Runs fetched artifacts through pluggable external scanners after they're
staged in storage but before they're committed to the cache DB, so a
block verdict deletes the object instead of ever exposing it to a
client. Scanners pull the staged bytes themselves via a short-lived
HMAC-signed internal route rather than the proxy pushing bytes to them,
keeping the mechanism storage-backend-agnostic and avoiding uploading
potentially huge artifacts through the proxy's own egress.
Hardening baked in from the start: the internal scan-fetch route is
gated both at router-mount time and in the handler so it's inert
whenever scanning is disabled or unsigned; the signing key is mandatory
whenever scanning is enabled, enforced directly in scanner.NewGroup
rather than relying on callers to invoke config validation; the scan
call and the delete-on-block cleanup both run on a context detached
from the client's, so a client disconnecting mid-scan can't be mistaken
for a scanner failure, doesn't cause a legitimate artifact to be
deleted, and doesn't leave a genuinely blocked artifact's bytes
orphaned in storage; and scanner infrastructure errors (connection
failures, internal hostnames) are never forwarded verbatim to anonymous
clients, only a generic message. The scan-error metric also correctly
distinguishes a scanner's own timeout from being cancelled because a
sibling scanner already decided the verdict.
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
