Non-deterministic zone selection in `getHostZoneID` when multiple zones match domain filter

[jelinek-wgs]

Bug Description

When multiple DNS zones in OpenStack Designate match the configured --domain-filter (for example, a parent zone example.com. and a subdomain zone sub.example.com.), the webhook non-deterministically selects different zones across reconcile cycles for hostnames where zone lengths evaluate equally or during map iteration.

This causes external-dns (running with --policy=sync) to continuously delete records in one zone and recreate them in another zone on "random" sync loop, leading to massive DNS record flapping/churn.

Root Cause Analysis

In internal/designate/provider/provider.go:

func getHostZoneID(hostname string, managedZones map[string]string) string {
    longestZoneLength := 0
    resultID := ""

    for zoneID, zoneName := range managedZones {
        if !strings.HasSuffix(hostname, "."+zoneName) && hostname != zoneName {
            continue
        }
        ln := len(zoneName)
        if ln > longestZoneLength {
            resultID = zoneID
            longestZoneLength = ln
        }
    }
    return resultID
}

Since managedZones is a Go map[string]string, iterating over it with for zoneID, zoneName := range managedZones yields a randomized iteration order in Go at runtime.
When multiple zones match with identical host/zone length criteria or overlap during filtering, the returned resultID changes randomly between reconcile cycles depending on map iteration order.