A cached rewrite made with cooldown on stays right until a version it held
back leaves cooldown, so it now expires then instead of after a fixed
time. Handlers ask cooldown through Proxy.cooldown, which reports each
withheld version to the rewrites running at that moment. A rewrite that
held nothing back is kept for the shortest cooldown, at most a day.
With cooldown on, the RubyGems compact index and versions API, the Hex
package and Hex API entry, and conda's repodata.json were fetched with
plain HTTP requests on every client request: no TTL, no revalidation,
no coalescing and no stale fallback, even with cache_metadata.
Gem and Hex now read both documents through the metadata cache and
filter the cached copies. The filtered result goes through the rewrite
cache and is served with an ETag. When the times document is older than
the index and misses a version, it is fetched again; if that fails the
version is left out. Conda filters a fresh stored repodata.json as it
reads it from storage, uses the metadata cache otherwise, and remembers
the ETag of the filtered copy until a withheld entry leaves cooldown.
