fix: Preprocessors may return promises, but not resolve to 'content'.

[seebees]

The original API for Preprocessors assumed callback.
Now promises are a valid return.
If an implementer returns a promise from process
but only resolves content via the callback,
then while the result of process is a promise,
it will not resolve to the intended content.
An easy example is for process to be an async function.
e.g.

async function process(content, file, done) {
   const newContent = await magic(content)
   done(newContent)
}

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[seebees]

A link to the "breaking" PR: #3376
for historical purposes.

[johnjbarton]

Thanks for finding this issue. The author of the async change has a similar fix in PR #3387 with unit tests for both paths.

[seebees]

even better! I would add a comment since this is a little edgy, but that is a style choice :)
Do you mind if I ask what the ETA for release is?

[johnjbarton]

4.4.1 is up