Output Caching: Correctly gate auto-registration of UseOutputCache() middleware

[AndyButland]

Description

In 17.4 we centralised the registration of the ASP.NET Core output cache middleware in UmbracoApplicationBuilder.RegisterDefaultRequiredMiddleware(), gated on the presence of IOutputCacheStore in the container. That gate is too broad — IOutputCacheStore is registered by any call to services.AddOutputCache(...), including calls made by the consuming application for its own (non-Umbraco) output caching needs.

The result: applications that had their own services.AddOutputCache(...) + app.UseOutputCache() in Program.cs (working fine pre-17.4) now get a second UseOutputCache() registration added by Umbraco, and requests fail with:

InvalidOperationException: Another instance of OutputCacheFeature already exists.
Only one instance of OutputCacheMiddleware can be configured for an application.

Fix

Replace the IOutputCacheStore gate with a dedicated marker, IUmbracoManagedOutputCacheMarker, that is registered only when Umbraco itself enables output caching — via Umbraco:CMS:Website:OutputCache:Enabled or Umbraco:CMS:DeliveryApi:OutputCache:Enabled. Applications calling services.AddOutputCache(...) for their own purposes are no longer affected by Umbraco's automatic middleware registration.

How to test

Reproduces with the UmbracoNewsDashboard sample project on 17.4.0:

1. services.AddOutputCache(...) + app.UseOutputCache() in Program.cs, no Umbraco output cache settings enabled → InvalidOperationException on every request before this fix.
2. After this fix → caching works, no exception, no behaviour change.

[claude]

Claude finished @AndyButland's task in 1m 52s —— View job

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PR Review

Target: origin/release/17.4.1 · Based on commit: 6ce7c8c9

Introduces IUmbracoManagedOutputCacheMarker (Core layer) to gate Umbraco's automatic UseOutputCache() middleware registration on its own caching being configured, rather than on the presence of IOutputCacheStore which any application-level AddOutputCache(...) call would also register.

• Modified public API: Added IUmbracoManagedOutputCacheMarker (interface) and UmbracoManagedOutputCacheMarker (class) to Umbraco.Cms.Core.DependencyInjection
• Breaking changes: None — purely additive public API, internal gate condition changed

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Suggestions

• src/Umbraco.Core/DependencyInjection/IUmbracoManagedOutputCacheMarker.cs:1: Both the interface and concrete class live in the same file. This matches the existing IBackOfficeEnabledMarker convention exactly, so no action needed — just noting the intentional deviation from "one class per file" for reviewers.

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Approved

The fix is correct and tightly scoped. The new marker pattern is a direct analogue of IBackOfficeEnabledMarker (same file, same structure, same namespace), so it's idiomatic for this codebase. TryAddSingleton is the right choice — it prevents double-registration when both Website and Delivery API caching are enabled simultaneously, and the first registration wins (which is fine since the marker carries no state). The old IOutputCacheStore gate was a legitimate bug: any application calling services.AddOutputCache(...) for its own purposes would have inadvertently triggered Umbraco's UseOutputCache() registration. No breaking changes detected.

Labels applied: area/backend, category/api

[Copilot]

Pull request overview

This PR adjusts Umbraco’s automatic UseOutputCache() middleware registration so it only happens when Umbraco itself has enabled output caching (via Website template caching or Delivery API caching), avoiding unintended middleware registration when an application calls services.AddOutputCache(...) for its own purposes.

Changes:

• Introduces an IUmbracoManagedOutputCacheMarker service as the explicit signal that Umbraco-managed output caching is enabled.
• Registers the marker when Website and/or Delivery API output caching is configured.
• Updates UmbracoApplicationBuilder to gate UseOutputCache() on the marker instead of IOutputCacheStore.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/Umbraco.Web.Website/DependencyInjection/UmbracoBuilderExtensions.cs	Registers the marker when Website output caching is enabled.
src/Umbraco.Web.Common/ApplicationBuilder/UmbracoApplicationBuilder.cs	Switches middleware auto-registration gate from IOutputCacheStore to the new marker.
src/Umbraco.Core/DependencyInjection/IUmbracoManagedOutputCacheMarker.cs	Adds the marker interface + implementation used to signal Umbraco-managed output caching.
src/Umbraco.Cms.Api.Delivery/DependencyInjection/UmbracoBuilderExtensions.cs	Registers the marker when Delivery API output caching is enabled.

[NillasKA]

Looks good and tests good to me!