fix: barman-to-barman sync for incremental backups (#1115)

[jonatas-rp]

Fix barman-to-barman sync for incremental backups #1115

When syncing incremental backups from a primary barman server to a passive barman server, a KeyError was raised in is_backup_locally_obsolete because the parent backup info did not contain the reference to the children_backup_ids and the field was not loaded correctly when building a BackupInfo object.

Changes:

• Added sync_parent_backup_info method to Server class
• Call sync_parent_backup_info in sync_cron before check_sync_required
• Fixed children_backup_ids deserialization in BackupInfo.from_json
• Added tests for sync_parent_backup_info

[mike-broughton]

I think this change does not deal with the situation where you have synced backups that are beyond the retention period on the primary? So even when this change gets merged, the only way to cleanup an existing barman backup server is to rerun the sync from scratch?

[barthisrael]

I think this change does not deal with the situation where you have synced backups that are beyond the retention period on the primary? So even when this change gets merged, the only way to cleanup an existing barman backup server is to rerun the sync from scratch?

@mike-broughton the code on master has an issue with synching block-level incremental backups: whenever barman cron runs on the passive node and attempts to synchronize a block-level incremental backup, it faces a KeyError exception because the children metadata of the parent backup was not being handled correctly by that code. As a consequence, you would never get block-level incremental backups in the passive node.

The fix will allow incremental backups to be synched from now own.

Could you please expand on what are your concerns? Is that related with block-level incremental backups?