Adds an internal test utility for e2e testing of requests initiated by
the Next.js Router, such as prefetches and navigations. Calls the given
async function then intercepts any router requests that are initiated as
a result. It will then wait for all the requests to complete before
exiting. Inspired by the React `act` API.
I generally dislike early testing abstractions, but getting all the
details right for intercepting the requests with Playwright ended up
being non-trivial enough that I relented and extracted it to a separate
module, in the `e2e/app-dir/segment-cache` folder. I'll hold off moving
it to somewhere even more generic until it's proven to be useful.
Example:
```js
// Asserts that rendering a link results in a prefetch response that
// contains some expected substring
await act(async () => {
await revealLink()
}, {
includes: 'subset of linked page content'
})
```
A few goals here:
- As much as possible, avoid coupling to internal implementation
details. For example, it will check for the presence of the "rsc" header
to determine if a request was initiated by the router, but that's about
it.
- No timers, no race conditions. It works by intercepting the requests
at the Playwright network layer. There should be no long pauses when
running the tests, and if an assertion fails, it should fail quickly.
- Account for Next.js's internal bandwidth throttling. The router does
not initiate more than a fixed number of requests at a time, so every
time a request is fulfilled, it has the potential to unblock more
prefetches. So we must wait a task to check for additional requests.
This is similar to what React's `act` implementation does to account for
asynchronous rendering tasks.
During a navigation, if all the data has been prefetched, and the target
route does not contain any dynamic data, then we should skip a request
to the server.
This uses the `isPartial` field I added in the previous PRs to track
whether the prefetched data is complete or not.
Andrew Clark