Prevent synchronous writes when using Razor (#9395)

* Do not perform synchronous writes to the Response TextWriter after a Razor FlushAsync
* Use ViewBuffer to perform async writes to the response when using ViewComponentResult

Related to #6397 

Fixes https://github.com/aspnet/AspNetCore/issues/4885

src/Mvc/Mvc.Razor/src/RazorPageBase.cs

@@ -370,12 +370,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             var encoder = HtmlEncoder;
             if (value is IHtmlContent htmlContent)
             {
-                var bufferedWriter = writer as ViewBufferTextWriter;
+                if (writer is ViewBufferTextWriter bufferedWriter)
-                if (bufferedWriter == null || !bufferedWriter.IsBuffering)
-                {
-                    htmlContent.WriteTo(writer, encoder);
-                }
-                else
                 {
                     if (value is IHtmlContentContainer htmlContentContainer)
                     {
@@ -389,6 +384,10 @@ namespace Microsoft.AspNetCore.Mvc.Razor
                         bufferedWriter.Buffer.AppendHtml(htmlContent);
                     }
                 }
+                else
+                {
+                    htmlContent.WriteTo(writer, encoder);
+                }
 
                 return;
             }

src/Mvc/Mvc.Razor/src/RazorView.cs

@@ -233,7 +233,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             // (including the layout page we just rendered).
             while (!string.IsNullOrEmpty(previousPage.Layout))
             {
-                if (!bodyWriter.IsBuffering)
+                if (bodyWriter.Flushed)
                 {
                     // Once a call to RazorPage.FlushAsync is made, we can no longer render Layout pages - content has
                     // already been written to the client and the layout content would be appended rather than surround
@@ -274,25 +274,22 @@ namespace Microsoft.AspNetCore.Mvc.Razor
                 layoutPage.EnsureRenderedBodyOrSections();
             }
 
-            if (bodyWriter.IsBuffering)
+            // We've got a bunch of content in the view buffer. How to best deal with it
+            // really depends on whether or not we're writing directly to the output or if we're writing to
+            // another buffer.
+            if (context.Writer is ViewBufferTextWriter viewBufferTextWriter)
             {
-                // If IsBuffering - then we've got a bunch of content in the view buffer. How to best deal with it
+                // This means we're writing to another buffer. Use MoveTo to combine them.
-                // really depends on whether or not we're writing directly to the output or if we're writing to
+                bodyWriter.Buffer.MoveTo(viewBufferTextWriter.Buffer);
-                // another buffer.
+            }
-                var viewBufferTextWriter = context.Writer as ViewBufferTextWriter;
+            else
-                if (viewBufferTextWriter == null || !viewBufferTextWriter.IsBuffering)
+            {
+                // This means we're writing to a 'real' writer, probably to the actual output stream.
+                // We're using PagedBufferedTextWriter here to 'smooth' synchronous writes of IHtmlContent values.
+                using (var writer = _bufferScope.CreateWriter(context.Writer))
                 {
-                    // This means we're writing to a 'real' writer, probably to the actual output stream.
+                    await bodyWriter.Buffer.WriteToAsync(writer, _htmlEncoder);
-                    // We're using PagedBufferedTextWriter here to 'smooth' synchronous writes of IHtmlContent values.
+                    await writer.FlushAsync();
-                    using (var writer = _bufferScope.CreateWriter(context.Writer))
-                    {
-                        await bodyWriter.Buffer.WriteToAsync(writer, _htmlEncoder);
-                    }
-                }
-                else
-                {
-                    // This means we're writing to another buffer. Use MoveTo to combine them.
-                    bodyWriter.Buffer.MoveTo(viewBufferTextWriter.Buffer);
                 }
             }
         }

src/Mvc/Mvc.ViewFeatures/ref/Microsoft.AspNetCore.Mvc.ViewFeatures.netcoreapp3.0.cs

@@ -1204,7 +1204,9 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
     }
     public partial class ViewComponentResultExecutor : Microsoft.AspNetCore.Mvc.Infrastructure.IActionResultExecutor<Microsoft.AspNetCore.Mvc.ViewComponentResult>
     {
+        [System.ObsoleteAttribute("This constructor is obsolete and will be removed in a future version.")]
         public ViewComponentResultExecutor(Microsoft.Extensions.Options.IOptions<Microsoft.AspNetCore.Mvc.MvcViewOptions> mvcHelperOptions, Microsoft.Extensions.Logging.ILoggerFactory loggerFactory, System.Text.Encodings.Web.HtmlEncoder htmlEncoder, Microsoft.AspNetCore.Mvc.ModelBinding.IModelMetadataProvider modelMetadataProvider, Microsoft.AspNetCore.Mvc.ViewFeatures.ITempDataDictionaryFactory tempDataDictionaryFactory) { }
+        public ViewComponentResultExecutor(Microsoft.Extensions.Options.IOptions<Microsoft.AspNetCore.Mvc.MvcViewOptions> mvcHelperOptions, Microsoft.Extensions.Logging.ILoggerFactory loggerFactory, System.Text.Encodings.Web.HtmlEncoder htmlEncoder, Microsoft.AspNetCore.Mvc.ModelBinding.IModelMetadataProvider modelMetadataProvider, Microsoft.AspNetCore.Mvc.ViewFeatures.ITempDataDictionaryFactory tempDataDictionaryFactory, Microsoft.AspNetCore.Mvc.Infrastructure.IHttpResponseStreamWriterFactory writerFactory) { }
         [System.Diagnostics.DebuggerStepThroughAttribute]
         public virtual System.Threading.Tasks.Task ExecuteAsync(Microsoft.AspNetCore.Mvc.ActionContext context, Microsoft.AspNetCore.Mvc.ViewComponentResult result) { throw null; }
     }

src/Mvc/Mvc.ViewFeatures/src/Buffers/ViewBufferTextWriter.cs

@@ -86,25 +86,20 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
         /// <inheritdoc />
         public override Encoding Encoding { get; }
 
-        /// <inheritdoc />
-        public bool IsBuffering { get; private set; } = true;
-
         /// <summary>
         /// Gets the <see cref="ViewBuffer"/>.
         /// </summary>
         public ViewBuffer Buffer { get; }
 
+        /// <summary>
+        /// Gets a value that indiciates if <see cref="Flush"/> or <see cref="FlushAsync" /> was invoked.
+        /// </summary>
+        public bool Flushed { get; private set; }
+
         /// <inheritdoc />
         public override void Write(char value)
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(value.ToString());
-            {
-                Buffer.AppendHtml(value.ToString());
-            }
-            else
-            {
-                _inner.Write(value);
-            }
         }
 
         /// <inheritdoc />
@@ -125,14 +120,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
                 throw new ArgumentOutOfRangeException(nameof(count));
             }
 
-            if (IsBuffering)
+            Buffer.AppendHtml(new string(buffer, index, count));
-            {
-                Buffer.AppendHtml(new string(buffer, index, count));
-            }
-            else
-            {
-                _inner.Write(buffer, index, count);
-            }
         }
 
         /// <inheritdoc />
@@ -143,14 +131,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
                 return;
             }
 
-            if (IsBuffering)
+            Buffer.AppendHtml(value);
-            {
-                Buffer.AppendHtml(value);
-            }
-            else
-            {
-                _inner.Write(value);
-            }
         }
 
         /// <inheritdoc />
@@ -186,14 +167,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
                 return;
             }
 
-            if (IsBuffering)
+            Buffer.AppendHtml(value);
-            {
-                Buffer.AppendHtml(value);
-            }
-            else
-            {
-                value.WriteTo(_inner, _htmlEncoder);
-            }
         }
 
         /// <summary>
@@ -207,14 +181,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
                 return;
             }
 
-            if (IsBuffering)
+            value.MoveTo(Buffer);
-            {
-                value.MoveTo(Buffer);
-            }
-            else
-            {
-                value.WriteTo(_inner, _htmlEncoder);
-            }
         }
 
         /// <inheritdoc />
@@ -245,15 +212,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
         /// <inheritdoc />
         public override Task WriteAsync(char value)
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(value.ToString());
-            {
+            return Task.CompletedTask;
-                Buffer.AppendHtml(value.ToString());
-                return Task.CompletedTask;
-            }
-            else
-            {
-                return _inner.WriteAsync(value);
-            }
         }
 
