configuration by copying the already built configuration providers over to final configuration builder.
+            // We wrap the existing provider in a configuration source to avoid re-bulding the already added configuration sources.
             _hostBuilder.ConfigureAppConfiguration(builder =>
             {
-                foreach (var source in ((IConfigurationBuilder)Configuration).Sources)
+                foreach (var provider in ((IConfigurationRoot)Configuration).Providers)
                 {
-                    builder.Sources.Add(source);
+                    builder.Sources.Add(new ConfigurationProviderSource(provider));
                 }
 
                 foreach (var (key, value) in ((IConfigurationBuilder)Configuration).Properties)
@@ -279,7 +278,7 @@ private void ConfigureApplication(WebHostBuilderContext context, IApplicationBui
             }
         }
 
-        private class LoggingBuilder : ILoggingBuilder
+        private sealed class LoggingBuilder : ILoggingBuilder
         {
             public LoggingBuilder(IServiceCollection services)
             {
@@ -288,5 +287,20 @@ public LoggingBuilder(IServiceCollection services)
 
             public IServiceCollection Services { get; }
         }
+
+        private sealed class ConfigurationProviderSource : IConfigurationSource
+        {
+            private readonly IConfigurationProvider _configurationProvider;
+
+            public ConfigurationProviderSource(IConfigurationProvider configurationProvider)
+            {
+                _configurationProvider = configurationProvider;
+            }
+
+            public IConfigurationProvider Build(IConfigurationBuilder builder)
+            {
+                return _configurationProvider;
+            }
+        }
     }
 }

src/DefaultBuilder/test/Microsoft.AspNetCore.Tests/WebApplicationTests.cs

@@ -1478,10 +1478,26 @@ public void ConfigurationCanBeReloaded()
             Assert.NotEqual(value0, value1);
         }
 
+        [Fact]
+        public void ConfigurationSourcesAreBuiltOnce()
+        {
+            var builder = WebApplication.CreateBuilder();
+
+            var configSource = new RandomConfigurationSource();
+            ((IConfigurationBuilder)builder.Configuration).Sources.Add(configSource);
+
+            var app = builder.Build();
+
+            Assert.Equal(1, configSource.ProvidersBuilt);
+        }
+
         public class RandomConfigurationSource : IConfigurationSource
         {
+            public int ProvidersBuilt { get; set; }
+
             public IConfigurationProvider Build(IConfigurationBuilder builder)
             {
+                ProvidersBuilt++;
                 return new RandomConfigurationProvider();
             }
         }