[DPE-2780] Test shard for unpermitted relations (#293)

## Issue
No tests for relations that are not allowed for shards

## Solution
add tests for checking unsupported relations

## extras
create a dummy legacy application. This should NOT be used inside the
legacy integration test suite as those tests should be done with
production legacy charms

.github/workflows/ci.yaml

@@ -82,6 +82,7 @@ jobs:
           - backup-integration
           - metric-integration
           - sharding-integration
+          - sharding-relation-integration
     name: ${{ matrix.tox-environments }}
     needs:
       - lint

tests/integration/dummy_legacy_app/charmcraft.yaml

@@ -0,0 +1,11 @@
+# Copyright 2023 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+type: charm
+bases:
+  - build-on:
+      - name: "ubuntu"
+        channel: "20.04"
+    run-on:
+      - name: "ubuntu"
+        channel: "20.04"

tests/integration/dummy_legacy_app/metadata.yaml

@@ -0,0 +1,14 @@
+# Copyright 2023 Canonical Ltd.
+# See LICENSE file for licensing details.
+name: legacy-application
+description: |
+  Dummy legacy application charm used in integration tests.
+summary: |
+  Dummy legacy application charm meant only to be used in non-legacy integration tests. Legacy 
+  integration tests should use a production legacy application.
+series:
+  - focal
+
+requires:
+  obsolete:
+    interface: mongodb

tests/integration/dummy_legacy_app/requirements.txt

@@ -0,0 +1 @@
+ops >= 1.4.0

tests/integration/dummy_legacy_app/src/charm.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+# Copyright 2023 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+"""Application charm that connects to database charms.
+
+This charm is meant to be used only for testing
+of the libraries in this repository.
+"""
+
+import logging
+
+from ops.charm import CharmBase
+from ops.main import main
+from ops.model import ActiveStatus
+
+logger = logging.getLogger(__name__)
+
+# Extra roles that this application needs when interacting with the database.
+EXTRA_USER_ROLES = "admin"
+
+
+class ApplicationCharm(CharmBase):
+    """Application charm that connects to database charms."""
+
+    def __init__(self, *args):
+        super().__init__(*args)
+
+        # Default charm events.
+        self.framework.observe(self.on.start, self._on_start)
+
+    def _on_start(self, _) -> None:
+        """Only sets an Active status."""
+        self.unit.status = ActiveStatus()
+
+
+if __name__ == "__main__":
+    main(ApplicationCharm)

tests/integration/sharding_tests/conftest.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+# Copyright 2023 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+import pytest
+from pytest_operator.plugin import OpsTest
+
+
+@pytest.fixture(scope="module")
+async def legacy_charm(ops_test: OpsTest):
+    """Build an application charm that uses the legacy interface.
+
+    Note: this should only be used outside of the legacy integration tests, as those tests should
+    test a real legacy application.
+    """
+    charm_path = "tests/integration/dummy_legacy_app"
+    charm = await ops_test.build_charm(charm_path)
+    return charm
+
+
+@pytest.fixture(scope="module")
+async def application_charm(ops_test: OpsTest):
+    """Build the application charm."""
+    charm_path = "tests/integration/relation_tests/new_relations/application-charm"
+    charm = await ops_test.build_charm(charm_path)
+    return charm
+
+
+@pytest.fixture(scope="module")
+async def database_charm(ops_test: OpsTest):
+    """Build the database charm."""
+    charm = await ops_test.build_charm(".")
+    return charm

