MB-47964: Stop retrieving samples when "main node" changes

This change fixes the badmatch error that happens in the
retrieve_samples_from_archive function:

 {true, AccNodes, Nodes} =
     {AccNodes =:= undefined orelse Nodes =:= AccNodes,
      AccNodes, Nodes},

It happens when 7.* nodes joins 6.5 cluster, and UI requests
stats from the 7.* node using old /_uistats API.

Function do_retrieve_samples_from_archive might return results
with nodes in different order. This means that "main node" that
was chosen by stats gatherer has changed, and this means that
the previous "main node" doesn't have samples for this archive.
In this case there is no need to continue and we can stop.

Change-Id: I4331763b44e732fdd394ae9f35f48f7559b51f31
Reviewed-on: http://review.couchbase.org/c/ns_server/+/161022
Well-Formed: Build Bot <[email protected]>
Well-Formed: Restriction Checker
Tested-by: Timofey Barmin <[email protected]>
Reviewed-by: Artem Stemkovski <[email protected]>

Author: timofey-barmin

src/menelaus_stats.erl

@@ -2917,29 +2917,36 @@ retrieve_samples_from_archive(Archive, Stat,
             {AccSamples, AccNodes, Kind, false};
         #gathered_stats{nodes = Nodes, kind = NewKind, extractor = Extractor,
                         samples = Samples} ->
-            {true, AccNodes, Nodes} =
-                {AccNodes =:= undefined orelse Nodes =:= AccNodes,
-                 AccNodes, Nodes},
-            NewContinue =
-                case latest_start_timestamp(Samples, StartTS) - StartTS > 1000 of
-                    true ->
-                        Continue;
-                    false ->
-                        %% we got all the samples we wanted, time to
-                        %% stop retrieveing
-                        false
-                end,
+            if
+                (AccNodes =:= undefined) orelse (Nodes =:= AccNodes) ->
+                    NewContinue =
+                        LatestStart = latest_start_timestamp(Samples, StartTS),
+                        case LatestStart - StartTS > 1000 of
+                            true ->
+                                Continue;
+                            false ->
+                                %% we got all the samples we wanted, time to
+                                %% stop retrieveing
+                                false
+                        end,
+
+                    MergedSamples =
+                        case Aggregate of
+                            true ->
+                                aggregate_and_merge(Samples, AccSamples,
+                                                    StartTS, EndTS, Extractor);
+                            false ->
+                                merge_samples(Samples, AccSamples, StartTS,
+                                              EndTS, Extractor)
+                        end,
+                    {MergedSamples, Nodes, NewKind, NewContinue};
+
+                true -> %% Main node has changed. It means it doesn't have any
+                        %% samples for this archive, which means we can stop
+                        %% and ignore the last result
+                    {AccSamples, AccNodes, Kind, false}
+            end
 
-            MergedSamples =
-                case Aggregate of
-                    true ->
-                        aggregate_and_merge(Samples, AccSamples, StartTS,
-                                            EndTS, Extractor);
-                    false ->
-                        merge_samples(Samples, AccSamples, StartTS, EndTS,
-                                      Extractor)
-                end,
-            {MergedSamples, Nodes, NewKind, NewContinue}
     end.
 
 do_retrieve_samples_from_archive({Period, Seconds, Count}, StatName,