Allow sync IO for NewtonsoftJsonOutputFormatter #8302

src/Mvc/Mvc.NewtonsoftJson/src/NewtonsoftJsonOutputFormatter.cs

@@ -111,6 +111,13 @@ public override async Task WriteResponseBodyAsync(OutputFormatterWriteContext co
                 throw new ArgumentNullException(nameof(selectedEncoding));
             }
 
+            // Opt into sync IO support until we can work out an alternative https://github.com/aspnet/AspNetCore/issues/6397
+            var syncIOFeature = context.HttpContext.Features.Get<Http.Features.IHttpBodyControlFeature>();
+            if (syncIOFeature != null)
+            {
+                syncIOFeature.AllowSynchronousIO = true;
+            }
+
             var response = context.HttpContext.Response;
             using (var writer = context.WriterFactory(response.Body, selectedEncoding))
             {

src/Mvc/test/Mvc.FunctionalTests/JsonOutputFormatterTests.cs

@@ -32,7 +32,7 @@ public async Task JsonOutputFormatter_ReturnsIndentedJson()
             {
                 Id = 1,
                 Alias = "john",
-                description = "Administrator",
+                description = "This is long so we can test large objects " + new string('a', 1024 * 65),
                 Designation = "Administrator",
                 Name = "John Williams"
             };

src/Mvc/test/WebSites/FormatterWebSite/Controllers/JsonFormatterController.cs

@@ -32,7 +32,7 @@ public IActionResult ReturnsIndentedJson()
             {
                 Id = 1,
                 Alias = "john",
-                description = "Administrator",
+                description = "This is long so we can test large objects " + new string('a', 1024 * 65),
                 Designation = "Administrator",
                 Name = "John Williams"
             };