Use System.Text.Json's IAsyncEnumerable support (#31894)

## Description

System.Text.Json recently added support for streaming IAsyncEnumerable types based on an ask from the ASP.NET Core team. This PR enables MVC to leverage this feature.

## Customer Impact
Using IAsyncEnumerable with System.Text.Json will result in data to be streamed rather than buffered and serialized.

## Regression?
- [ ] Yes
- [x] No

[If yes, specify the version the behavior has regressed from]

## Risk
- [ ] High
- [ ] Medium
- [x] Low

[Justify the selection above]
The feature has been well-tested in the runtime. This is simply removing additional buffering that was previously performed by MVC before invoking the serializer.

## Verification
- [x] Manual (required)
- [x] Automated

## Packaging changes reviewed?
- [ ] Yes
- [x] No
- [ ] N/A


Addresses #11558 #23203

global.json

@@ -1,9 +1,9 @@
 {
   "sdk": {
-    "version": "6.0.100-preview.3.21168.19"
+    "version": "6.0.100-preview.4.21216.8"
   },
   "tools": {
-    "dotnet": "6.0.100-preview.3.21168.19",
+    "dotnet": "6.0.100-preview.4.21216.8",
     "runtimes": {
       "dotnet/x64": [
         "2.1.25",

src/Components/WebAssembly/testassets/WasmLinkerTest/WasmLinkerTest.csproj

@@ -3,6 +3,7 @@
     <TargetFramework>$(DefaultNetCoreTargetFramework)</TargetFramework>
     <RuntimeIdentifier>browser-wasm</RuntimeIdentifier>
     <SelfContained>true</SelfContained>
+    <UseMonoRuntime>true</UseMonoRuntime>
   </PropertyGroup>
   <ItemGroup>
     <Reference Include="Microsoft.AspNetCore.Metadata" />

src/Mvc/Mvc.Core/src/Infrastructure/ObjectResultExecutor.cs

@@ -21,8 +21,6 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
     /// </summary>
     public class ObjectResultExecutor : IActionResultExecutor<ObjectResult>
     {
-        private readonly AsyncEnumerableReader _asyncEnumerableReaderFactory;
-
         /// <summary>
         /// Creates a new <see cref="ObjectResultExecutor"/>.
         /// </summary>
@@ -54,8 +52,6 @@ public ObjectResultExecutor(
             FormatterSelector = formatterSelector;
             WriterFactory = writerFactory.CreateWriter;
             Logger = loggerFactory.CreateLogger<ObjectResultExecutor>();
-            var options = mvcOptions?.Value ?? throw new ArgumentNullException(nameof(mvcOptions));
-            _asyncEnumerableReaderFactory = new AsyncEnumerableReader(options);
         }
 
         /// <summary>
@@ -103,23 +99,9 @@ public virtual Task ExecuteAsync(ActionContext context, ObjectResult result)
             }
 
             var value = result.Value;
-
-            if (value != null && _asyncEnumerableReaderFactory.TryGetReader(value.GetType(), out var reader))
-            {
-                return ExecuteAsyncEnumerable(context, result, value, reader);
-            }
-
             return ExecuteAsyncCore(context, result, objectType, value);
         }
 
-        private async Task ExecuteAsyncEnumerable(ActionContext context, ObjectResult result, object asyncEnumerable, Func<object, Task<ICollection>> reader)
-        {
-            Log.BufferingAsyncEnumerable(Logger, asyncEnumerable);
-
-            var enumerated = await reader(asyncEnumerable);
-            await ExecuteAsyncCore(context, result, enumerated.GetType(), enumerated);
-        }
-
         private Task ExecuteAsyncCore(ActionContext context, ObjectResult result, Type? objectType, object? value)
         {
             var formatterContext = new OutputFormatterWriteContext(
@@ -166,21 +148,7 @@ private static void InferContentTypes(ActionContext context, ObjectResult result
             }
         }
 
-        private static class Log
-        {
-            private static readonly Action<ILogger, string?, Exception?> _bufferingAsyncEnumerable = LoggerMessage.Define<string?>(
-                LogLevel.Debug,
-                new EventId(1, "BufferingAsyncEnumerable"),
-                "Buffering IAsyncEnumerable instance of type '{Type}'.",
-                skipEnabledCheck: true);
-
-            public static void BufferingAsyncEnumerable(ILogger logger, object asyncEnumerable)
-            {
-                if (logger.IsEnabled(LogLevel.Debug))
-                {
-                    _bufferingAsyncEnumerable(logger, asyncEnumerable.GetType().FullName, null);
-                }
-            }
-        }
+        // Removed Log.
+        // new EventId(1, "BufferingAsyncEnumerable")
     }
 }

src/Mvc/Mvc.Core/src/Infrastructure/SystemTextJsonResultExecutor.cs

@@ -69,12 +69,6 @@ public async Task ExecuteAsync(ActionContext context, JsonResult result)
             Log.JsonResultExecuting(_logger, result.Value);
 
             var value = result.Value;
-            if (value != null && _asyncEnumerableReaderFactory.TryGetReader(value.GetType(), out var reader))
-            {
-                Log.BufferingAsyncEnumerable(_logger, value);
-                value = await reader(value);
-            }
-
             var objectType = value?.GetType() ?? typeof(object);
 
