This adds some missing fields to the `NextConfig` type we expose under `next` and also adds a `NextConfigComplete` internal type that allows us to not treat all fields as optional like you would when using the type in `next.config.js`.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes
By itself, `withCoalescedInvoke` with a separate `this.incrementalCache.set(...)` isn't really suitable for streaming responses. Since streaming is asynchronous, updating the cache separately introduces a gap where another origin request for the same resource could be made.
This could potentially be addressed by moving the cache update, but then `IncrementalCache` itself would need to be made to support streaming, in addition to the many other responsibilities it has. In this case, it seemed best to just use composition to add another caching layer in front of it, which is a familiar and understandable concept. Eventually, we might want to move this cache to the HTTP layer, which will also be simpler with this change.
As an added bonus, `renderToResponseWithComponents` becomes significantly simpler, and we delete some duplication.
Refactors the internals of `next-server` to use `ResponsePayload` instead of `string | null` and manual `sendPayload` calls. This is the first step toward streaming support.
I split `renderToResponseWithComponents` into a separate `renderToResponseWithComponentsInternal` function for ease of review: GitHub's diff rendering was highly misleading, making it seem as though more of the function had changed. The separate function just makes the actual change clearer: we split `renderToHTMLWithComponents` into two promises; one that represents the actual render result, and one that represents all of the work (including background work for e.g. revalidation) that needs to be done as part of generating the result.
These changes make it easier to bubble up a `ResponsePayload`, instead of sometimes calling `sendPayload` out-of-band, centralizing all payload handling in `sendResponse` and eventually a similar function for public APIs that return a string. This centralization will make it much easier to handle a response that needs to be streamed, which is coming soon in another PR.
