22 Zone data as an indexed semantic model

Zone is more than a parser for tinydns text. It is the semantic index used by authoritative answers:

pub struct Zone {
    records: BTreeMap<Name, Vec<Record>>,
    metadata: BTreeMap<Name, Vec<RecordMetadata>>,
    authoritative: BTreeSet<Name>,
    delegations: BTreeSet<Name>,
    locations: Vec<(Vec<u8>, [u8; 2])>,
    current_metadata: RecordMetadata,
    default_serial: u32,
    nodes: BTreeSet<Name>,
    unqualified_nodes: BTreeSet<Name>,
}

The maps hold records and djbdns location metadata. The sets encode facts that would otherwise require scans: zone apexes, delegation cuts, all existing nodes, and nodes that exist independently of location-qualified records. This all-node index is why the optimized NXDOMAIN benchmark is about eleven times faster than the earlier scan-based implementation.

The type also prevents semantic drift. Parsing validates CNAME exclusivity once. Lookup returns a Lookup enum, so absence cannot collapse into one null result:

• Answer carries the matching RRset.
• Referral carries delegation NS records and in-bailiwick glue.
• NoData says the name exists but the requested type does not.
• NxDomain says the name itself does not exist.
• Refused says the server is not authoritative for the question.

Wildcard processing uses the nodes index to find the closest encloser. Delegation processing uses the explicit delegations set. Transfer processing walks the same model while excluding child-zone contents. One representation therefore supplies ordinary answers, negative answers, wildcards, referrals, and AXFR without five subtly different interpretations of a zone.