tests/integration/sharding_tests/test_sharding_relations.py

@@ -0,0 +1,140 @@
+#!/usr/bin/env python3
+# Copyright 2023 Canonical Ltd.
+# See LICENSE file for licensing details.
+import pytest
+from juju.errors import JujuAPIError
+from pytest_operator.plugin import OpsTest
+
+SHARD_ONE_APP_NAME = "shard-one"
+CONFIG_SERVER_ONE_APP_NAME = "config-server-one"
+CONFIG_SERVER_TWO_APP_NAME = "config-server-two"
+APP_CHARM_NAME = "application"
+LEGACY_APP_CHARM_NAME = "legacy-application"
+
+CONFIG_SERVER_REL_NAME = "config-server"
+SHARD_REL_NAME = "sharding"
+DATABASE_REL_NAME = "first-database"
+LEGACY_RELATION_NAME = "obsolete"
+
+RELATION_LIMIT_MESSAGE = 'cannot add relation "shard-one:sharding config-server-two:config-server": establishing a new relation for shard-one:sharding would exceed its maximum relation limit of 1'
+# for now we have a large timeout due to the slow drainage of the `config.system.sessions`
+# collection. More info here:
+# https://stackoverflow.com/questions/77364840/mongodb-slow-chunk-migration-for-collection-config-system-sessions-with-remov
+TIMEOUT = 30 * 60
+
+
+@pytest.mark.abort_on_fail
+async def test_build_and_deploy(
+    ops_test: OpsTest, application_charm, legacy_charm, database_charm
+) -> None:
+    """Build and deploy a sharded cluster."""
+    await ops_test.model.deploy(
+        database_charm,
+        config={"role": "config-server"},
+        application_name=CONFIG_SERVER_ONE_APP_NAME,
+    )
+    await ops_test.model.deploy(
+        database_charm,
+        config={"role": "config-server"},
+        application_name=CONFIG_SERVER_TWO_APP_NAME,
+    )
+    await ops_test.model.deploy(
+        database_charm, num_units=1, config={"role": "shard"}, application_name=SHARD_ONE_APP_NAME
+    )
+    await ops_test.model.deploy(application_charm, application_name=APP_CHARM_NAME)
+    await ops_test.model.deploy(legacy_charm, application_name=LEGACY_APP_CHARM_NAME)
+
+    await ops_test.model.wait_for_idle(
+        apps=[
+            CONFIG_SERVER_ONE_APP_NAME,
+            CONFIG_SERVER_TWO_APP_NAME,
+            SHARD_ONE_APP_NAME,
+        ],
+        idle_period=20,
+        raise_on_blocked=False,
+        timeout=TIMEOUT,
+    )
+
+
+async def test_only_one_config_server_relation(ops_test: OpsTest) -> None:
+    """Verify that a shard can only be related to one config server."""
+    await ops_test.model.integrate(
+        f"{SHARD_ONE_APP_NAME}:{SHARD_REL_NAME}",
+        f"{CONFIG_SERVER_ONE_APP_NAME}:{CONFIG_SERVER_REL_NAME}",
+    )
+
+    with pytest.raises(JujuAPIError) as juju_error:
+        await ops_test.model.integrate(
+            f"{SHARD_ONE_APP_NAME}:{SHARD_REL_NAME}",
+            f"{CONFIG_SERVER_TWO_APP_NAME}:{CONFIG_SERVER_REL_NAME}",
+        )
+
+    assert (
+        juju_error.value.args[0] == RELATION_LIMIT_MESSAGE
+    ), "Shard can relate to multiple config servers."
+
+
+async def test_cannot_use_db_relation(ops_test: OpsTest) -> None:
+    """Verify that a shard cannot use the DB relation."""
+    await ops_test.model.integrate(
+        f"{APP_CHARM_NAME}:{DATABASE_REL_NAME}",
+        SHARD_ONE_APP_NAME,
+    )
+
+    await ops_test.model.wait_for_idle(
+        apps=[SHARD_ONE_APP_NAME],
+        idle_period=20,
+        raise_on_blocked=False,
+        timeout=TIMEOUT,
+    )
+
+    shard_unit = ops_test.model.applications[SHARD_ONE_APP_NAME].units[0]
+    assert (
+        shard_unit.workload_status_message == "Sharding roles do not support database interface."
+    ), "Shard cannot be related using the database relation"
+
+    # clean up relation
+    await ops_test.model.applications[SHARD_ONE_APP_NAME].remove_relation(
+        f"{APP_CHARM_NAME}:{DATABASE_REL_NAME}",
+        SHARD_ONE_APP_NAME,
+    )
+
+    await ops_test.model.wait_for_idle(
+        apps=[SHARD_ONE_APP_NAME],
+        idle_period=20,
+        raise_on_blocked=False,
+        timeout=TIMEOUT,
+    )
+
+
+async def test_cannot_use_legacy_db_relation(ops_test: OpsTest) -> None:
+    """Verify that a shard cannot use the legcy DB relation."""
+    await ops_test.model.integrate(
+        LEGACY_APP_CHARM_NAME,
+        SHARD_ONE_APP_NAME,
+    )
+
+    await ops_test.model.wait_for_idle(
+        apps=[SHARD_ONE_APP_NAME],
+        idle_period=20,
+        raise_on_blocked=False,
+        timeout=TIMEOUT,
+    )
+
+    shard_unit = ops_test.model.applications[SHARD_ONE_APP_NAME].units[0]
+    assert (
+        shard_unit.workload_status_message == "Sharding roles do not support obsolete interface."
+    ), "Shard cannot be related using the mongodb relation"
+
+    # clean up relation
+    await ops_test.model.applications[SHARD_ONE_APP_NAME].remove_relation(
+        f"{SHARD_ONE_APP_NAME}:{LEGACY_RELATION_NAME}",
+        f"{LEGACY_APP_CHARM_NAME}:{LEGACY_RELATION_NAME}",
+    )
+
+    await ops_test.model.wait_for_idle(
+        apps=[SHARD_ONE_APP_NAME],
+        idle_period=20,
+        raise_on_blocked=False,
+        timeout=TIMEOUT,
+    )

tox.ini

@@ -192,6 +192,21 @@ deps =
 commands =
     pytest -v --tb native --log-cli-level=INFO -s --durations=0 {posargs} {[vars]tests_path}/integration/sharding_tests/test_sharding.py
 
+[testenv:sharding-relation-integration]
+description = Run sharding integration tests
+pass_env =
+    {[testenv]pass_env}
+    CI
+deps =
+    pytest
+    juju==3.2.0.1
+    pytest-mock
+    pytest-operator
+    git+https://github.com/canonical/[email protected]\#subdirectory=python/pytest_plugins/pytest_operator_cache
+    -r {tox_root}/requirements.txt
+commands =
+    pytest -v --tb native --log-cli-level=INFO -s --durations=0 {posargs} {[vars]tests_path}/integration/sharding_tests/test_sharding_relations.py
+
 
 [testenv:integration]
 description = Run all integration tests