             // Keep this code in sync with SystemTextJsonOutputFormatter
@@ -147,11 +141,7 @@ private static class Log
                 "Executing JsonResult, writing value of type '{Type}'.",
                 skipEnabledCheck: true);
 
-            private static readonly Action<ILogger, string?, Exception?> _bufferingAsyncEnumerable = LoggerMessage.Define<string?>(
-               LogLevel.Debug,
-               new EventId(2, "BufferingAsyncEnumerable"),
-               "Buffering IAsyncEnumerable instance of type '{Type}'.",
-                skipEnabledCheck: true);
+            // EventId 2 BufferingAsyncEnumerable
 
             public static void JsonResultExecuting(ILogger logger, object? value)
             {
@@ -161,14 +151,6 @@ public static void JsonResultExecuting(ILogger logger, object? value)
                     _jsonResultExecuting(logger, type, null);
                 }
             }
-
-            public static void BufferingAsyncEnumerable(ILogger logger, object asyncEnumerable)
-            {
-                if (logger.IsEnabled(LogLevel.Debug))
-                {
-                    _bufferingAsyncEnumerable(logger, asyncEnumerable.GetType().FullName, null);
-                }
-            }
         }
     }
 }

src/Mvc/Mvc.Core/test/Formatters/SystemTextJsonOutputFormatterTest.cs

@@ -1,8 +1,10 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// 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.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Text;
 using System.Text.Json;
 using System.Text.Json.Serialization;
@@ -81,6 +83,36 @@ public async Task WriteResponseBodyAsync_WithNonUtf8Encoding_FormattingErrorsAre
             await Assert.ThrowsAsync<TimeZoneNotFoundException>(() => formatter.WriteResponseBodyAsync(outputFormatterContext, Encoding.GetEncoding("utf-16")));
         }
 
+        [Fact]
+        public async Task WriteResponseBodyAsync_ForLargeAsyncEnumerable()
+        {
+            // Arrange
+            var expected = new MemoryStream();
+            await JsonSerializer.SerializeAsync(expected, LargeAsync(), new JsonSerializerOptions(JsonSerializerDefaults.Web));
+            var formatter = GetOutputFormatter();
+            var mediaType = MediaTypeHeaderValue.Parse("application/json; charset=utf-8");
+            var encoding = CreateOrGetSupportedEncoding(formatter, "utf-8", isDefaultEncoding: true);
+
+            var body = new MemoryStream();
+            var actionContext = GetActionContext(mediaType, body);
+
+            var asyncEnumerable = LargeAsync();
+            var outputFormatterContext = new OutputFormatterWriteContext(
+                actionContext.HttpContext,
+                new TestHttpResponseStreamWriterFactory().CreateWriter,
+                asyncEnumerable.GetType(),
+                asyncEnumerable)
+            {
+                ContentType = new StringSegment(mediaType.ToString()),
+            };
+
+            // Act
+            await formatter.WriteResponseBodyAsync(outputFormatterContext, Encoding.GetEncoding("utf-8"));
+
+            // Assert
+            Assert.Equal(expected.ToArray(), body.ToArray());
+        }
+
         private class Person
         {
             public string Name { get; set; }
@@ -108,5 +140,15 @@ public override void Write(Utf8JsonWriter writer, ThrowingFormatterModel value,
                 throw new TimeZoneNotFoundException();
             }
         }
+
+        private static async IAsyncEnumerable<int> LargeAsync()
+        {
+            await Task.Yield();
+            // MvcOptions.MaxIAsyncEnumerableBufferLimit is 8192. Pick some value larger than that.
+            foreach (var i in Enumerable.Range(0, 9000))
+            {
+                yield return i;
+            }
+        }
     }
 }

src/Mvc/Mvc.Core/test/Infrastructure/AsyncEnumerableReaderTest.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// 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;
@@ -190,6 +190,19 @@ public async Task Reader_ThrowsIfBufferLimitIsReached()
             Assert.Equal(expected, ex.Message);
         }
 
+        [Fact]
+        public async Task Reader_ThrowsIfIAsyncEnumerableThrows()
+        {
+            // Arrange
+            var enumerable = ThrowingAsyncEnumerable();
+            var options = new MvcOptions();
+            var readerFactory = new AsyncEnumerableReader(options);
+
+            // Act & Assert
+            Assert.True(readerFactory.TryGetReader(enumerable.GetType(), out var reader));
+            await Assert.ThrowsAsync<TimeZoneNotFoundException>(() => reader(enumerable));
+        }
+
         public static async IAsyncEnumerable<string> TestEnumerable(int count = 3)
         {
             await Task.Yield();
@@ -225,5 +238,18 @@ public IAsyncEnumerator<string> GetAsyncEnumerator(CancellationToken cancellatio
             IAsyncEnumerator<object> IAsyncEnumerable<object>.GetAsyncEnumerator(CancellationToken cancellationToken)
                 => GetAsyncEnumerator(cancellationToken);
         }
+
+        private static async IAsyncEnumerable<string> ThrowingAsyncEnumerable()
+        {
+            await Task.Yield();
+            for (var i = 0; i < 10; i++)
+            {
+                yield return $"Hello {i}";
+                if (i == 5)
+                {
+                   throw new TimeZoneNotFoundException();
+                }
+            }
+        }
     }
 }

