feat: cache .next/cache between builds

[lindsaylevine]

Fixes #63

questions:

1. how will this behave with the pizzafox plugin? answer: works
2. how can this be tested within the plugin, if at all? answer: mock/spy

[lindsaylevine]

@ehmicky any specific thoughts on these questions in the PR desc? 🙏

how will this behave with the pizzafox plugin?
how can this be tested within the plugin, if at all?

[ehmicky]

The cache utility deletes the destination file/directory before both saving and restoring. So I believe it should be idempotent, and running both netlify-plugin-cache-nextjs and the logic in this PR should work.

However, I would suggest manually testing it before merging this PR. The production behavior of the caching utility is different from local builds, so I would suggest testing both (or at least production). If it turned out to be incompatible, I would suggest:

• Detecting whether netlify-plugin-cache-nextjs is used. This could be done by using require('netlify-plugin-cache-nextjs').
• If detected, the logic of this PR could be a noop.

When it comes to testing it, I would suggest using a fixture directory then:

• Delete .next/ if already exists (from running the test previously)
• Create .next/ with a .next/dummy in it
• Run the plugin
• Delete .next/ and .next/dummy
• Run the plugin again
• Assert that .next/dummy exists
• Delete .next/ and .next/dummy

[lindsaylevine]

i manually tested. as for automated tests, i'm not sure how we'd "run the plugin" with the real valid utils.cache hook?

[ehmicky]

The above was assuming the tests would be run using @netlify/build programmatically.

Another option would be to spy on the mocked utils.cache.* methods, for example using jest.spyOn(). This would be simpler to implement, but would also not catch every type of bug.