cannot use cache provider in React.StrictMode

[bash-wealthkernel]

Bug report

Description / Observed Behavior

When using SWRConfig to set the cache provider it is ignored when used in React.StrictMode. I believe this has to do with StrictMode rendering function components twice (docs). When initCache is called the second time around with the same cache it returns undefined and therefore the provider never ends up using the value (here).

Expected Behavior

The provided configuration should be respected.

Repro Steps / Code Example

https://codesandbox.io/s/awesome-moon-4d8xb
Toggle strict to trigger the issue.

Additional Context

"react": "17.0.2",
"react-dom": "17.0.2",
"react-scripts": "4.0.3",
"swr": "1.0.1",

[shuding]

You don’t need to use an extra useState to initialize the cache, just use:

provider: () => new Map()

It already has state reusing inside.

[shuding]

(Removed “good first issue” as well since I don’t think strict mode and lazy state initialization are friendly for new contributors cc @huozhi)

[shuding]

I noticed that there's still a bug here, opening a PR to address.