src/Mvc/Mvc.Core/test/Infrastructure/JsonResultExecutorTestBase.cs

@@ -333,7 +333,7 @@ public async Task ExecuteAsync_WithNullValue()
         public async Task ExecuteAsync_SerializesAsyncEnumerables()
         {
             // Arrange
-            var expected = Encoding.UTF8.GetBytes(JsonSerializer.Serialize(new[] { "Hello", "world" }));
+            var expected = JsonSerializer.Serialize(new[] { "Hello", "world" });
 
             var context = GetActionContext();
             var result = new JsonResult(TestAsyncEnumerable());
@@ -344,7 +344,7 @@ public async Task ExecuteAsync_SerializesAsyncEnumerables()
 
             // Assert
             var written = GetWrittenBytes(context.HttpContext);
-            Assert.Equal(expected, written);
+            Assert.Equal(expected, Encoding.UTF8.GetString(written));
         }
 
         [Fact]

src/Mvc/Mvc.Core/test/Infrastructure/ObjectResultExecutorTest.cs

@@ -459,106 +459,6 @@ public async Task ObjectResult_NullValue()
             Assert.Null(formatterContext.Object);
         }
 
-        [Fact]
-        public async Task ObjectResult_ReadsAsyncEnumerables()
-        {
-            // Arrange
-            var executor = CreateExecutor();
-            var result = new ObjectResult(AsyncEnumerable());
-            var formatter = new TestJsonOutputFormatter();
-            result.Formatters.Add(formatter);
-
-            var actionContext = new ActionContext()
-            {
-                HttpContext = GetHttpContext(),
-            };
-
-            // Act
-            await executor.ExecuteAsync(actionContext, result);
-
-            // Assert
-            var formatterContext = formatter.LastOutputFormatterContext;
-            Assert.Equal(typeof(List<string>), formatterContext.ObjectType);
-            var value = Assert.IsType<List<string>>(formatterContext.Object);
-            Assert.Equal(new[] { "Hello 0", "Hello 1", "Hello 2", "Hello 3", }, value);
-        }
-
-        [Fact]
-        public async Task ObjectResult_Throws_IfEnumerableThrows()
-        {
-            // Arrange
-            var executor = CreateExecutor();
-            var result = new ObjectResult(AsyncEnumerable(throwError: true));
-            var formatter = new TestJsonOutputFormatter();
-            result.Formatters.Add(formatter);
-
-            var actionContext = new ActionContext()
-            {
-                HttpContext = GetHttpContext(),
-            };
-
-            // Act & Assert
-            await Assert.ThrowsAsync<TimeZoneNotFoundException>(() => executor.ExecuteAsync(actionContext, result));
-        }
-
-        [Fact]
-        public async Task ObjectResult_AsyncEnumeration_AtLimit()
-        {
-            // Arrange
-            var count = 24;
-            var executor = CreateExecutor(options: new MvcOptions { MaxIAsyncEnumerableBufferLimit = count });
-            var result = new ObjectResult(AsyncEnumerable(count: count));
-            var formatter = new TestJsonOutputFormatter();
-            result.Formatters.Add(formatter);
-
-            var actionContext = new ActionContext()
-            {
-                HttpContext = GetHttpContext(),
-            };
-
-            // Act
-            await executor.ExecuteAsync(actionContext, result);
-
-            // Assert
-            var formatterContext = formatter.LastOutputFormatterContext;
-            var value = Assert.IsType<List<string>>(formatterContext.Object);
-            Assert.Equal(24, value.Count);
-        }
-
-        [Theory]
-        [InlineData(25)]
-        [InlineData(1024)]
-        public async Task ObjectResult_Throws_IfEnumerationExceedsLimit(int count)
-        {
-            // Arrange
-            var executor = CreateExecutor(options: new MvcOptions { MaxIAsyncEnumerableBufferLimit = 24 });
-            var result = new ObjectResult(AsyncEnumerable(count: count));
-            var formatter = new TestJsonOutputFormatter();
-            result.Formatters.Add(formatter);
-
-            var actionContext = new ActionContext()
-            {
-                HttpContext = GetHttpContext(),
-            };
-
-            // Act & Assert
-            var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => executor.ExecuteAsync(actionContext, result));
-        }
-
-        private static async IAsyncEnumerable<string> AsyncEnumerable(int count = 4, bool throwError = false)
-        {
-            await Task.Yield();
-            for (var i = 0; i < count; i++)
-            {
-                yield return $"Hello {i}";
-            }
-
-            if (throwError)
-            {
-                throw new TimeZoneNotFoundException();
-            }
-        }
-
         private static IServiceCollection CreateServices()
         {
             var services = new ServiceCollection();