The lookup took any non-200 metadata response for a version that isn't
listed, so a Packagist outage or rate limit answered the archive request
with 404. Composer treats that as the version being gone.
Only 404 and 410 now mean the document is missing. A 429 is passed on as
503 with Retry-After while the upstream is held, and any other status
answers 502.
Every Composer archive download resolved its dist from the package
metadata before checking the artifact cache. With cache_metadata off,
the default, that meant an uncoalesced upstream GET of the p2 document
for every download of an archive the proxy already had: three downloads
of a cached archive made three metadata requests. With the cache on,
each download after the TTL revalidated and rescanned the document.
The lookup is only needed for the publish time cooldown checks
downloads against, since lock files name archive URLs directly. Like
the npm handler, record that time in the versions table when the dist
is first resolved, and on later downloads of a tagged release read it
from there and serve the cached archive straight away. Without a stored
time, as for archives cached by an older proxy, the download takes the
metadata path, which then records it. The time is stored even with
cooldown off, so enabling cooldown later keeps the fast path.
The cache still enforces the denylist and verifies the stored digest,
and an archive is only ever cached under a name that was checked
against the metadata, so those guarantees hold. Branches, whose
archive and time change with every push, routed packages, whose
versions row would not say which repository the time came from, and
requests with a query string such as ?sha= keep resolving from the
metadata.
Every Composer archive download fetched the package's p2 metadata straight
from upstream to find the archive's URL, before the archive cache was even
checked. A cached archive cost one or two uncached Packagist requests per
package and install, and failed with a 502 while Packagist was down.
With cache_metadata on, downloads now look the dist up in the metadata the
proxy already caches for Composer, under the keys handlePackageMetadata uses.
Within the TTL that needs no upstream request, and when upstream fails the
stale copy still resolves the archive. A version or file the cached
documents do not list, such as one published since they were cached, is
looked up upstream as before.
