Unite uniter state files; charm upgrades

Unite uniter state files; charm upgrades

The most notable common feature of the two preceding rejections was that
their mechanisms for smoothly managing the switch from deployment to hook-
running were clunky and inelegant. This prompted me to switch over to a
single uniter state file, which is written atomically, and can compactly
express everything the uniter needs to persist (excluding relation state).

The immediate effects of this change were to move significant chunks of code
from the hook and charm packages directly into uniter/state.go; it also
naturally induced minor fixes across uniter.go and modes.go (to stay
compatible).

Doing this made it clear that the original hook state storage strategy was
flawed: saving even a membership list for a 100k-scale relation is a bad
idea even when we're only persisting it at relation hook time, but it's
just plain awful if we might have to save it repeatedly. So: we no longer
save hook.Info.Members, and we depend on the Relationer to induce the
appropriate changes to relation state. This was pleasing; it required only a
single else branch in Relationer.PrepareHook.

In turn, it allowed me to write a charm.Deployer which uses charm bundles
and URLs to deploy to (slightly enhanced) GitDirs, and enabled a reasonably
straightforward rewrite of Uniter.changeCharm (now deploy), which subsumes
a small but significant chunk of charm.Manager.

The Uniter modes also underwent related changes; most notably, ModeInit is
now almost vestigial, but I expect to add to it very soon, so I would prefer
to keep it around. ModeContinue has been rewritten but still works the
same; ModeStarted and ModeHookError now upgrade charms, while ModeUpgrading
and ModeConflicted have been added to implement this behaviour.

I'm really pretty happy with this as it stands, and hope the above reduces
the WTF-is-he-doing mental load a little; the only thing I'm sure *won't*
make it through review is the not-doing-anything-right-now Op, which I
called "Abide" after some time with a thesaurus, but I'm pretty sure is
wrong.

https://code.launchpad.net/~fwereade/juju-core/uniter-operation-state/+merge/122662

(do not edit description out of merge proposal)

[fwereade, 2012-09-04 12:25:28]

Please take a look.