fix: never lower a serial or cap it at 99

tests/test_signals.py

@@ -1,6 +1,7 @@
 import pytest
+from freezegun import freeze_time
 
-from mdb.models import DhcpConfig, Ip4Subnet, Ip6Subnet
+from mdb.models import DhcpConfig, Domain, Interface, Ip4Subnet, Ip6Subnet
 
 # ---------------------------------------------------------------------------
 # pre_save signals: populate domain_name once, then leave it frozen
@@ -65,3 +66,21 @@ def test_ip6_signal_sets_domain_name():
         domain_filename="db.reverse-v6",
     )
     assert subnet.domain_name == "0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa"
+
+
+# ---------------------------------------------------------------------------
+# pre_delete Interface: the zones it was in get a new serial
+# ---------------------------------------------------------------------------
+
+
+@freeze_time("2026-11-02")
+def test_deleting_interface_raises_zone_serials(domain):
+    subnet = Interface.objects.get(pk=1).ip4address.subnet
+    Domain.objects.filter(pk=domain.pk).update(domain_serial=2026110299)
+    Ip4Subnet.objects.filter(pk=subnet.pk).update(domain_serial=2015010101)
+
+    Interface.objects.get(pk=1).delete()
+
+    # Past 99 the serial carries into the date instead of going back.
+    assert Domain.objects.get(pk=domain.pk).domain_serial == 2026110300
+    assert Ip4Subnet.objects.get(pk=subnet.pk).domain_serial == 2026110201

tests/test_utils.py

@@ -0,0 +1,32 @@
+import pytest
+from freezegun import freeze_time
+
+from mdb.utils import format_domain_serial_and_add_one
+
+
+@freeze_time("2026-11-02")
+@pytest.mark.parametrize(
+    ("serial", "expected"),
+    [
+        (2026110205, 2026110206),  # today: next number
+        (2025010107, 2026110201),  # an earlier day: today's first
+        (1, 2026110201),  # not a date at all
+        (2026110298, 2026110299),
+        (2026110299, 2026110300),  # used to stay at 99, so the change never synced
+        (2026110300, 2026110301),  # ahead of today: never goes back
+        (2026110505, 2026110506),
+        (None, 2026110201),
+        ("garbage", 2026110201),
+    ],
+)
+def test_next_serial(serial, expected):
+    assert format_domain_serial_and_add_one(serial) == expected
+
+
+@freeze_time("2026-11-02")
+def test_serial_always_increases():
+    serial = 2026110290
+    for _ in range(30):
+        new = format_domain_serial_and_add_one(serial)
+        assert new > serial
+        serial = new