         /// <inheritdoc />
@@ -273,121 +233,64 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
                 throw new ArgumentOutOfRangeException(nameof(count));
             }
 
-            if (IsBuffering)
+            Buffer.AppendHtml(new string(buffer, index, count));
-            {
+            return Task.CompletedTask;
-                Buffer.AppendHtml(new string(buffer, index, count));
-                return Task.CompletedTask;
-            }
-            else
-            {
-                return _inner.WriteAsync(buffer, index, count);
-            }
         }
 
         /// <inheritdoc />
         public override Task WriteAsync(string value)
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(value);
-            {
+            return Task.CompletedTask;
-                Buffer.AppendHtml(value);
-                return Task.CompletedTask;
-            }
-            else
-            {
-                return _inner.WriteAsync(value);
-            }
         }
 
         /// <inheritdoc />
         public override void WriteLine()
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(NewLine);
-            {
-                Buffer.AppendHtml(NewLine);
-            }
-            else
-            {
-                _inner.WriteLine();
-            }
         }
 
         /// <inheritdoc />
         public override void WriteLine(string value)
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(value);
-            {
+            Buffer.AppendHtml(NewLine);
-                Buffer.AppendHtml(value);
-                Buffer.AppendHtml(NewLine);
-            }
-            else
-            {
-                _inner.WriteLine(value);
-            }
         }
 
         /// <inheritdoc />
         public override Task WriteLineAsync(char value)
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(value.ToString());
-            {
+            Buffer.AppendHtml(NewLine);
-                Buffer.AppendHtml(value.ToString());
+            return Task.CompletedTask;
-                Buffer.AppendHtml(NewLine);
-                return Task.CompletedTask;
-            }
-            else
-            {
-                return _inner.WriteLineAsync(value);
-            }
         }
 
         /// <inheritdoc />
         public override Task WriteLineAsync(char[] value, int start, int offset)
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(new string(value, start, offset));
-            {
+            Buffer.AppendHtml(NewLine);
-                Buffer.AppendHtml(new string(value, start, offset));
+            return Task.CompletedTask;
-                Buffer.AppendHtml(NewLine);
+
-                return Task.CompletedTask;
-            }
-            else
-            {
-                return _inner.WriteLineAsync(value, start, offset);
-            }
         }
 
         /// <inheritdoc />
         public override Task WriteLineAsync(string value)
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(value);
-            {
+            Buffer.AppendHtml(NewLine);
-                Buffer.AppendHtml(value);
+            return Task.CompletedTask;
-                Buffer.AppendHtml(NewLine);
-                return Task.CompletedTask;
-            }
-            else
-            {
-                return _inner.WriteLineAsync(value);
-            }
         }
 
         /// <inheritdoc />
         public override Task WriteLineAsync()
         {
-            if (IsBuffering)
+            Buffer.AppendHtml(NewLine);
-            {
+            return Task.CompletedTask;
-                Buffer.AppendHtml(NewLine);
-                return Task.CompletedTask;
-            }
-            else
-            {
-                return _inner.WriteLineAsync();
-            }
         }
 
         /// <summary>
         /// Copies the buffered content to the unbuffered writer and invokes flush on it.
-        /// Additionally causes this instance to no longer buffer and direct all write operations
-        /// to the unbuffered writer.
         /// </summary>
         public override void Flush()
         {
@@ -396,20 +299,16 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
                 return;
             }
 
-            if (IsBuffering)
+            Flushed = true;
-            {
+
-                IsBuffering = false;
+            Buffer.WriteTo(_inner, _htmlEncoder);
-                Buffer.WriteTo(_inner, _htmlEncoder);
+            Buffer.Clear();
-                Buffer.Clear();
-            }
 
             _inner.Flush();
         }
 
         /// <summary>
         /// Copies the buffered content to the unbuffered writer and invokes flush on it.
-        /// Additionally causes this instance to no longer buffer and direct all write operations
-        /// to the unbuffered writer.
         /// </summary>
         /// <returns>A <see cref="Task"/> that represents the asynchronous copy and flush operations.</returns>
         public override async Task FlushAsync()
@@ -419,12 +318,10 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
                 return;
             }
 
