Dell SC: Reject thaw call when backend is failed over

Implements thaw_backend function. Driver has limited support
in a failed over state so thaw_backend has been implemented to
reject the thaw call when in such a state.

Change-Id: I3552f67f0c39c1d1e77e4fa8111ae7a4be165b0d

cinder/tests/unit/volume/drivers/dell/test_dellsc.py

@@ -4003,3 +4003,20 @@ class DellSCSanISCSIDriverTestCase(test.TestCase):
         ret = self.driver._get_qos(12345)
         self.assertEqual('cinderqos', ret)
         self.driver.backends = backends
+
+    def test_thaw_backend(self,
+                          mock_close_connection,
+                          mock_open_connection,
+                          mock_init):
+        self.driver.failed_over = False
+        ret = self.driver.thaw_backend(self._context)
+        self.assertTrue(ret)
+
+    def test_thaw_backend_failed_over(self,
+                                      mock_close_connection,
+                                      mock_open_connection,
+                                      mock_init):
+        self.driver.failed_over = True
+        self.assertRaises(exception.Invalid,
+                          self.driver.thaw_backend,
+                          self._context)

cinder/volume/drivers/dell/dell_storagecenter_common.py

@@ -1730,3 +1730,24 @@ class DellCommonDriver(driver.ConsistencyGroupVD, driver.ManageableVD,
             # Free our snapshot.
             api.unmanage_replay(screplay)
             # Do not check our result.
+
+    def thaw_backend(self, context):
+        """Notify the backend that it's unfrozen/thawed.
+
+        This is a gate. We do not allow the backend to be thawed if
+        it is still failed over.
+
+        :param context: security context
+        :response: True on success
+        :raises Invalid: if it cannot be thawed.
+        """
+        # We shouldn't be called if we are not failed over.
+        if self.failed_over:
+            msg = _('The Dell SC array does not support thawing a failed over'
+                    ' replication. Please migrate volumes to an operational '
+                    'back-end or resolve primary system issues and '
+                    'fail back to reenable full functionality.')
+            LOG.error(msg)
+            raise exception.Invalid(reason=msg)
+
+        return True

releasenotes/notes/Dell-SC-thaw_backend-b9362d381fabd4c9.yaml

@@ -0,0 +1,5 @@
+---
+issues:
+  - Dell SC Cinder driver has limited support in a failed
+    over state so thaw_backend has been implemented to
+    reject the thaw call when in such a state.