Listing backups fails on some installations against Azure

[adejanovski]

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While we couldn't reproduce the issue, we're seeing installations where backups are taken correctly but listing them fails with the following error:

Traceback (most recent call last):
 File "/usr/local/bin/medusa", line 8, in <module>
  sys.exit(cli())
 File "/usr/local/lib/python3.6/site-packages/click/core.py", line 1128, in __call__
  return self.main(*args, **kwargs)
 File "/usr/local/lib/python3.6/site-packages/click/core.py", line 1053, in main
  rv = self.invoke(ctx)
 File "/usr/local/lib/python3.6/site-packages/click/core.py", line 1659, in invoke
  return _process_result(sub_ctx.command.invoke(sub_ctx))
 File "/usr/local/lib/python3.6/site-packages/click/core.py", line 1395, in invoke
  return ctx.invoke(self.callback, **ctx.params)
 File "/usr/local/lib/python3.6/site-packages/click/core.py", line 754, in invoke
  return __callback(*args, **kwargs)
 File "/usr/local/lib/python3.6/site-packages/click/decorators.py", line 84, in new_func
  return ctx.invoke(f, obj, *args, **kwargs)
 File "/usr/local/lib/python3.6/site-packages/click/core.py", line 754, in invoke
  return __callback(*args, **kwargs)
 File "/usr/local/lib/python3.6/site-packages/medusa/medusacli.py", line 185, in list_backups
  medusa.listing.list_backups(medusaconfig, show_all)
 File "/usr/local/lib/python3.6/site-packages/medusa/listing.py", line 41, in list_backups
  cluster_backups = get_backups(config, show_all)
 File "/usr/local/lib/python3.6/site-packages/medusa/listing.py", line 29, in get_backups
  key=lambda b: b.started
 File "/usr/local/lib/python3.6/site-packages/medusa/storage/__init__.py", line 355, in list_cluster_backups
  key=lambda b: (b.name, b.started)
 File "/usr/local/lib/python3.6/site-packages/medusa/storage/__init__.py", line 190, in list_node_backups
  blobs_by_backup = self.group_backup_index_by_backup_and_node(backup_index_blobs)
 File "/usr/local/lib/python3.6/site-packages/medusa/storage/__init__.py", line 299, in group_backup_index_by_backup_and_node
  key=name_and_fqdn
 File "/usr/local/lib/python3.6/site-packages/medusa/storage/__init__.py", line 288, in name_and_fqdn
  return get_backup_name(blob), Storage.get_fqdn_from_any_index_blob(blob)
 File "/usr/local/lib/python3.6/site-packages/medusa/storage/__init__.py", line 285, in get_backup_name
  return blob.name.split('/')[2] if len(str(self.prefix_path)) <= 1 else blob.name.split('/')[3]
IndexError: list index out of range

After adding some debugging outputs, we see that the folders are returned when listing backups, giving a hierarchical view of the blobs instead of a flat one:

[2023-06-29 20:29:25,480] DEBUG: Loading storage_provider: azure_blobs
[2023-06-29 20:29:26,966] DEBUG: [Storage] Listing objects in index/backup_index
[2023-06-29 20:29:26,991] DEBUG: Found backup index blob: index/backup_index
[2023-06-29 20:29:26,991] DEBUG: Found backup index blob: index/backup_index/cassandra_backup_20230628
[2023-06-29 20:29:26,991] DEBUG: Found backup index blob: index/backup_index/cassandra_backup_20230628/differential....
...

This is unexpected and not dealt with correctly in the code.
We either need to filter out the folders when listing blobs, which should be made possible thanks to their 0 size.
We could also make the get_backup_name() method more resilient to such issues and do a precheck before trying to access the split array indices. If we don't have the index we're looking for, we can issue a warning and skip the file without erroring.

Definition of Done

• when listing blobs in the list-backups command, empty files are filtered out
• get_backup_name() will detect blob that aren't named as expected and ignore them