fix(gatsby-transformer-sharp): prevent duplicate copy of the same file

[denisbabineau]

a race condition existed between existsSync() and .copy() for the same file causing the underlying fs-extra to randomly fail on unlink / chmod syscalls

now keeping track of copies in progress to prevent multiple parallel copies of the same file.

fixes #20602

thanks to @pieh for the proposed fix

[pieh]

I didn't try the proposed fix - just wrote up something that made some sense (IMO).

Did you try this approach and it fixed the FS issues you were seeing?

[denisbabineau]

Yes, tested locally with extra logging to confirm that it copies each files once.

[pieh]

Thanks @denisbabineau!

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