Fix HPE3PAR not returning cached stats

All Cinder drivers must honor this:

   stats = driver.get_volume_stats(refresh=True)
   cached_stats = driver.get_volume_stats(refresh=False)
   assert stats == cached_stats

But HPE 3PAR drivers don't.  If you try to get cached stats values
you'll get an empty dictionary instead.

This is caused by the get_volume_stats method calling the _login method
which in turn calls _init_common, which returns a new HPE3PARCommon
instance on each call, so it won't have any cached values.

Closes-Bug: #1813930
Change-Id: I011e7142db5d54882fd2cd9c07276ac71a07b4b7

cinder/tests/unit/volume/drivers/hpe/test_hpe3par.py

@@ -7632,6 +7632,32 @@ class TestHPE3PARFCDriver(HPE3PARBaseDriver):
                 expected +
                 self.standard_logout)
 
+    def test_get_volume_stats5(self):
+        # Testing get_volume_stats(refresh=False) for cached values
+        config = self.setup_configuration()
+        self.setup_driver(config=config)
+        with mock.patch.object(self.driver, '_login') as login_mock, \
+                mock.patch.object(self.driver, '_logout') as logout_mock:
+            stats_mock = login_mock.return_value.get_volume_stats
+
+            stats = self.driver.get_volume_stats(True)
+
+            self.assertEqual(stats_mock.return_value, stats)
+            login_mock.assert_called_once_with()
+            stats_mock.assert_called_once_with(True, FILTER_FUNCTION,
+                                               GOODNESS_FUNCTION)
+            logout_mock.assert_called_once_with(login_mock.return_value)
+
+            login_mock.reset_mock()
+            stats_mock.reset_mock()
+            logout_mock.reset_mock()
+
+            cached_stats = self.driver.get_volume_stats(False)
+            self.assertEqual(stats, cached_stats)
+            login_mock.assert_not_called()
+            stats_mock.assert_not_called()
+            logout_mock.assert_not_called()
+
     def test_create_host_with_unmanage_fc_and_manage_iscsi_hosts(self):
         # setup_mock_client drive with default configuration
         # and return the mock HTTP 3PAR client

cinder/volume/drivers/hpe/hpe_3par_base.py

@@ -115,6 +115,14 @@ class HPE3PARDriverBase(driver.ManageableVD,
 
     @utils.trace
     def get_volume_stats(self, refresh=False):
+        # NOTE(geguileo): We don't need to login to the backed if we are not
+        # going to refresh the stats, furthermore if we login, then we'll
+        # return an empty dict, because the _login method calls calls
+        # _init_common which returns a new HPE3PARCommon instance each time,
+        # so it won't have any cached values.
+        if not refresh:
+            return self._stats
+
         common = self._login()
         try:
             self._stats = common.get_volume_stats(