SFTP and FTP: allow logging in with an arbitrary cap URI as root directory

[davidsarah]

The SFTP and FTP frontends should allow logging in with username uri, and password an arbitrary cap URI.

Implementing this for SFTP can then be used to support a tahoe mount command (#1357), as suggested in ticket:1353#comment:3. For both SFTP and FTP, it is potentially useful to be able to log in with a root URI without having set up an account for it in the ftp.accounts file. (SFTP and FTP use the same code in src/allmydata/frontends/auth.py​ to handle logins, so it is simpler for them to behave the same.)

Note that you can already access an arbitrary cap URI via the /uri/ directory, but that does not have nearly as nice usability properties, because you can't access aliases that way. (Allowing access to aliases would provide ambient authority and so is not capability-secure.)

[davidsarah]

SFTP and FTP: allow logging in with an arbitrary cap URI as root directory. refs #1356

[davidsarah]

I have manually tested that this patch works for SFTP with FileZilla, and the OpenSSH command-line sftp client. I have not yet checked that it works with sshfs. I have not checked FTP (which I can't get to work at all because of #1360).

Note that if you give an incorrect root URI as the password, the login will succeed but subsequent operations will fail. This is the same behaviour as when an incorrect root URI is given for the account in the ftp.accounts file. Perhaps the login should fail in both cases.

[Zarutian]

Replying to davidsarah:

-snip-

Note that if you give an incorrect root URI as the password, the login will succeed but subsequent operations will fail. This is the same behaviour as when an incorrect root URI is given for the account in the ftp.accounts file. Perhaps the login should fail in both cases.

It is a good behaviour to fail the soonest when some problem occurs. If login failed then it should be reported there and then and not at the next operation. So, I say Aye to the last sentance in the quoted comment.

[zooko]

Removing the tag design-reviewed because it matches this search for reviewed: ​http://tahoe-lafs.org/trac/tahoe-lafs/query?status=!closed&keywords=~reviewed&order=priority , which query I use to find patches that I should apply to trunk.

[davidsarah]

Since the main motivation for this is the proposed tahoe mount command, which is not ready for 1.9, I'm bumping this out of 1.9 as well.