exometer_slide: normalise last_two

If a slide has been idle for some time then sees a single sample
the interval between the samples is much greater than the
slide interval which would result in an articifially low rate.

deps/rabbitmq_management_agent/src/exometer_slide.erl

@@ -294,21 +294,24 @@ first_max(_, X) -> X.
 %% @doc Returns the newest 2 elements on the sample
 last_two(#slide{buf1 = [{TS, Evt} = H1, {_, drop} | _], interval = Interval}) ->
     [H1, {TS - Interval, Evt}];
-last_two(#slide{buf1 = [H1, H2 | _]}) ->
+last_two(#slide{buf1 = [H1, H2_0 | _], interval = Interval}) ->
+    H2 = adjust_timestamp(H1, H2_0, Interval),
     [H1, H2];
-last_two(#slide{buf1 = [H1], buf2 = [H2 | _]}) ->
+last_two(#slide{buf1 = [H1], buf2 = [H2_0 | _],
+                interval = Interval}) ->
+    H2 = adjust_timestamp(H1, H2_0, Interval),
     [H1, H2];
 last_two(#slide{buf1 = [H1], buf2 = []}) ->
     [H1];
-last_two(#slide{buf1 = [], buf2 = [{TS, Evt} = H1, {_, drop} | _], interval = Interval}) ->
-    [H1, {TS - Interval, Evt}];
-last_two(#slide{buf1 = [], buf2 = [H1, H2 | _]}) ->
-    [H1, H2];
-last_two(#slide{buf1 = [], buf2 = [H1]}) ->
-    [H1];
 last_two(_) ->
     [].
 
+adjust_timestamp({TS1, _}, {TS2, V2}, Interval) ->
+    case TS1 - TS2 > Interval of
+        true -> {TS1 - Interval, V2};
+        false -> {TS2, V2}
+    end.
+
 -spec last(slide()) -> value() | undefined.
 last(#slide{total = T}) when T =/= undefined ->
     T;

deps/rabbitmq_management_agent/test/exometer_slide_SUITE.erl

@@ -27,6 +27,7 @@ groups() ->
     [
      {parallel_tests, [parallel], [
                   incremental_add_element_basics,
+                  last_two_normalises_old_sample_timestamp,
                   incremental_last_two_returns_last_two_completed_samples,
                   incremental_sum,
                   incremental_sum_stale,
@@ -110,6 +111,17 @@ incremental_add_element_basics(_Config) ->
     % contains single element with incremented value
     [{Then, {2}}] = exometer_slide:to_list(Then, S2).
 
+last_two_normalises_old_sample_timestamp(_Config) ->
+    Now = 0,
+    S0 = exometer_slide:new(Now, 10, [{incremental, true},
+                                      {interval, 100}]),
+
+    S1 = exometer_slide:add_element(100, {1}, S0),
+    S2 = exometer_slide:add_element(500, {1}, S1),
+
+    [{500, {2}}, {400, {1}}] = exometer_slide:last_two(S2).
+
+
 incremental_last_two_returns_last_two_completed_samples(_Config) ->
     Now = exometer_slide:timestamp(),
     S0 = exometer_slide:new(Now, 10, [{incremental, true},
@@ -121,7 +133,7 @@ incremental_last_two_returns_last_two_completed_samples(_Config) ->
     S1 = exometer_slide:add_element(Now100, {1}, S0),
     S2 = exometer_slide:add_element(Now200, {1}, S1),
     S3 = exometer_slide:add_element(Now + 210, {1}, S2),
-    %% to_list is empty
+
     [{Now200, {2}}, {Now100, {1}}] = exometer_slide:last_two(S3).
 
 incremental_sum(_Config) ->