fix(dhcp): de-duplicate DHCP lease label sets - #76
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AdGuard can return duplicate DHCP lease rows (e.g. an in-place self-update
rebuilds the lease table, or several clients behind a MAC-NAT'ing wifi
repeater share one upstream MAC). DhcpLeasesServer.Collect emitted one
metric per lease with no de-duplication, so a repeated label tuple made the
registry fail the entire /metrics scrape with:
collected metric "adguard_dhcp_leases" { ... } was collected before with
the same name and label values
i.e. a single duplicate lease 500s the whole exporter and reads as down,
even though AdGuard itself is healthy.
De-duplicate on the full label set before emitting. The metric set is built
under the existing mutex (Collect previously read the leases map unlocked
while the worker's Record writes it, which is a data race) and sent to the
channel after unlocking so a slow scrape cannot stall Record. Adds a
regression test using a pedantic registry.
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When AdGuard returns duplicate DHCP lease rows,
/metricsfails with HTTP 500:A single duplicate lease takes down the entire scrape (every metric, not just leases), so Prometheus reads the exporter as down even though AdGuard itself is healthy. I have seen AdGuard emit duplicate rows after an in-place self-update (lease-table rebuild) and when several clients behind a MAC-NAT'ing wifi repeater share one upstream MAC.
Fix
De-duplicate on the full label set in
DhcpLeasesServer.Collectbefore emitting. The metric set is built under the existing mutex (Collectpreviously read the leases map unlocked while the worker'sRecordwrites it, whichgo test -raceflags as a data race) and sent to the channel after unlocking so a slow scrape cannot stallRecord.Adds a regression test using a pedantic registry; it fails on the current collector and passes with the fix.