fix: delay revalidation if a key is already active and has error

[promer94]

close #2336

The reason for #2336 is that

every time a revalidation is finished, a new swr hook will mount and trigger a new revalidation.

isValidating will also cause similar problem here.

https://codesandbox.io/s/isvalidating-infinite-render-xgwyyz

[codesandbox-ci]

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Latest deployment of this branch, based on commit fe8a088:

Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
SWR-SSR	Configuration
isValidating-infinite-render	PR
frosty-silence-k2hgx5	Issue #2336

[promer94]

@promer94 Thank you! I understand the intention 👍

Let me share another solution I came up with. I feel the issue is caused by an incorrect isLoading state on an error, and I'd say isLoading should be true only when both data and error are undefined.

swr/core/use-swr.ts

Lines 302 to 304 in 3cec4bd

	if (isUndefined(getCache().data)) {
	initialState.isLoading = true
	}

should be

      if (isUndefined(getCache().data) && isUndefined(getCache().error)) {
        initialState.isLoading = true
      }

What do you think?

After reviewing #1928, I think it would be batter to discuss this change in a spreate pr.

[koba04]

I'd say it's a bit tricky checking hasValidator, but as @promer94 mentioned at #2338 (comment), this is a way to fix the problem without changing existing behaviors. LGTM 👍