Infinite loop redirecting fix

[DovydasGusarovas]

Background
In certain edge cases, the 404handler was causing an infinite redirect loop resulting in long, repeated paths like:

/om-brand/hvorfor-vaelge-os/hvorfor-vaelge-os/hvorfor-vaelge-os/.../rabataftaler/fdm/foo

This occurred when:

A redirect exists for a "not found" URL (e.g., /brand/.../foo)

The redirect target URL (NewUrl) matched the referrer of the current request

The logic did not properly prevent redirection back to the referrer, which triggered another 404, causing a loop

✅ What's been fixed
The logic in HandleRequest() already guards against redirect loops where:

NewUrl == urlNotFound.PathAndQuery ✅

This PR adds an additional safeguard:

Block redirect if NewUrl == referrer.PathAndQuery ❗

This prevents redirecting back to the source, which would repeat the same failure and retry cycle indefinitely.

🔬 What was added
A new unit test:

HandleRequest_returns_false_when_redirect_matches_referrer()

Ensures HandleRequest() returns false if the redirect would loop back to the referring URL

The logic to block referrer-based loops already existed, this test explicitly verifies that behavior and protects it against regression

🔄 Why it matters
Prevents unnecessary 404 redirects and infinite loops

Protects end users and search crawlers from broken navigation

Saves server resources and ensures cleaner error handling

Makes redirect logic more robust in content-rich environments where URL overlaps can happen

[gemini-code-assist]

Summary of Changes

Hello @DovydasGusarovas, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the stability and reliability of the 404 handler by addressing potential infinite redirect loops. My changes introduce explicit checks to prevent redirects that would lead back to the referring URL, and also ensure that only one redirect attempt is made per request, making the system more resilient to complex redirection scenarios and misconfigurations.

Highlights

• Infinite Loop Prevention (Referrer-based): I've implemented a crucial safeguard within the HandleRequest method to prevent infinite redirect loops. Specifically, if a determined redirect's target URL (NewUrl) matches the referrer URL of the current request, the redirect will now be blocked. This addresses edge cases where a 404 handler could redirect back to the source, causing an endless cycle of 404s and redirects.
• Single Request Redirect Limit: To further enhance robustness, I've added a mechanism in the Handle method to ensure that a redirect is only attempted once per HTTP request. A new RedirectedOnceKey is used in the HttpContext.Items collection to track if a redirect has already been processed for the current request, preventing redundant or recursive redirect attempts within the same request lifecycle.
• New Unit Test Coverage: A new unit test, HandleRequest_returns_false_when_redirect_matches_referrer(), has been added to explicitly verify the new referrer-based loop prevention logic. This ensures the intended behavior is correctly implemented and protected against future regressions.

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[gemini-code-assist]

Code Review

This pull request effectively fixes a potential infinite redirect loop by adding checks to prevent redirecting to the same URL or back to the referrer. It also introduces a safeguard against re-entrant calls to the handler within the same request. The changes are logical and include a new unit test to cover the fix. My review includes a few suggestions to improve logging clarity and code style.

[gemini-code-assist]

This line has a trailing whitespace character. Please remove it to maintain a consistent code style.

LogDebug("Handled saved URL", context);

[DovydasGusarovas]

@jevgenijsp are you up for some code review? Thanks in advance