mounts: support reliably detecting and using Azure ephemeral disks

Azure's ephemeral disks are not guaranteed to be assigned the same name by
the kernel every boot. This causes problems on ~2% of Azure instances, and
can be fixed by using udev rules to give us a deterministic path to mount;
this patch introduces those udev rules and modifies the Azure data source
to use them.

Changes to a couple of config modules were also required. In some places,
they just needed to learn to dereference symlinks. In cc_mounts this
wasn't sufficient because the dereferenced device would have been put in
/etc/fstab (rather defeating the point of using the udev rules in the
first place). A fairly hefty refactor was required to separate "is this a
valid block device?" from "what shall I put in fstab?".

LP: #1411582

1 files changed, 4 insertions, 2 deletions

diff --git a/tests/unittests/test_datasource/test_azure.py b/tests/unittests/test_datasource/test_azure.py
index d632bcb9..8952374f 100644
--- a/tests/unittests/test_datasource/test_azure.py
+++ b/tests/unittests/test_datasource/test_azure.py
@@ -475,10 +475,12 @@ class TestAzureBounce(TestCase):
             mock.patch.object(DataSourceAzure, 'list_possible_azure_ds_devs',
                               mock.MagicMock(return_value=[])))
         self.patches.enter_context(
-            mock.patch.object(DataSourceAzure, 'find_ephemeral_disk',
+            mock.patch.object(DataSourceAzure,
+                              'find_fabric_formatted_ephemeral_disk',
                               mock.MagicMock(return_value=None)))
         self.patches.enter_context(
-            mock.patch.object(DataSourceAzure, 'find_ephemeral_part',
+            mock.patch.object(DataSourceAzure,
+                              'find_fabric_formatted_ephemeral_part',
                               mock.MagicMock(return_value=None)))
         self.patches.enter_context(
             mock.patch.object(DataSourceAzure, 'get_metadata_from_fabric',