improve oauth skew to base off uptime

This bug was originally filed in Launchpad as [LP: #1531233](https://bugs.launchpad.net/cloud-init/+bug/1531233)

Launchpad details

affected_projects = ['curtin']

assignee = None

assignee_name = None

date_closed = None

date_created = 2016-01-05T18:26:54.077738+00:00

date_fix_committed = None

date_fix_released = None

id = 1531233

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1531233

milestone = None

owner = smoser

owner_name = Scott Moser

private = False

status = confirmed

submitter = smoser

submitter_name = Scott Moser

tags = []

duplicates = []

_Launchpad user **Scott Moser(smoser)** wrote on 2016-01-05T18:26:54.077738+00:00_

when doing oauth, each request needs to include a timestamp that matches a window of the host's clock.
So, if our local clock is broken we need to adjust it. Cloud-init and curtin handle this by adding a 'skew', and recently store that skew in a dictionary for later reference. The issue though is that currently we store the skew off the current local time. This works fine until the local time is updated, and then it would result in us having a *bad* offset where using none would work.

During boot, ntp or systemd-network-timed might update the clock, so we have to be aware of that.

My plan for a fix is to use uptime as an always increasing/stable clock.

To address the fact that a clock could be wildly inaccurate, we can discard the stored skew if it is older than 10 minutes or some reasonably large value.