Merge ~ack/maas:1908552-3.0 into maas:3.0

[Alberto Donato]

LP: #1908552 - only run DB migrations on init if DB is empty

[Alberto Donato (ack)]

review: Approve

diff --git a/src/maascli/snap.py b/src/maascli/snap.py
index bef3af1..5a16465 100644
--- a/src/maascli/snap.py
+++ b/src/maascli/snap.py
@@ -315,6 +315,21 @@ def change_user(username, effective=False):
         os.setuid(running_uid)
 
 
+def db_need_init(connection=None) -> bool:
+    """Whether the database needs initializing.
+
+    It assumes the database is set up if there's any table in it.
+    """
+    if connection is None:
+        # local import since the CLI shouldn't unconditionally depend on Django
+        from django.db import connection
+
+    try:
+        return not connection.introspection.table_names()
+    except Exception:
+        return True
+
+
 def migrate_db(capture=False):
     """Migrate the database."""
     if capture:
@@ -645,7 +660,7 @@ class cmd_init(SnapCommand):
             start_services,
         )
 
-        if mode in ("region", "region+rack"):
+        if mode in ("region", "region+rack") and db_need_init():
             # When in 'region' or 'region+rack' the migrations for the database
             # must be at the same level as this controller.
             perform_work(
diff --git a/src/maascli/tests/test_snap.py b/src/maascli/tests/test_snap.py
index a0cf740..aee41db 100644
--- a/src/maascli/tests/test_snap.py
+++ b/src/maascli/tests/test_snap.py
@@ -598,3 +598,25 @@ class TestCmdConfig(MAASTestCase):
         config_manager.update.assert_not_called()
         self.assertEqual(stdout.getvalue(), "")
         mock_sighup_supervisord.assert_not_called()
+
+
+class TestDBNeedInit(MAASTestCase):
+    def test_has_tables(self):
+        connection = MagicMock()
+        connection.introspection.table_names.return_value = [
+            "table1",
+            "table2",
+        ]
+        self.assertFalse(snap.db_need_init(connection))
+
+    def test_no_tables(self):
+        connection = MagicMock()
+        connection.introspection.table_names.return_value = []
+        self.assertTrue(snap.db_need_init(connection))
+
+    def test_fail(self):
+        connection = MagicMock()
+        connection.introspection.table_names.side_effect = Exception(
+            "connection failed"
+        )
+        self.assertTrue(snap.db_need_init(connection))