Fix/action forward loop guard

[LiamBoz]

What?

Fixes #84504

Add a minimal loop-detection guard to createForwardedActionResponse so a Server Action request that has already been forwarded (marked by x-action-forwarded=1) is not forwarded again. Instead, we return early (no second forward).

• File: packages/next/src/server/app-render/action-handler.ts
• Behavior change: if x-action-forwarded=1 is present on the incoming request, return from createForwardedActionResponse without forwarding again.

Why?

In apps that use Middleware rewrites, a forwarded Server Action request can be rewritten to a path that triggers forwarding again, causing an infinite loop (e.g. /rewrite/other → /rewrite/... → /rewrite/rewrite/... → …). This leads to repeated logs and a server action that never resolves.

This guard breaks the cycle with very low risk by only affecting requests that were already forwarded once.

• Repro repo: https://github.com/blairmcalpine/forwarded-server-action-request-loop-repro
• Steps:
  1. npm i && npm run dev
  2. Visit http://localhost:3000
  3. Click Run action, then quickly (<500ms) click Navigate to other page
  4. Before this change: infinite loop/log spam; action never resolves.
  5. After this change: no loop; request is not re-forwarded again.

How?

At the start of createForwardedActionResponse, inspect the incoming headers:

• If x-action-forwarded=1 is present → return (do not forward again).
• Otherwise, proceed as before and set x-action-forwarded=1 on the outgoing forwarded request.

Works in both Node and Edge:

• If req.headers is already a Headers (Edge), use it.
• Otherwise adapt Node headers via HeadersAdapter.

Test Plan

Manual (using the public repro)

• With this patch built and packed into the repro app:
  • Trigger the fast Run action → Navigate sequence.
  • Confirm:
    • No repeated /rewrite/... requests or log spam.
    • The page no longer gets stuck due to an infinite forwarding loop.

Notes: This PR intentionally focuses on loop prevention. A broader follow-up could make forwarded requests mirror the pre-rewrite target to ensure resolution semantics remain identical under rewrites. That would be a separate change.

Risk

Low:

• Only affects requests that already bear x-action-forwarded=1.
• Does not change the initial legitimate forward.
• Keeps all other server action behavior intact.

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For Contributors Checklist

• Bug fix
• Related issue linked (Fixes #84504)
• Repro steps included (public repo)
• Ran pnpm prettier-fix locally
• Tests: this is a minimal guard; happy to add a small e2e that simulates a rewrite + quick navigation if maintainers prefer (guidance welcome)

[ijjk]

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