[OpenApi] Fix thread static use and write directly to PipeWriter (#61700)

Author: BrennanConroy

src/OpenApi/src/Extensions/OpenApiEndpointRouteBuilderExtensions.cs

@@ -53,29 +53,23 @@ public static IEndpointConventionBuilder MapOpenApi(this IEndpointRouteBuilder e
                 {
                     var document = await documentService.GetOpenApiDocumentAsync(context.RequestServices, context.Request, context.RequestAborted);
                     var documentOptions = options.Get(lowercasedDocumentName);
-                    using var output = MemoryBufferWriter.Get();
-                    using var writer = Utf8BufferTextWriter.Get(output);
-                    try
+
+                    using var writer = new Utf8BufferTextWriter();
+                    writer.SetWriter(context.Response.BodyWriter);
+
+                    if (UseYaml(pattern))
                     {
-                        if (UseYaml(pattern))
-                        {
-                            await document.SerializeAsync(new OpenApiYamlWriter(writer), documentOptions.OpenApiVersion);
-                            context.Response.ContentType = "text/plain+yaml;charset=utf-8";
-                        }
-                        else
-                        {
-                            await document.SerializeAsync(new OpenApiJsonWriter(writer), documentOptions.OpenApiVersion);
-                            context.Response.ContentType = "application/json;charset=utf-8";
-                        }
-                        await context.Response.BodyWriter.WriteAsync(output.ToArray(), context.RequestAborted);
-                        await context.Response.BodyWriter.FlushAsync(context.RequestAborted);
+                        context.Response.ContentType = "text/plain+yaml;charset=utf-8";
+                        await context.Response.StartAsync();
+                        await document.SerializeAsync(new OpenApiYamlWriter(writer), documentOptions.OpenApiVersion, context.RequestAborted);
                     }
-                    finally
+                    else
                     {
-                        MemoryBufferWriter.Return(output);
-                        Utf8BufferTextWriter.Return(writer);
+                        context.Response.ContentType = "application/json;charset=utf-8";
+                        await context.Response.StartAsync();
+                        await document.SerializeAsync(new OpenApiJsonWriter(writer), documentOptions.OpenApiVersion, context.RequestAborted);
                     }
-
+                    await context.Response.BodyWriter.FlushAsync(context.RequestAborted);
                 }
             }).ExcludeFromDescription();
     }