id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	launchpad_bug
1351	Use extended attributes to expose metadata	ScottD	somebody	"A number of users want to be able to mount Tahoe dircaps into their file system (via FUSE, for instance). Once mounted this way, it becomes difficult and error-prone to retrieve any metadata that Tahoe has about files and directories (e.g. a read-only cap).

One way that this could be made better would be to expose those values as extended attributes in the file system. For instance, FUSE supports getxattr, which could be used for this purpose. For instance,

{{{
% getfattr -n readcap /mnt/pubgrid
# file: /mnt/pubgrid
readcap=""URI:DIR2-RO:xxx...:yyy...""
}}}

At the moment, Tahoe does not support storing extended attributes, so there are no conflicts. However, ticket:947 proposes to add support for storing extended attributes to Tahoe, which would create a namespace conflict for this enhancement. Despite that, we could reserve a prefix for attributes (e.g., ""`tahoe.readcap`"") and raise an error/warning when such an attribute is asked to be stored.

There are currently a few different implementations of Tahoe where a specification for getxattr would be useful. We have a couple FUSE implementations in the source tree and the TahoeFS implementation in PyFS (ticket:1111) can expose extended attributes in a variety of ways.

I have personally patched PyFS to expose attributes like I have described and find it useful. Where ""useful"" means that I previously punted entirely on using readonly/verify caps because they were too difficult to retrieve. Additionally, being able to retrieve caps like this allows perform other ""`tahoe`"" commands with ease:

{{{
% tahoe check `getfattr -n readcap --only-values somefile`
Summary: Healthy
 storage index: qggq5w4ynbxo6x2ns3ydv3uyq4
 good-shares: 12 (encoding is 3-of-12)
 wrong-shares: 0
}}}

As and aside, if we reserved a prefix like ""`tahoe.`"" for extended attributes, then we could even make ""`tahoe`"" aware of these attributes to create a shortcut around running ""`getfattr`"".

Prior to pushing this into my local version of PyFS, I would've had to resort to the WUI and copy/pasting sensitive data, which means I just didn't."	enhancement	new	minor	undecided	code	1.8.2		fuse metadata unix usability