fix: handle notFound in Suspense with cacheComponents enabled

[Rani367]

What?

This PR fixes an issue where notFound() (and other HTTP access fallback errors like forbidden()/unauthorized()) would cause "Connection closed" errors when cacheComponents: true is enabled and the page has a Suspense boundary with an async component.

Why?

When cacheComponents is enabled and a page is prerendered, subsequent requests use DynamicState.DATA mode which only sends RSC data without re-rendering the HTML shell. However, when notFound() is thrown during this RSC render, the prerendered HTML doesn't contain the not-found component, causing the client to receive an incomplete stream and display "Connection closed" errors.

How?

This fix buffers the RSC stream in DynamicState.DATA mode to detect HTTP access fallback errors. After consuming the stream:

1. Check reactServerErrorsByDigest for any errors with the HTTP_ERROR_FALLBACK_ERROR_CODE prefix
2. If no such errors exist, send the buffered RSC data as before
3. If such errors are found, set the appropriate status code and fall through to the full dynamic render path which properly handles the not-found page

Test Plan

Added two test cases:

• not-found-suspense: Tests notFound() thrown inside a Suspense boundary
• not-found-with-layout-suspense: Exact reproduction of the issue - notFound() in page with async component in layout's Suspense

Fixes #86251

[nextjs-bot]

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[harikapadia999]

Excellent fix for a tricky edge case! 🎯

Problem Identified: When cacheComponents: true is enabled and notFound() is thrown inside a Suspense boundary, the RSC stream was being sent directly without detecting the HTTP access fallback error, causing "Connection closed" errors on the client.

Solution Approach: Your fix properly consumes the entire RSC stream first to detect HTTP access fallback errors (like notFound()), then:

• If no errors → sends the buffered RSC data as before
• If HTTP error detected → falls through to full dynamic render to properly display the error page

Strengths: ✅ Comprehensive test coverage with two test cases covering different scenarios
✅ Proper error detection by checking reactServerErrorsByDigest
✅ Maintains backward compatibility for non-error cases
✅ Sets correct HTTP status codes
✅ Clean fallback to dynamic rendering when needed

Minor Suggestions:

1. Performance consideration: Buffering the entire RSC stream in memory (chunks array) could be memory-intensive for large responses. Consider adding a comment about this trade-off or exploring streaming detection if possible.

2. Code comment clarity: The comment "We need to consume the RSC stream first..." is great, but could also mention why we can't detect this during streaming (because Suspense boundaries can catch errors).

3. Test robustness: Consider adding a test case for a large page to ensure the buffering approach doesn't cause memory issues.

Overall, this is a solid fix for a complex streaming + error handling interaction! 🚀