local juju users: bob vs bob at local

[roger peppe]

We've been working on adding external user support to juju
and a discussion arose from a throwaway comment on the
IRC channel:

"The difference between Canonical() and Id() seems confusing to me"

The comment referred to two names.UserTag methods
(defined in the package github.com/juju/names).

The Canonical method (†) returns the "canonical" form of
a user always with an @ symbol. For example bob at local (a local user),
bob at somewhere (an external user).

The Id method returns whatever id the UserTag was created with, which
preserves a basic invariant of the Tag interface - NewUserTag(x).Id() == x
for any valid user id x.

It seemed to us that the reason for the confusing Canonical method
was entirely because we have two alternative spellings
for a local user, "bob" and "bob at local".

As a solution to this, we propose that there be exactly
one spelling of a local user - plain "bob", and
that *any* username with an @ domain should be considered
to be external.

If we do this, there is no need for a Canonical method
because every user name is inherently in canonical form.
That is, there is no way to get it wrong.

It is also consistent with the way that user names are currently stored
in the database - they are stored without the @local suffix,
and we will soon want to store information about external
users in the same database, so using the above suggested
canonical form means that no database migration is necessary.

We believe that this is also a very intuitive model:

*    A username is external if and only if it has an @domain suffix.*

We have a working feature branch with this change applied,
but we'd be grateful for any feedback on the proposal.

  cheers,
    rog.

(†) until this morning, this was named Username, but we
changed it because we found the fact that Username()
doesn't return a string that satisfies IsValidUserName
somewhat confusing.
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