-            if (IsBuffering)
+            Flushed = true;
-            {
+
-                IsBuffering = false;
+            await Buffer.WriteToAsync(_inner, _htmlEncoder);
-                await Buffer.WriteToAsync(_inner, _htmlEncoder);
+            Buffer.Clear();
-                Buffer.Clear();
-            }
 
             await _inner.FlushAsync();
         }

src/Mvc/Mvc.ViewFeatures/src/ViewComponentResultExecutor.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.IO;
 using System.Text.Encodings.Web;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Html;
@@ -9,8 +10,8 @@ using Microsoft.AspNetCore.Mvc.Formatters;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 using Microsoft.AspNetCore.Mvc.ModelBinding;
 using Microsoft.AspNetCore.Mvc.Rendering;
+using Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers;
 using Microsoft.AspNetCore.Mvc.ViewFeatures.Filters;
-using Microsoft.AspNetCore.WebUtilities;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
@@ -24,13 +25,26 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
         private readonly ILogger<ViewComponentResult> _logger;
         private readonly IModelMetadataProvider _modelMetadataProvider;
         private readonly ITempDataDictionaryFactory _tempDataDictionaryFactory;
+        private IHttpResponseStreamWriterFactory _writerFactory;
 
+        [Obsolete("This constructor is obsolete and will be removed in a future version.")]
         public ViewComponentResultExecutor(
             IOptions<MvcViewOptions> mvcHelperOptions,
             ILoggerFactory loggerFactory,
             HtmlEncoder htmlEncoder,
             IModelMetadataProvider modelMetadataProvider,
             ITempDataDictionaryFactory tempDataDictionaryFactory)
+            : this(mvcHelperOptions, loggerFactory, htmlEncoder, modelMetadataProvider, tempDataDictionaryFactory, null)
+        {
+        }
+
+        public ViewComponentResultExecutor(
+            IOptions<MvcViewOptions> mvcHelperOptions,
+            ILoggerFactory loggerFactory,
+            HtmlEncoder htmlEncoder,
+            IModelMetadataProvider modelMetadataProvider,
+            ITempDataDictionaryFactory tempDataDictionaryFactory,
+            IHttpResponseStreamWriterFactory writerFactory)
         {
             if (mvcHelperOptions == null)
             {
@@ -62,6 +76,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             _htmlEncoder = htmlEncoder;
             _modelMetadataProvider = modelMetadataProvider;
             _tempDataDictionaryFactory = tempDataDictionaryFactory;
+            _writerFactory = writerFactory;
         }
 
         /// <inheritdoc />
@@ -105,14 +120,9 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 response.StatusCode = result.StatusCode.Value;
             }
 
-            // Opt into sync IO support until we can work out an alternative https://github.com/aspnet/AspNetCore/issues/6397
+            _writerFactory ??= context.HttpContext.RequestServices.GetRequiredService<IHttpResponseStreamWriterFactory>();
-            var syncIOFeature = context.HttpContext.Features.Get<Http.Features.IHttpBodyControlFeature>();
-            if (syncIOFeature != null)
-            {
-                syncIOFeature.AllowSynchronousIO = true;
-            }
 
-            using (var writer = new HttpResponseStreamWriter(response.Body, resolvedContentTypeEncoding))
+            using (var writer = _writerFactory.CreateWriter(response.Body, resolvedContentTypeEncoding))
             {
                 var viewContext = new ViewContext(
                     context,
@@ -127,9 +137,26 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 // IViewComponentHelper is stateful, we want to make sure to retrieve it every time we need it.
                 var viewComponentHelper = context.HttpContext.RequestServices.GetRequiredService<IViewComponentHelper>();
                 (viewComponentHelper as IViewContextAware)?.Contextualize(viewContext);
-
                 var viewComponentResult = await GetViewComponentResult(viewComponentHelper, _logger, result);
-                viewComponentResult.WriteTo(writer, _htmlEncoder);
+
+                if (viewComponentResult is ViewBuffer viewBuffer)
+                {
+                    // In the ordinary case, DefaultViewComponentHelper will return an instance of ViewBuffer. We can simply
+                    // invoke WriteToAsync on it.
+                    await viewBuffer.WriteToAsync(writer, _htmlEncoder);
+                    await writer.FlushAsync();
+                }
+                else
+                {
+                    using var memoryStream = new MemoryStream();
+                    using (var intermediateWriter = _writerFactory.CreateWriter(response.Body, resolvedContentTypeEncoding))
+                    {
+                        viewComponentResult.WriteTo(intermediateWriter, _htmlEncoder);
+                    }
+
+                    memoryStream.Position = 0;
+                    await memoryStream.CopyToAsync(response.Body);
+                }
             }
         }
 

