fix: don't include private-address in default testing.Relation databags on Juju 4

testing/src/scenario/_runtime.py

@@ -28,6 +28,7 @@
     PeerRelation,
     Relation,
     SubordinateRelation,
+    _strip_juju4_defaults,
 )
 
 if TYPE_CHECKING:  # pragma: no cover
@@ -313,6 +314,7 @@ def exec(
 
         # we make a copy to avoid mutating the input state
         output_state = copy.deepcopy(state)
+        _strip_juju4_defaults(output_state, self._juju_version)
 
         logger.info(' - generating virtual charm root')
         with self._virtual_charm_root() as temporary_charm_root:

testing/src/scenario/state.py

@@ -716,12 +716,35 @@ def _validate_databag(self, databag: dict[str, str]):
 
 
 _DEFAULT_IP = '192.0.2.0'
+# Juju 3.x populates `private-address` in every relation databag, but Juju 4
+# does not. The dataclass `default_factory` runs at `Relation()` construction
+# time, before the test's `juju_version` is known, so we always include
+# `private-address` here and strip it later in `_strip_juju4_defaults` if the
+# `Context.juju_version` is 4.0 or later.
 _DEFAULT_JUJU_DATABAG = {
     'egress-subnets': _DEFAULT_IP,
     'ingress-address': _DEFAULT_IP,
     'private-address': _DEFAULT_IP,
 }
 
+# Keys in `_DEFAULT_JUJU_DATABAG` that Juju 4 no longer auto-populates.
+_JUJU_4_REMOVED_DATABAG_KEYS = ('private-address',)
+
+
+def _strip_juju4_defaults(state: State, juju_version: str) -> None:
+    """Strip Juju-3-only default databag keys from `state` for Juju 4+.
+
+    Mutates the databag dicts in place; only keys still set to the default
+    sentinel IP are removed, so explicit user-set values are preserved.
+    """
+    if ops.JujuVersion(juju_version).major < 4:
+        return
+    for relation in state.relations:
+        for databag in relation._databags:
+            for key in _JUJU_4_REMOVED_DATABAG_KEYS:
+                if databag.get(key) == _DEFAULT_IP:
+                    del databag[key]
+
 
 @dataclasses.dataclass(frozen=True, kw_only=True)
 class Relation(RelationBase):

testing/tests/test_e2e/test_relations.py

@@ -487,6 +487,70 @@ def test_relation_default_unit_data_peer():
     assert relation.local_unit_data == _DEFAULT_JUJU_DATABAG
 
 
+@pytest.mark.parametrize(
+    'juju_version,expect_private_address',
+    [('3.6.14', True), ('4.0.0', False), ('4.1.0', False)],
+)
+def test_private_address_default_dropped_on_juju_4(
+    juju_version: str, expect_private_address: bool
+):
+    """Juju 4 no longer auto-populates `private-address` in relation databags.
+
+    Verify the output state reflects that — the default key is preserved on
+    Juju 3, and stripped on Juju 4+.
+    """
+    ctx = Context(
+        Charm,
+        meta={
+            'name': 'foo',
+            'requires': {'foo': {'interface': 'foo'}},
+            'peers': {'p': {'interface': 'p'}},
+            'provides': {'sub': {'interface': 'sub', 'scope': 'container'}},
+        },
+        juju_version=juju_version,
+    )
+    relation = Relation('foo')
+    peer = PeerRelation('p')
+    sub = SubordinateRelation('sub')
+    state_in = State(leader=True, relations={relation, peer, sub})
+
+    state_out = ctx.run(ctx.on.start(), state_in)
+
+    rel = state_out.get_relation(relation.id)
+    peer_rel = state_out.get_relation(peer.id)
+    sub_rel = state_out.get_relation(sub.id)
+    assert isinstance(rel, Relation)
+    assert isinstance(peer_rel, PeerRelation)
+    assert isinstance(sub_rel, SubordinateRelation)
+    assert ('private-address' in rel.local_unit_data) is expect_private_address
+    assert ('private-address' in rel.remote_units_data[0]) is expect_private_address
+    assert ('private-address' in peer_rel.local_unit_data) is expect_private_address
+    assert ('private-address' in sub_rel.local_unit_data) is expect_private_address
+    assert ('private-address' in sub_rel.remote_unit_data) is expect_private_address
+    # `egress-subnets` and `ingress-address` are still set on Juju 4.
+    assert rel.local_unit_data['ingress-address'] == '192.0.2.0'
+    assert rel.local_unit_data['egress-subnets'] == '192.0.2.0'
+
+
+def test_private_address_explicit_value_preserved_on_juju_4():
+    """Only the default sentinel IP is stripped — explicit user values stay."""
+    ctx = Context(
+        Charm,
+        meta={'name': 'foo', 'requires': {'foo': {'interface': 'foo'}}},
+        juju_version='4.0.0',
+    )
+    relation = Relation(
+        'foo',
+        local_unit_data={'private-address': '10.0.0.5', 'extra': 'kept'},
+    )
+    state_out = ctx.run(ctx.on.start(), State(leader=True, relations={relation}))
+
+    rel = state_out.get_relation(relation.id)
+    assert isinstance(rel, Relation)
+    assert rel.local_unit_data['private-address'] == '10.0.0.5'
+    assert rel.local_unit_data['extra'] == 'kept'
+
+
 @pytest.mark.parametrize('evt_name', ('broken', 'created'))
 def test_relation_events_no_remote_units(evt_name: str, caplog: pytest.LogCaptureFixture):
     relation = Relation(