commit 7243217b80ab755615dab266ac242f7b5a8ade96
author Josh Lehan <krellan@google.com> Wed Mar 17 13:35:43 2021 -0700
committer Josh Lehan <krellan@google.com> Fri Apr 02 03:19:58 2021 -0700
tree 7049e25050a24a06b106d00ae329297a92f875a2
parent ffe1834826cd45d3021ca489ada7fe7c5278769e

Add timer expiration feature to ExternalSensor

ExternalSensor now functions as intended, wholly within dbus-sensors,
without requiring any modification to the IPMI or Redfish servers,
to provide the feature of timeout expiration of external data, so that
stale/lost external connections can be properly indicated as such.

A "Timeout" parameter is added, in decimal seconds, providing a
watchdog for the arrival of external data. The expectation is that the
external source will provide D-Bus updates, to the sensor Value
property, at regular intervals, repeating indefinitely.
If this external source stops doing this, the watchdog barks, and
the Value of this Sensor will become set to "NaN". This provides
an indication to consumers of this Sensor, to realize that the Value
of this sensor has became stale/disconnected.

A practical application of this is fan control. Upon loss of external
temperature notification, the fans could be thrown into failsafe
mode, instead of risking the system overheating by wrongly continuing
to believe an old temperature value that has become stale.

Tested: Works for me. I started an external data source, data arrived
into the Value of the sensor. I stopped that external data source,
after the Timeout period, the Value became "NaN". I started the
external source again, the Value became correct again, as soon as
external data started to arrive again. I repeated this stop and start
procedure a few times, verifying that it operated as intended.

Signed-off-by: Josh Lehan <krellan@google.com>
Change-Id: I53b9ff4c0aa771aff4aaf3449fcab23c07afa296