NFS: Update connection info on online snap create

The NFS snapshot creation for an attached volume requires interaction
between Nova and Cinder, and a new qcow2 file is used after the
attachment completes.

This means that the connection properties stored in the Attachment is no
longer valid, as it is pointing to the old qcow2 file, so if Nova tries
to use that attachment it will start writing on the old qcow2 file.

A flow showing this issue is:

- Attach NFS volume
- Create snapshot
- Hard reboot

After that the VM will start using the base image, breaking the qcow2
chain.

If we delete the snapshot in the meantime, then the VM will fail to
reboot.

This patch fixes this inconsistency by updating the connection info
field inside the remotefs driver.

We usually prefer that drivers don't to touch the DB, directly or
indirectly (using OVOs), but in this case we are using OVOs methods
instead of the usual model update on the volume manager because there
are cases in the driver where a snapshot is created (cloning via
snapshot) and we have to update the attachment without the manager, as
it is unaware that a temporary snapshot is being created.

Besides that main reason there are other less critical reasons to do the
attachment update in the driver:

- Smaller code change
- Easier to backport
- Limit change impact on other areas (better for backport)
- The snapshot_create model update code in the manager does not support
  updating attachments.
- There are cases in the cinder volume manager where the model update
  values returned by snapshot_create are not being applied.

Snapshot deletion belonging to in-use volumes are not affected by this
issue because we only do block commit when the snapshot file we are
deleting is not the active file.  In _delete_snapshot_online:

        if utils.paths_normcase_equal(info['active_file'],
                                      info['snapshot_file']):

Closes-Bug: #1860913
Change-Id: I62fcef3169dcb9f4363a5344af4b2711edfef632

cinder/tests/unit/volume/drivers/test_remotefs.py

@@ -74,7 +74,7 @@ class RemoteFsSnapDriverTestCase(test.TestCase):
         self._fake_snap_c = fake_snapshot.fake_snapshot_obj(self.context)
         self._fake_snap_c.volume = self.volume_c
         self.volume_c_path = os.path.join(self._FAKE_MNT_POINT,
-                                          self._fake_snap_c.name)
+                                          self.volume_c.name)
         self._fake_snap_c_path = (self.volume_c_path + '.' +
                                   self._fake_snap_c.id)
 
@@ -356,6 +356,18 @@ class RemoteFsSnapDriverTestCase(test.TestCase):
             snapshot.volume.status = 'backing-up'
             snapshot.volume.attach_status = 'attached'
             expected_method_called = '_create_snapshot_online'
+            conn_info = ('{"driver_volume_type": "nfs",'
+                         '"export": "localhost:/srv/nfs1",'
+                         '"name": "old_name"}')
+            attachment = fake_volume.volume_attachment_ovo(
+                self.context, connection_info=conn_info)
+            snapshot.volume.volume_attachment.objects.append(attachment)
+            mock_save = self.mock_object(attachment, 'save')
+
+            # After the snapshot the connection info should change the name of
+            # the file
+            expected = copy.deepcopy(attachment.connection_info)
+            expected['name'] = snapshot.volume.name + '.' + snapshot.id
         else:
             expected_method_called = '_do_create_snapshot'
 
@@ -372,6 +384,11 @@ class RemoteFsSnapDriverTestCase(test.TestCase):
             mock.sentinel.fake_info_path,
             expected_snapshot_info)
 
+        if volume_in_use:
+            mock_save.assert_called_once()
+            changed_fields = attachment.cinder_obj_get_changes()
+            self.assertEqual(expected, changed_fields['connection_info'])
+
     @ddt.data({'encryption': True}, {'encryption': False})
     def test_create_snapshot_volume_available(self, encryption):
         self._test_create_snapshot(encryption=encryption)

cinder/volume/drivers/remotefs.py

@@ -1653,18 +1653,25 @@ class RemoteFSSnapDriverBase(RemoteFSDriver):
         backing_filename = self.get_active_image_from_info(
             snapshot.volume)
         new_snap_path = self._get_new_snap_path(snapshot)
+        active = os.path.basename(new_snap_path)
 
         if self._is_volume_attached(snapshot.volume):
             self._create_snapshot_online(snapshot,
                                          backing_filename,
                                          new_snap_path)
+            # Update reference in the only attachment (no multi-attach support)
+            attachment = snapshot.volume.volume_attachment[0]
+            attachment.connection_info['name'] = active
+            # Let OVO know it has been updated
+            attachment.connection_info = attachment.connection_info
+            attachment.save()
         else:
             self._do_create_snapshot(snapshot,
                                      backing_filename,
                                      new_snap_path)
 
-        snap_info['active'] = os.path.basename(new_snap_path)
-        snap_info[snapshot.id] = os.path.basename(new_snap_path)
+        snap_info['active'] = active
+        snap_info[snapshot.id] = active
         self._write_info_file(info_path, snap_info)
 
     def _create_snapshot_online(self, snapshot, backing_filename,

releasenotes/notes/nfs-online-snapshot-c05e6c8113bbded6.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    NFS driver `bug #1860913
+    <https://bugs.launchpad.net/cinder/+bug/1860913>`_: Fixed instance uses
+    base image file when it is rebooted after online snapshot creation.