commit 11da4a4abde8fa3db07492e7f6695667fe4c4be9
author Oliver Smith <osmith@sysmocom.de> Thu Aug 19 11:58:09 2021 +0200
committer osmith <osmith@sysmocom.de> Fri Aug 20 14:04:54 2021 +0000
tree fa2b4886ed0fc338aa7517b33f9dd7c21737c5b8
parent a79a549273de9fbd014f2602ebe1885d308e531e

stats: send real last value if no new values come

Background:
* Individual values can be added to osmo_stat_item.values at any time.
* Stats are reported at a fixed interval (see vty 'stats interval'),
  e.g. every 10 seconds.
* In order to report a new stat value, we use the maximum of all
  osmo_stat_item.values added since the last report.
* By default, we do not send new stat values if they did not change
  (see vty 'config-stats' -> 'flush-period' default of 0).

Fix the following bug:
* If 'flush-period' is 0, and no new osmo_stat_item.values are coming
  in, the last value that gets reported is not necessarily the last
  entry in osmo_stat_item.values.
* For attached reporters (statsd), it could then be that the given stat
  stays at the wrong value for a long stretch of time (think of several
  hours/days/forever).

Explanation of how the test shows that it is fixed:
* stats get reported (value is irrelevant)
* osmo_stat_item gets a new value: 20
* osmo_stat_item gets a new value: 10
* stats get reported (value: 20, the maximum of both new values)
* osmo_stat_item gets no new values
* stats get reported (value: 10, this is new because of the bug fix,
  the real last value in osmo_stat_item, different from the 20 sent
  earlier, without the fix it would not send anything here and the last
  sent value would be 20)
* osmo_stat_item gets no new values
* stats get reported (nothing gets sent, since the real last value was
  already sent and 'flush-period' is 0)

Fixes: OS#5215
Change-Id: Ibeefd0e3d1dbe4be454ff05a21df4848b2abfabe