Fix fetch inside use cache in ISR

[conico974]

Using fetch (or I/O in general) inside of a use cache in an ISR/SSG route would make it crash with a Cannot perform I/O on behalf of a different request error during revalidation request if the instance is warm.

This is because by default it will use AsyncLocalStorage.snapshot() and it will bind everything to the initial request context. The downsides is that use cache function will have access to the full request ALS context from next (i.e. cookies, headers ...)

[changeset-bot]

🦋 Changeset detected

Latest commit: f4a744a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@opennextjs/cloudflare	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[pkg-pr-new]

Open in StackBlitz

pnpm add https://pkg.pr.new/@opennextjs/cloudflare@666

commit: f4a744a

[vicb]

Thanks for the PR nico 🙏

I'll review tomorrow.

Do you understand why we "perform I/O on behalf of a different request"?

[conico974]

Do you understand why we "perform I/O on behalf of a different request"?

I think i do, basically a function using use cache will be wrapped in this https://github.com/vercel/next.js/blob/62de494f3c28e3a6e949a434047ea3023121d239/packages/next/src/server/app-render/async-local-storage.ts#L55-L65. This is done only once at import time (so on the first request to the instance), subsequent request will then get bound to this initial AsyncLocalStorage context which is associated with the first request.

It might be possible to fix it at the workerd level so that ALS bind or snapshot is not actually bound to the request context, but this is likely tricky.

[vicb]

Could you please add more details from the PR discussion:

https://github.com/vercel/next.js/blob/62de494f/packages/next/src/server/app-render/async-local-storage.ts#L55-L65

This is because by default it will use AsyncLocalStorage.snapshot() and it will bind everything to the initial request context. The downsides is that use cache function will have access to the full request ALS context from next (i.e. cookies, headers ...)

[vicb]

Thanks for the fix!