A snapshot was recorded after the response by reading the metadata cache
again, so a refresh that replaced the cached copy in between lost the
version the request served. Record it from the served bytes before
responding instead, and compare a new snapshot with the version current
at its own time, so one recorded late keeps its place in the history.
AddMetadataSnapshot takes a transaction-scoped advisory lock on the
document before its check and insert, so two replicas recording the same
version can't both read the previous digest and both insert. SQLite runs
one write at a time, so it keeps the single statement.
Alpine mirrors keep only the current build of each package, so once a
package is upgraded `apk add curl=8.5.0-r0` fails: the current index no
longer lists the old build, even when the proxy still has it cached.
With upstream.apk_snapshots set (e.g. "90d"), each version of an index or
detached signature the metadata cache stores is copied to
_snapshots/alpine/ and recorded in a new metadata_snapshots table with the
time it was first served. /apk/{name}@{YYYY-MM-DD}/ serves the version that
was current at the end of that day; packages below it are the plain path's
requests. The kept bytes are the signed upstream bytes, so apk verifies
them unchanged.
An hourly job deletes versions replaced longer ago than the configured
duration and queues their blobs on the pending-delete queue; the newest
version of each index stays. Packages listed by kept snapshots are not
protected from eviction or retention; the docs say so.
