iiuc, update_engine is supposed to be started during boot and lives since then. 

If update_engine was not on after reboot (some times it takes a while to be brought up, but eventually it should) then we have to figure out what was the underlying problem that caused UE not to be brought up.

@dhaddock: almost all UE operations that we have now do a restart of UE at the beginning. So if for some reason UE is not brought up automatically during boot, it will be problematic because if it is shipped to users in that state, they will never get update. Can we add some test to just check if update_engine is running and healthy without actually restarting it after reboot? We also can have this as a part of another autotest.

i think it's fine to pursue healthy system/autotests, but that wouldn't be in place of making cros flash smarter about recovering a system regardless of its current state

I will work on this!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/1a4893f29618959cbf00212596a2c5a7bcdb774b

commit 1a4893f29618959cbf00212596a2c5a7bcdb774b
Author: Amin Hassani <ahassani@google.com>
Date: Fri Mar 09 03:29:08 2018

auto_updater: cros flash starts update_engine

If the update_engine is not running in the device, the cros flash fails
badly. This patch starts the update-engine service on the DUT if it was not
running.

BUG= chromium:818201 
TEST=stopped the update-engine service, and cros flashed succeeded.

Change-Id: I4797f8f26185f0487e486379caa72b3b73b2f2a5
Reviewed-on: https://chromium-review.googlesource.com/954413
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>

[modify] https://crrev.com/1a4893f29618959cbf00212596a2c5a7bcdb774b/lib/auto_updater_unittest.py
[modify] https://crrev.com/1a4893f29618959cbf00212596a2c5a7bcdb774b/lib/auto_updater.py