src/Mvc/Mvc.ViewFeatures/test/Buffers/ViewBufferTextWriterTest.cs

@@ -7,7 +7,6 @@ using System.IO;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Html;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.WebEncoders.Testing;
 using Moq;
@@ -19,7 +18,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
     {
         [Fact]
         [ReplaceCulture]
-        public void Write_WritesDataTypes_ToBuffer()
+        public void Write_WritesDataTypes()
         {
             // Arrange
             var expected = new object[] { "True", "3", "18446744073709551615", "Hello world", "3.14", "2.718", "m" };
@@ -41,86 +40,31 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
 
         [Fact]
         [ReplaceCulture]
-        public void Write_WritesDataTypes_ToUnderlyingStream_WhenNotBuffering()
+        public async Task Write_WritesDataTypes_AfterFlush()
         {
             // Arrange
-            var expected = new[] { "True", "3", "18446744073709551615", "Hello world", "3.14", "2.718" };
+            var expected = new object[] { "True", "3", "18446744073709551615", "Hello world", "3.14", "2.718", "m" };
-            var inner = new Mock<TextWriter>();
             var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner.Object);
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8);
-            var testClass = new TestClass();
-
-            // Act
-            writer.Flush();
-            writer.Write(true);
-            writer.Write(3);
-            writer.Write(ulong.MaxValue);
-            writer.Write(testClass);
-            writer.Write(3.14);
-            writer.Write(2.718m);
-
-            // Assert
-            Assert.Equal(0, buffer.Count);
-            foreach (var item in expected)
-            {
-                inner.Verify(v => v.Write(item), Times.Once());
-            }
-        }
-
-        [Fact]
-        [ReplaceCulture]
-        public async Task Write_WritesCharValues_ToUnderlyingStream_WhenNotBuffering()
-        {
-            // Arrange
-            var inner = new Mock<TextWriter> { CallBase = true };
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner.Object);
-            var buffer1 = new[] { 'a', 'b', 'c', 'd' };
-            var buffer2 = new[] { 'd', 'e', 'f' };
-
-            // Act
-            writer.Flush();
-            writer.Write('x');
-            writer.Write(buffer1, 1, 2);
-            writer.Write(buffer2);
-            await writer.WriteAsync(buffer2, 1, 1);
-            await writer.WriteLineAsync(buffer1);
-
-            // Assert
-            inner.Verify(v => v.Write('x'), Times.Once());
-            inner.Verify(v => v.Write(buffer1, 1, 2), Times.Once());
-            inner.Verify(v => v.Write(buffer1, 0, 4), Times.Once());
-            inner.Verify(v => v.Write(buffer2, 0, 3), Times.Once());
-            inner.Verify(v => v.WriteAsync(buffer2, 1, 1), Times.Once());
-            inner.Verify(v => v.WriteLine(), Times.Once());
-        }
-
-        [Fact]
-        [ReplaceCulture]
-        public async Task Write_WritesStringValues_ToUnbufferedStream_WhenNotBuffering()
-        {
-            // Arrange
-            var inner = new Mock<TextWriter>();
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner.Object);
 
             // Act
             await writer.FlushAsync();
-            writer.Write("a");
+
-            writer.WriteLine("ab");
+            writer.Write(true);
-            await writer.WriteAsync("ef");
+            writer.Write(3);
-            await writer.WriteLineAsync("gh");
+            writer.Write(ulong.MaxValue);
+            writer.Write(new TestClass());
+            writer.Write(3.14);
+            writer.Write(2.718m);
+            writer.Write('m');
 
