Merge ~bjornt/maas:reverse-dns-3.0-backport into maas:3.0

[Björn Tillenius]

LP #1931838: Reverse DNS lookup fails for subnets smaller than /24

pull the skip exclusions logic into its own function

correct the rfc2317 glue exclusions

(cherry picked from commit fdd0c41bba6b758416f460829b83d03e86200f36)

[Björn Tillenius (bjornt)]

Self-approve backport

review: Approve

[MAAS Lander (maas-lander)]

UNIT TESTS
-b reverse-dns-3.0-backport lp:~bjornt/maas/+git/maas into -b 3.0 lp:~maas-committers/maas

STATUS: SUCCESS
COMMIT: dad809bbc9e36f92f45cb12ac78b8f3b411e6975

review: Approve

diff --git a/src/maasserver/dns/tests/test_zonegenerator.py b/src/maasserver/dns/tests/test_zonegenerator.py
index 0d96159..c012e9c 100644
--- a/src/maasserver/dns/tests/test_zonegenerator.py
+++ b/src/maasserver/dns/tests/test_zonegenerator.py
@@ -384,6 +384,9 @@ class TestZoneGenerator(MAASServerTestCase):
             "0-25.33.168.192.in-addr.arpa",
             "0-26.35.168.192.in-addr.arpa",
             "0-26.36.168.192.in-addr.arpa",
+            "33.168.192.in-addr.arpa",
+            "35.168.192.in-addr.arpa",
+            "36.168.192.in-addr.arpa",
             "overlap",
         ]
         zone_names = [
diff --git a/src/maasserver/dns/zonegenerator.py b/src/maasserver/dns/zonegenerator.py
index bd6401a..40af246 100644
--- a/src/maasserver/dns/zonegenerator.py
+++ b/src/maasserver/dns/zonegenerator.py
@@ -429,7 +429,7 @@ class ZoneGenerator:
                 dynamic_ranges=dynamic_ranges,
                 rfc2317_ranges=glue,
                 exclude=set(
-                    [IPNetwork(s.cidr) for s in subnets if s is not subnet]
+                    IPNetwork(s.cidr) for s in subnets if s is not subnet
                 ),
             )
         # Now provide any remaining rfc2317 glue networks.
@@ -441,7 +441,11 @@ class ZoneGenerator:
                 network=network,
                 ns_host_name=ns_host_name,
                 rfc2317_ranges=ranges,
-                exclude=set([IPNetwork(s.cidr) for s in subnets]),
+                exclude=set(
+                    IPNetwork(s.cidr)
+                    for s in subnets
+                    if network in IPNetwork(s.cidr)
+                ),
             )
 
     def __iter__(self):
diff --git a/src/provisioningserver/dns/zoneconfig.py b/src/provisioningserver/dns/zoneconfig.py
index c88a9ba..24d5547 100644
--- a/src/provisioningserver/dns/zoneconfig.py
+++ b/src/provisioningserver/dns/zoneconfig.py
@@ -346,6 +346,22 @@ class DNSReverseZoneConfig(DomainConfigBase):
         super().__init__(domain, zone_info=zone_info, **kwargs)
 
     @classmethod
+    def _skip_if_overlaps(cls, first, base, step, network, exclude):
+        for other_network in exclude:
+            if (
+                first in other_network
+                and network.prefixlen < other_network.prefixlen
+            ):  # allow the more specific overlapping subnet to create the zone config
+                try:
+                    base += 1
+                    first += step
+                except IndexError:
+                    # IndexError occurs when we go from 255.255.255.255 to
+                    # 0.0.0.0.  If we hit that, we're all fine and done.
+                    break
+        return (first, base)
+
+    @classmethod
     def compose_zone_info(cls, network, exclude=()):
         """Return the names of the reverse zones."""
         # Generate the name of the reverse zone file:
@@ -400,23 +416,13 @@ class DNSReverseZoneConfig(DomainConfigBase):
             base = int(split_zone[rest_limit - 1])
         while first <= last:
 
-            for other_network in exclude:
-                if (
-                    first in other_network
-                    and network.prefixlen < other_network.prefixlen
-                ):  # allow the more specific overlapping subnet to create the zone config
-                    try:
-                        base += 1
-                        first += step
-                        if (
-                            first > last
-                        ):  # if the excluding subnet pushes the base IP beyond the bounds of the generating subnet, we've reached the end and return early
-                            return info
-                        continue
-                    except IndexError:
-                        # IndexError occurs when we go from 255.255.255.255 to
-                        # 0.0.0.0.  If we hit that, we're all fine and done.
-                        break
+            (first, base) = cls._skip_if_overlaps(
+                first, base, step, network, exclude
+            )
+            if first > last:
+                # if the excluding subnet pushes the base IP beyond the bounds of the generating subnet, we've reached the end and return early
+                return info
+
             # Rest_limit has bounds of 1..labelcount+1 (5 or 33).
             # If we're stripping any elements, then we just want base.name.
             if rest_limit > 1: