Add support of a common volume_backend_name

Previously, we didn't have a control over volume_backend_name other than
the default app name in the Juju model. A common backend name to
multiple backends with the same character is useful because those can be
treated as a single virtual backend associated with a single volume
type.

Change-Id: I4b57f7979837d21a1b116007f3da707ee154792b
Closes-Bug: #1884511

config.yaml

@@ -24,6 +24,16 @@ options:
       created for the pool. The number of placement groups for a pool can
       only be increased, never decreased - so it is important to identify the
       percent of data that will likely reside in the pool.
+  volume-backend-name:
+    default:
+    type: string
+    description: |
+      Volume backend name for the backend. The default value is the
+      application name in the Juju model, e.g. "cinder-ceph-mybackend"
+      if it's deployed as `juju deploy cinder-ceph cinder-ceph-mybackend`.
+      A common backend name can be set to multiple backends with the
+      same characters so that those can be treated as a single virtual
+      backend associated with a single volume type.
   backend-availability-zone:
     default:
     type: string

hooks/cinder_contexts.py

@@ -73,6 +73,11 @@ class CephSubordinateContext(OSContextGenerator):
         else:
             volume_driver = 'cinder.volume.driver.RBDDriver'
 
+        if config('volume-backend-name'):
+            volume_backend_name = config('volume-backend-name')
+        else:
+            volume_backend_name = service
+
         if config('pool-type') == 'erasure-coded':
             pool_name = (
                 config('ec-rbd-metadata-pool') or
@@ -81,8 +86,7 @@ class CephSubordinateContext(OSContextGenerator):
             )
         else:
             pool_name = config('rbd-pool-name') or service
-
-        section = {service: [('volume_backend_name', service),
+        section = {service: [('volume_backend_name', volume_backend_name),
                              ('volume_driver', volume_driver),
                              ('rbd_pool', pool_name),
                              ('rbd_user', service),

unit_tests/test_cinder_contexts.py

@@ -118,6 +118,32 @@ class TestCinderContext(CharmTestCase):
                 }
             }})
 
+    def test_ceph_explicit_volume_backend_name(self):
+        self.test_config.set('volume-backend-name', 'special-backend-name')
+        self.is_relation_made.return_value = True
+        self.get_os_codename_package.return_value = "mitaka"
+        service = 'mycinder'
+        self.service_name.return_value = service
+        self.assertEqual(
+            contexts.CephSubordinateContext()(),
+            {"cinder": {
+                "/etc/cinder/cinder.conf": {
+                    "sections": {
+                        service: [
+                            ('volume_backend_name', 'special-backend-name'),
+                            ('volume_driver',
+                             'cinder.volume.drivers.rbd.RBDDriver'),
+                            ('rbd_pool', service),
+                            ('rbd_user', service),
+                            ('rbd_secret_uuid', 'libvirt-uuid'),
+                            ('rbd_ceph_conf',
+                             '/var/lib/charm/mycinder/ceph.conf'),
+                            ('report_discard_supported', True)
+                        ]
+                    }
+                }
+            }})
+
     def test_ceph_related_erasure_coded(self):
         self.is_relation_made.return_value = True
         self.get_os_codename_package.return_value = "queens"