             // Assert
-            inner.Verify(v => v.Write("a"), Times.Once());
+            Assert.Equal(expected, GetValues(buffer));
-            inner.Verify(v => v.WriteLine("ab"), Times.Once());
-            inner.Verify(v => v.WriteAsync("ef"), Times.Once());
-            inner.Verify(v => v.WriteLineAsync("gh"), Times.Once());
         }
 
         [Fact]
         [ReplaceCulture]
-        public void WriteLine_WritesDataTypes_ToBuffer()
+        public void WriteLine_WritesDataTypes()
         {
             // Arrange
             var newLine = Environment.NewLine;
@@ -139,9 +83,11 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
 
         [Fact]
         [ReplaceCulture]
-        public void WriteLine_WritesDataTypes_ToUnbufferedStream_WhenNotBuffering()
+        public void WriteLine_WritesDataType_AfterFlush()
         {
             // Arrange
+            var newLine = Environment.NewLine;
+            var expected = new List<object> { "False", newLine, "1.1", newLine, "3", newLine };
             var inner = new Mock<TextWriter>();
             var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
             var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner.Object);
@@ -153,10 +99,12 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
             writer.WriteLine(3L);
 
             // Assert
-            inner.Verify(v => v.Write("False"), Times.Once());
+            inner.Verify(v => v.Write("False"), Times.Never());
-            inner.Verify(v => v.Write("1.1"), Times.Once());
+            inner.Verify(v => v.Write("1.1"), Times.Never());
-            inner.Verify(v => v.Write("3"), Times.Once());
+            inner.Verify(v => v.Write("3"), Times.Never());
-            inner.Verify(v => v.WriteLine(), Times.Exactly(3));
+            inner.Verify(v => v.WriteLine(), Times.Never());
+
+            Assert.Equal(expected, GetValues(buffer));
         }
 
         [Fact]
@@ -199,25 +147,6 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers
             Assert.Equal<object>(new[] { input1, input2, newLine, input3, input4, newLine }, actual);
         }
 
-        [Fact]
-        public void Write_HtmlContent_AfterFlush_GoesToStream()
-        {
-            // Arrange
-            var inner = new StringWriter();
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner);
-
-            writer.Flush();
-
-            var content = new HtmlString("Hello, world!");
-
-            // Act
-            writer.Write(content);
-
-            // Assert
-            Assert.Equal("Hello, world!", inner.ToString());
-        }
-
         private static object[] GetValues(ViewBuffer buffer)
         {
             var pages = new List<ViewBufferPage>();

src/Mvc/Mvc.ViewFeatures/test/ViewComponentResultTest.cs

@@ -397,6 +397,48 @@ namespace Microsoft.AspNetCore.Mvc
             Assert.Equal("Hello, World!", body);
         }
 
+        [Fact]
+        public async Task ExecuteResultAsync_WithCustomViewComponentHelper()
+        {
+            // Arrange
+            var expected = "Hello from custom helper";
+            var methodInfo = typeof(TextViewComponent).GetMethod(nameof(TextViewComponent.Invoke));
+            var descriptor = new ViewComponentDescriptor()
+            {
+                FullName = "Full.Name.Text",
+                ShortName = "Text",
+                TypeInfo = typeof(TextViewComponent).GetTypeInfo(),
+                MethodInfo = methodInfo,
+                Parameters = methodInfo.GetParameters(),
+            };
+            var result = Task.FromResult<IHtmlContent>(new HtmlContentBuilder().AppendHtml(expected));
+
+            var helper = Mock.Of<IViewComponentHelper>(h => h.InvokeAsync(It.IsAny<Type>(), It.IsAny<object>()) == result);
+
+            var httpContext = new DefaultHttpContext();
+            var services = CreateServices(diagnosticListener: null, httpContext, new[] { descriptor });
+            services.AddSingleton<IViewComponentHelper>(helper);
+
+            httpContext.RequestServices = services.BuildServiceProvider();
+            httpContext.Response.Body = new MemoryStream();
+
+            var actionContext = new ActionContext(httpContext, new RouteData(), new ActionDescriptor());
+
+            var viewComponentResult = new ViewComponentResult()
+            {
+                Arguments = new { name = "World!" },
+                ViewComponentType = typeof(TextViewComponent),
+                TempData = _tempDataDictionary,
+            };
+
+            // Act
+            await viewComponentResult.ExecuteResultAsync(actionContext);
+
+            // Assert
+            var body = ReadBody(actionContext.HttpContext.Response);
+            Assert.Equal(expected, body);
+        }
+
         [Fact]
         public async Task ExecuteResultAsync_SetsStatusCode()
         {
@@ -600,6 +642,7 @@ namespace Microsoft.AspNetCore.Mvc
             services.AddSingleton<HtmlEncoder, HtmlTestEncoder>();
             services.AddSingleton<IViewBufferScope, TestViewBufferScope>();
             services.AddSingleton<IActionResultExecutor<ViewComponentResult>, ViewComponentResultExecutor>();
+            services.AddSingleton<IHttpResponseStreamWriterFactory, TestHttpResponseStreamWriterFactory>();
 
