Downgrade of 2.9.1 to 2.9.0 triggers deletion WinVFS

[TheOneRing]

2.9.1 uses a branded registration id for WinVFS sync roots.
Clients < 2.9.1 try to unregister previous ids.
An unregistration causes all placeholders to vanish.
In a small testing scenario this doesn't look too bad as all files will disappear at once and we will be presented with the
Delete all/keep all dialog.
In a real world scenario however will will have some hydrated files, so it appears to the client as if those files where explicitly deleted.

TODO:

• Add version check for the registration, prevent downgrade in the future.
• Bump the 2.9.1 account config version, this will also prevent downgrades.

Logs: https://github.com/owncloud/enterprise/issues/4777

[gabi18]

Behavior with test scenario downgrade testpilotcloud 2.9.1rc2 to testpilotcloud 2.9.0:

• install testpilot 2.9.1rc2 and create 2 accounts, one with VFS ON and another with VFS OFF
• deinstall 2.9.1rc2 (otherwise a popup appears "No downgrades allowed, please uninstall older version..." when trying to install 2.9.0)
• install testpilotcloud 2.9.0

Result:

• Popup with warning -> okay

• 'Continue' results in VFS sync connection is removed, cfg-file is saved -> okay

• Connection without VFS remains -> okay

[gabi18]

Additionally tested: downgrade owncloud-client 2.9.2daily to 2.9.1rc2

Result: red error message "Downgrading ... not supported" -> okay

[gabi18]

Additionally tested: downgrade testpilot 2.9.1rc2 to 2.8.2

Result:
Previously configured accounts aren't available any longer

Cfg-files:
2_8_2.zip

[TheOneRing]

Additionally tested: downgrade testpilot 2.9.1rc2 to 2.8.2

Result: Previously configured accounts aren't available any longer

Cfg-files: 2_8_2.zip

This happens as the client config location for some branded clients changed with 2.9 compared to 2.8

[gabi18]

This happens as the client config location for some branded clients changed with 2.9 compared to 2.8

When updating to testpilot 2.9.1rc2 again the accounts are available again, everything is like before.

I will retest with the unbranded client.

[gabi18]

Confirmed that the downgrade for the unbranded client from client 2.9.1rc2 to 2.8.2 behaves like downgrade to 2.9.0, the sync folder connection gets removed, see #9114 (comment). Only difference is that on have to enter the credentials for th accounts.

Further testing: it's possible to choose the previous folder on 'Add Folder Sync Connection' and reuse it.

[gabi18]

Retest downgrade 2.9.1rc2 to 2.9.0 with owncloud-client and check behaviour after 'Continue'

• install 2.9.1rc2 and configure 2 accounts (VFS on/off)
• deinstall 2.9.1rc2, install and start 2.9.0
• popup about some settings not available
  
• 'Continue' -> client hides
• account without VFS remains
• account with VFS shows 'Add Sync Folder Connection'
• open Explorer: all data remain (folder, dehydrated + local available files), .sync_journal.db there
• 'Add Sync Folder Connection' and choose previous sync folder
  Result:
  
• sync connection has to be added to new sync folder root -> okay

[TheOneRing]

As far as possible we prevent a downgrade, in branded scenarios a downgrade is not supported at all.