[s1gr1d]

Stacked on #26301.

Why: Vercel ISR (config.isr) caches the HTML of one request and serves it to every visitor until it expires. That HTML holds the live sentry-trace and baggage tags of the render, so every visitor continued the same old server trace. Next.js had the same bug, fixed in #25141. Now a page that Vercel renders for its ISR cache gets a sentry-cache-origin tag with the render's sentry-trace in place of the trace tags. The pageload starts its own trace, gets page.server_rendering.type: static, and links the render with a cache_origin link, as in Next.js (#25318).

To detect an ISR render, sentryHandle reads the x-vercel-isr request header. Vercel sends it to every invocation of an ISR function, also for background revalidations and for routes that get config.isr from a layout. The adapter's own __pathname marker is gone before Kit sees the request, and handle cannot see the route config (tested both on a Vercel deployment).

Like a use cache hit, the pageload links only a sampled render. An unsampled render still marks the pageload as static. A page with ssr = false gets no tags, as in the build.

How to review: Start with getCacheOrigin in handle.ts. It returns the render's trace (ISR), '' (build) or undefined (rendered for this request only). Then withCacheOrigin in client/traceMetaTags.ts. The two browser-tracing variants only wrap their existing span options.

Unsure about: A client can send x-vercel-isr to a normal SSR route, and the function sees it. That only changes the pageload of the request that sends it: it links its own server request instead of continuing it. I think that is acceptable. I did not check draft mode (bypassToken), where Vercel does not cache the render.

Example with the UI changes from getsentry/sentry#130771 (a visitor that got an already happened ISR render):

https://sentry-4x3dkn58u.sentry.dev/explore/traces/trace/63b1dbe5fe3849f2b13df81530857c03/

Linear: JSSDK-37

[s1gr1d]

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[mydea]

what does a '' cache origin mean? 🤔 I wonder if we can simplify this a bit, I always find it a bit hard to follow if something can deliberately return '' or undefined 🤔