             return services;
         }
@@ -625,7 +668,6 @@ namespace Microsoft.AspNetCore.Mvc
             return CreateActionContext(null, descriptors);
         }
 
-
         private class FixedSetViewComponentDescriptorProvider : IViewComponentDescriptorProvider
         {
             private readonly ViewComponentDescriptor[] _descriptors;

src/Mvc/test/Mvc.FunctionalTests/FlushPointTest.cs

@@ -35,6 +35,33 @@ RenderBody content
             Assert.Equal(expected, body, ignoreLineEndingDifferences: true);
         }
 
+        [Fact]
+        public async Task FlushFollowedByLargeContent()
+        {
+            // Arrange
+            var expected = new string('a', 1024 * 1024);
+
+            // Act
+            var document = await Client.GetHtmlDocumentAsync("http://localhost/FlushPoint/FlushFollowedByLargeContent");
+
+            // Assert
+            var largeContent = document.RequiredQuerySelector("#large-content");
+            Assert.StartsWith(expected, largeContent.TextContent);
+        }
+
+        [Fact]
+        public async Task FlushInvokedInComponent()
+        {
+            var expected = new string('a', 1024 * 1024);
+
+            // Act
+            var document = await Client.GetHtmlDocumentAsync("http://localhost/FlushPoint/FlushInvokedInComponent");
+
+            // Assert
+            var largeContent = document.RequiredQuerySelector("#large-content");
+            Assert.StartsWith(expected, largeContent.TextContent);
+        }
+
         [Fact]
         public async Task FlushPointsAreExecutedForPagesWithoutLayouts()
         {

src/Mvc/test/WebSites/RazorWebSite/Components/ComponentWithFlush.cs

@@ -0,0 +1,16 @@
+// 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 Microsoft.AspNetCore.Mvc;
+
+namespace MvcSample.Web.Components
+{
+    [ViewComponent(Name = "ComponentWithFlush")]
+    public class ComponentWithFlush : ViewComponent
+    {
+        public IViewComponentResult Invoke()
+        {
+            return View();
+        }
+    }
+}

src/Mvc/test/WebSites/RazorWebSite/Controllers/FlushPoint.cs

@@ -12,6 +12,10 @@ namespace RazorWebSite
             return View();
         }
 
+        public IActionResult FlushFollowedByLargeContent() => View();
+
+        public IActionResult FlushInvokedInComponent() => View();
+
         public IActionResult PageWithoutLayout()
         {
             return View();

src/Mvc/test/WebSites/RazorWebSite/Views/FlushPoint/FlushFollowedByLargeContent.cshtml

@@ -0,0 +1,7 @@
+Header content
+@{
+    await FlushAsync();
+    var largeContent = new string('a', 1024 * 1024);
+}
+
+<span id="large-content">@largeContent</span>

src/Mvc/test/WebSites/RazorWebSite/Views/FlushPoint/FlushInvokedInComponent.cshtml

@@ -0,0 +1 @@
+@await Component.InvokeAsync("ComponentWithFlush")

src/Mvc/test/WebSites/RazorWebSite/Views/Shared/Components/ComponentWithFlush/Default.cshtml

@@ -0,0 +1,8 @@
+Hello from component
+@{
+    await FlushAsync();
+    var largeContent = new string('a', 1024 * 1024);
+}
+
+<span id="large-content">@largeContent</span>
+