Disallow synchronous handlers in type definition

[Pimm]

Which problem is this pull request solving?

The type definition of Handler (or BaseHandler, more specifically) allows handlers to by synchronous.

I wrote some Netlify functions, which worked as expected locally through Netlify CLI. On production, however, the function produced the following:

error decoding lambda response: invalid status code returned from lambda: 0

This seems to have been caused by the functions returning a Response (directly) rather than a Promise<Response> in some branches. Synchronous handlers are allowed by (Netlify CLI, as well as) the type definition, but not actually by Netlify.

List other issues or pull requests related to this problem

(none that I could find)

Describe the solution you've chosen

Removed the synchronous version from the union that is the return type of BaseHandler.

Describe alternatives you've considered

Add support for synchronous handlers to the Netlify platform.

Checklist

• I have read the contribution guidelines.
• The status checks are successful (continuous integration). Those can be seen below.

[minivan]

Thank you for this, Pimm! This is great, we'll check if it breaks anything on our end and get back to this PR 🙌

[Skn0tt]

LGTM! I agree with your reasoning. The fact that ntl dev supports it, I consider a bug in itself. I'll see what we can do to mirror Lambda's behaviour in ntl dev as well.