Moving files in/out of shared storage is broken

[rullzer]

1. have two users user1 and user2
2. user1 creates a folder foo
3. user1 creates test.txt inside foo
4. user1 shares foo with user2 with fill access
5. user2 tries to move test.txt out of foo

Expected:
File is moved and all is right with the world

Actual:
Forbidden. And not all is right in the world

[rullzer]

My bet is this is introduced with the disabling of the encryption wrapper. Since that turned a move in/out of a shared storage in a copy (which is not right!).

@icewind1991 I need your FS knowledge to solve this properly.

[rullzer]

As far as I can tell the issue is that the rootpath is set relative to the owner of a shared storage in https://github.com/nextcloud/server/blob/master/apps/files_sharing/lib/SharedStorage.php#L119

Which then breaks https://github.com/nextcloud/server/blob/master/lib/private/Files/Storage/Local.php#L435

Since the rename breaks on $sourceStorage->getSourcePath($sourceInternalPath) because that returns somethings like files/foo/test.txt which is basically missing the owner part.

[icewind1991]

Fix is here: #4329