Stop recording update-status runs.

[howbazaar]

The key purpose of the show-status-history command is to show historical events for a unit.

For any long running model, most units just show 'running update-status hook' followed by 'idle'. This isn't overly useful. So, now we don't record the 'running update-status hook' when the uniter runs it. The logging is still there, so it shows the hook running. When there is an error, this is still shown to the user.

For the workload status, if this value doesn't change, new history entries are no longer added. The current workload status is updated to reflect the timestamp change, but this history isn't added.

QA steps

juju deploy ./acceptancetests/repository/charms/update-status-tester tester
# wait 15 minutes or so so multiple update-status calls are made
juju show-status-history tester/0

During the testing for this branch, I found that show-status-history itself is a little broken in its display logic, so can check the database for the history values:

# agent status
db.statuseshistory.find({globalkey: "u#tester/0#charm"}).sort({updated:-1}).pretty()
# workload status
db.statuseshistory.find({globalkey: "u#tester/0"}).sort({updated:-1}).pretty()

Documentation changes

We should document that the update-status hook is no longer shown in juju status or the status history for a unit.

Bug reference

https://bugs.launchpad.net/juju/+bug/1530840

[howbazaar]

persist update-status only on effective change

[jameinel]

One small tweak. The general behavior of omitting duplicates and then not going into Executing seems ok to me.

[jameinel]

should we have an
} else if !mgo.NotFound(err) {
return err
}

I'm concerned that some other types of errors like "mongo died" would get swallowed here.

[howbazaar]

There is no error response from this method. It is a "maybeUpdate".

[howbazaar]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://ci.jujucharms.com/job/github-merge-juju