Consider unreadable remote folders (e.g. negative ACLs)

[moscicki]

This is partially related to #4621.

When accessing externally mounted storage (or in our case parts of the underlying filesystem) certain folders may not be readable by the current user (because of underlying ACLs preventing the user to read the folder). This case should be properly recognized by the sync client.

The implementation should consider two cases:

• the PROPFIND request on unreadable folder returns a specific error code (would 403 be OK?) and the sync client knows that it should not be looking into this folder anymore (ignore)
• the PROPFIND of the parent directory should return a property indicating the child folder is or is not readible. This may be simply done by adding one more value to oc:permissions (https://github.com/owncloud/client/blob/master/doc/architecture.rst#server-side--permissions)

[guruz]

This discussion should include @DeepDiver1975

[moscicki]

Here is a related problem to solve: suppose that I change the ACL to give access to a directory which was not readible before. I would assume that a change in ACL should result in an ETAG change and client would PROPFIND again the parent and read the new ACL and access the directory.

[hodyroff]

What happens today? What is the current user experience?

[moscicki]

The current user experience is bad: in this case the client shows 403 error and aborts current sync run. The it restarts a new run which shows 403 and aborts. And so on.

[guruz]

BTW, we already have handling for temporarily unavailable storages in the client:

https://github.com/owncloud/client/blob/master/csync/src/csync_update.c#L695

[felixboehm]

1. Given a folder is unreadable in a mounted external storage, the sync client must not stop syncing.
2. In the filecache mark the folder as unreadable, so a PROPFIND or other access will not provide contents. Ideally including information about the permissions.
   @butonic

[michaelstingl]

@butonic @tomneedham How can the server-side be improved to handle such situations?