Ensure response can be buffered if already buffered (#483)

There are times where buffering may be called but it is already buffering and it would throw. Now, if it is already buffering, it will continue to buffer and not throw.

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/Features/HttpResponseAdapterFeature.cs

@@ -58,7 +58,11 @@ void IHttpResponseBodyFeature.DisableBuffering()
 
     void IHttpResponseBufferingFeature.EnableBuffering(int memoryThreshold, long? bufferLimit)
     {
-        if (_state == StreamState.NotStarted)
+        if (_state == StreamState.Buffering)
+        {
+            return;
+        }
+        else if (_state == StreamState.NotStarted)
         {
             Debug.Assert(_bufferedStream is null);
 

test/Microsoft.AspNetCore.SystemWebAdapters.CoreServices.Tests/InputStreamTests.cs

@@ -7,6 +7,8 @@
 using System.Web;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Http.Features;
+using Microsoft.AspNetCore.SystemWebAdapters.Features;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
@@ -82,6 +84,24 @@ public async Task InputStreamWithMetadata()
         Assert.Equal(ContentValue, result);
     }
 
+    [Fact]
+    public async Task BufferMultipleTimes()
+    {
+        // Act
+        var result = await RunAsync(ContentValue, async context =>
+        {
+            var feature = context.AsAspNetCore().Features.GetRequired<IHttpRequestInputStreamFeature>();
+
+            await feature.BufferInputStreamAsync(default);
+            await feature.BufferInputStreamAsync(default);
+
+            context.Request.InputStream.CopyTo(context.Response.OutputStream);
+        });
+
+        // Assert
+        Assert.Equal(ContentValue, result);
+    }
+
     [Fact]
     public async Task InputStreamCanSeek()
     {

test/Microsoft.AspNetCore.SystemWebAdapters.CoreServices.Tests/ResponseStreamTests.cs

@@ -5,9 +5,11 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Text;
+using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.SystemWebAdapters.Features;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
@@ -212,6 +214,25 @@ public async Task MultipleClearContent()
         Assert.Equal("part4", result);
     }
 
+    [Fact]
+    public async Task BufferMultipleTimes()
+    {
+        const string Result = "text";
+
+        // Act
+        var result = await RunAsync(context =>
+        {
+            var feature = context.AsAspNetCore().Features.GetRequired<IHttpResponseBufferingFeature>();
+
+            feature.EnableBuffering(1024, default);
+            feature.EnableBuffering(1024, default);
+
+            context.Response.Write(Result);
+        });
+
+        Assert.Equal(Result, result);
+    }
+
     [Fact]
     public async Task BufferOutputIsNotEnabled()
     {