Global mutate with cache provider unexpectedly provides undefined for optimisticData callback

[sartak]

Bug report

Description / Observed Behavior

I'm seeing that, in specific configurations, using global mutate unexpectedly provides undefined for the old value in an optimisticData callback.

Expected Behavior

I expected global mutate to behave similarly to a bound mutate with the same key. This is what the documentation suggests (with the caveat that "mounted SWR hook using the same key" is satisfied)

Either switching to a bound mutate (which would require restructuring a lot of my code), or disabling the cache provider (which would significantly impact UX), both cause the optimisticData callback to correctly provide the old value.

Repro Steps / Code Example

https://codesandbox.io/s/distracted-leavitt-kgjpk4?file=/src/App.js

Observe that clicking "Bound mutate" increments the counter, but "Global mutate" does not (because, at the very least, the old value provided in optimisticData is surprisingly undefined rather than the current value of the counter). If you comment out SWRConfig provider config on line 44, provider: () => cache, then both buttons work.

Additional Context

Affects at least swr 2.2.4 and 2.2.2.

The root cause may be the same as https://github.com/vercel/swr/issues/2701 or https://github.com/vercel/swr/issues/2799. The shape of this is slightly different from those PRs' descriptions, so I figured I'd file this separately.

[koba04]

@sartak Thank you for reporting this. You are right. Optimistic updating by the global mutate doesn't work with custom cache providers. We should fix this or document it as a limitation of the custom cache provider.

UPDATE: We already have documented it as a warning. Our current recommendation is to use mutate returned from useSWRConfig() rather than the global mutate. https://swr.vercel.app/docs/advanced/cache#create-cache-provider

[sartak]

It's unfortunate that these two features are subtly broken when used together.