[gatsby-source-wordpress] prevent duplicate nodes for remote media files

[Fetten]

This fixes #3562

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[pieh]

Shouldn't we fix that in gatsby-source-filesystem ? To not replicate it in every source plugin?

We can check there if node for that remote file is already created and return that node? Am I missing something?

[Fetten]

Yes, I first thought of caching the url in gatsby-source-filesystem's create-remote-file-node.js, but then I realized, that multiple requests can run in parallel. This makes it hard to check for a cached local-file node before a duplicate request, as the first one might not be finished yet.

It would be great if anybody had an idea how to solve this.

[pieh]

we can store promises - like that pieh@af21319 (would just need better comments)

[Fetten]

Wow, never thought of caching a whole promise :) But if this works, let's go for it.
I'll close this PR if you would like to send one for your version.

[pieh]

We can keep that PR open for now. My code does work and fix fetching problem, but not sure yet if we won't have similar problem when we try to process those images (using transformer-sharp for example) and will check it tomorrow.

[KyleAMathews]

We cache promises like @pieh's PR elsewhere in core and it works great so let's go with his more general solution.

Thanks @Fetten for the PR all the same!