Do not remove root user on disable

The existence of the root user (with remote access)
is used to determine whether root was ever enabled on a restored
instance.
Do not remove it, just generate a new random password for it.

Change-Id: I8a4321ac062b1ec565945b49dbb7c619b6da867f
Closes-Bug: 1549600

releasenotes/notes/mysql-root-fix-35079552e25170ca.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - Do not remove MySQL root user on root-disable so that the
+    proper status can be reported on restore. Bug 1549600

trove/guestagent/datastore/mysql_common/service.py

@@ -1062,7 +1062,6 @@ class BaseMySqlRootAccess(object):
             return user.serialize()
 
     def disable_root(self):
-        """Disable the root user global access
+        """Reset the root password to an unknown value.
         """
-        with self.local_sql_client(self.mysql_app.get_engine()) as client:
-            client.execute(text(sql_query.REMOVE_ROOT))
+        self.enable_root(root_password=None)

trove/tests/unittests/guestagent/test_dbaas.py

@@ -42,7 +42,6 @@ from trove.conductor import api as conductor_api
 from trove.guestagent.common.configuration import ConfigurationManager
 from trove.guestagent.common.configuration import ImportOverrideStrategy
 from trove.guestagent.common import operating_system
-from trove.guestagent.common import sql_query
 from trove.guestagent.datastore.experimental.cassandra import (
     service as cass_service)
 from trove.guestagent.datastore.experimental.couchbase import (
@@ -1669,14 +1668,12 @@ class MySqlRootStatusTest(trove_testtools.TestCase):
             mock_execute.assert_any_call(TextClauseMatcher(
                 'UPDATE mysql.user'))
 
-    def test_root_disable(self):
-        with patch.object(self.mock_client,
-                          'execute', return_value=None) as mock_execute:
-            # invocation
-            MySqlRootAccess().disable_root()
-            # verification
-            mock_execute.assert_any_call(TextClauseMatcher(
-                sql_query.REMOVE_ROOT))
+    @patch.object(MySqlRootAccess, 'enable_root')
+    def test_root_disable(self, enable_root_mock):
+        # invocation
+        MySqlRootAccess().disable_root()
+        # verification
+        enable_root_mock.assert_called_once_with(root_password=None)
 
 
 class MockStats: