[Segment Cache] Interception routes (#73434)
Implements prefetching support for interception routes using the Segment
Cache.
The overall flow is the same as the previous prefetch cache
implementation. If a page varies based on the Next-URL — in other words,
if it might possibly be intercepted — we include the Next-URL as part of
the cache key. However, since most pages do not vary on the Next-URL,
and this is known at build time, for most pages we can omit the Next-URL
from the cache key for all but the first request.
We do this by checking the Vary header of the first response, and if the
Next-URL is not included, we re-key the cache entry to remove the
Next-URL. All subsequent requests for the same page will match this
entry regardless of the Next-URL.
---
One difference from the previous prefetch cache implementation: when an
entry varies by Next-URL, rather than concatenating the Next-URL to the
href to create a combined cache key, we store the entries in a tiered
map structure whose keys are tuples of the href and Next-URL. Then we
compare each key part separately. This might end up being overkill but
it's nice because we don't have to worry about escaping the values, nor
do we have to store an encoded cache key separately from its individual
parts. We will likely use the same approach for storing segment cache
entries, which vary on both the segment path and (in some cases; not yet
implemented) the search params.
