Add Vary header to all content API operations (#17962)

* Add Vary header to all content API operations

* Make attribute sealed

src/Umbraco.Cms.Api.Delivery/Controllers/Content/ContentApiControllerBase.cs

@@ -15,6 +15,7 @@ namespace Umbraco.Cms.Api.Delivery.Controllers.Content;
 [ApiExplorerSettings(GroupName = "Content")]
 [LocalizeFromAcceptLanguageHeader]
 [ValidateStartItem]
+[AddVaryHeader]
 [OutputCache(PolicyName = Constants.DeliveryApi.OutputCache.ContentCachePolicy)]
 public abstract class ContentApiControllerBase : DeliveryApiControllerBase
 {

src/Umbraco.Cms.Api.Delivery/Filters/AddVaryHeaderAttribute.cs

@@ -0,0 +1,13 @@
+using Microsoft.AspNetCore.Mvc.Filters;
+
+namespace Umbraco.Cms.Api.Delivery.Filters;
+
+public sealed class AddVaryHeaderAttribute : ActionFilterAttribute
+{
+    private const string Vary = "Accept-Language, Preview, Start-Item";
+
+    public override void OnResultExecuting(ResultExecutingContext context)
+        => context.HttpContext.Response.Headers.Vary = context.HttpContext.Response.Headers.Vary.Count > 0
+            ? $"{context.HttpContext.Response.Headers.Vary}, {Vary}"
+            : Vary;
+}