Random SSR invariant dehydration errors with bun

[ci]

Which project does this relate to?

Router

Describe the bug

I had some random Invariant failures in

router/packages/start-client-core/src/ssr-client.tsx

Lines 83 to 86 in 6e183ba

	invariant(
	window.__TSR_SSR__?.dehydrated,
	'Expected to find a dehydrated data on window.__TSR_SSR__.dehydrated... but we did not. Please file an issue!',
	)

with a blank page.

I've tried to debug it a bit, unsure exactly why that happens yet - it seemed to re-render an additional time when this crashed.

Here's a fun one: this only seems to happen using bun, and happens on the examples too.

Your Example Website or App

start-basic-react-query

Steps to Reproduce the Bug or Issue

bunx gitpick TanStack/router/tree/main/examples/react/start-basic-react-query repro
cd repro
bun install
SERVER_PRESET=bun bun run build
SERVER_PRESET=bun bun start

now go to http://localhost:3000 with "fast 4g" throttling in the network tab (makes it a bit more consistent for me, not necessary though), refresh a few times with it open

Expected behavior

Shouldn't run into the Invariant issues.

Screenshots or Videos

No response

Platform

OSX locally, Linux (inside Docker) in prod

Additional context

No response

[gabriel1ll7]

It sounds like you might be running into a race condition with the SSR setup—sometimes the server sends the response before window.TSR_SSR.dehydrated is fully set, leaving the client with undefined and causing a crash. A quick way to debug it is to log window.TSR_SSR.dehydrated right before the response goes out and see if it’s missing. If it is, you could tweak the SSR logic to wait for everything—like data fetching or dehydration—to finish up. Maybe toss in a Promise.all or adjust the pipeline so the state’s locked and loaded before sending. Let me know how it goes!

[abdelmalekkkkk]

Also having this issue with Bun 1.2.4 and @tanstack/react-start 1.116.1. However, the issue disappears when I build using the node-server preset and run using node. So I am not sure if this is an issue with TanStack or Bun.

[adrifer]

I'm facing the same issue when running it with Bun.

What I experienced in my case is the script with TSR_SSR.dehydrated is not complete when the invariant fails. Having a breakpoint in the error, I can see on the html response how the dehydrated property is not entirely there, something like:

 <script class='tsr-once'>__TSR_SSR__.dehydrated = "{\"manifest\":{\"

So probably Bun is chunking the response and when we are trying to hydrate, as the hydrated script is not fully in the document, we get the undefined error.

In my case I cannot reproduce it all the times, just like 5-10% of the times I will face this error, but not always