Bug 2992

Since OpenSSH 5.1, OpenSSH's sftp-server does support a openssh-specific extension to query for file system statistics (#1399). This feature is nice for users accessing their own filesystem (e.g. user's home is located on a distinct, size-limited ZFS filesystem).

There are also many other cases (CDN push zones, file or web hosting) where the user is hosted on a shared, quota-enabled filesystem (UFS/FFS, NFS, EXT3/4, XFS,…) and so this information is less meaningful to the user and may easily result in mis-assumptions.

For example, a user might assume sftp's "df" output represents information not on the actual filesystem, but on their specific own account: they'd either spot plenty of space available (even though they might be running short on quota) or wonder why their almost-empty account is consuming terabytes of space, assuming "some malware" or "being hacked".

I do suggest an option to configure the behavior of the sftp-server's statvfs-extension:
- report statvfs information (default, current behavior)
- report current user's (soft) user quota usage and limits
- report current user's (soft) group quota usage and limits

Introducing such an option enables a system administrator to configure sftp-server according to the specific storage subsystem and the system's configuration. By doing so, the statfs-extension becomes much more useful to sftp users accessing shared, quota-enabled filesystems.