Ensure cohort is never nil:
- NewDomainSet stores an empty Cohort before rebuildCohort, so a parse
failure leaves an empty (non-nil) cohort instead of nil
- Contains uses CompareAndSwap to lazily self-initialise a zero-value
struct, making nil impossible after first access — no explicit nil
check needed
Extract the cache-validity check into a testable cachedCohortValid()
helper and add filesPresent() to detect when data files exist on disk.
Use strings.Builder in commaify() for efficiency.
Tests:
- TestPrefixSetContainsZeroValue: zero-value self-heals via CAS,
cohort is non-nil after first Contains call
- TestCachedCohortValidRequiresFiles: cache invalidation logic
PrefixSet: git-backed CIDR files via gitshallow, refreshed every 47 min.
DomainSet: takes pre-parsed (staticPrefixes, domains []string); resolves
hostnames every 5 min, retaining stale IPs on failure. Both use
atomic-swapped ipcohort.Cohort for lock-free reads.
ParseDomainSet(RecordReader) parses rows into prefixes and hostnames;
caller constructs the record reader (e.g. csv.Reader with delimiter,
comment char). RecordReader interface is satisfied by *csv.Reader.
commaify uses plain string concat (integers max ~7 groups).
