fix(historian): #691.2 serve reverse native TimeAverage/Total on Gateway via forward reads on the later-anchored #677 grid

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/GatewayHistorianDataSource.cs

@@ -249,7 +249,9 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
     ///     <para>
     ///         <b>TimeAverage / Total</b> stay native AVEVA retrieval modes (<see cref="AggregateModeMapper"/>), stamped with
     ///         the interval start and the Calculated bit; AVEVA's extra row for the interval before the window is dropped and
-    ///         an Uncertain interval keeps its value (#685, <see cref="SampleMapper.ToAggregateSnapshots"/>). Everything else is <see cref="HistoryAggregateNotSupportedException"/>
+    ///         an Uncertain interval keeps its value (#685, <see cref="SampleMapper.ToAggregateSnapshots"/>). A reverse read
+    ///         (<c>start &gt; end</c>, which the gateway refuses) is served by forward requests on the later-anchored #677 grid
+    ///         (#691.2, <see cref="NativeReverseAggregatePlan"/>). Everything else is <see cref="HistoryAggregateNotSupportedException"/>
     ///         (see <see cref="ServedAggregates"/>).
     ///     </para>
     /// </remarks>
@@ -266,6 +268,9 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         // Mapped OUTSIDE the failure-recording try: an aggregate the gateway has no mode for is a client
         // request the server answers BadAggregateNotSupported, not a backend fault to count (#285).
         var mode = AggregateModeMapper.ToRetrievalMode(aggregate);
+        if (startUtc > endUtc)
+            return await ReadNativeReverseAsync(fullReference, BucketPlan.Create(startUtc, endUtc, interval), mode, cancellationToken)
+                .ConfigureAwait(false);
         try
         {
             // TimeAverage / Total are native gateway retrieval modes (TimeWeightedAverage / Integral) — no
@@ -294,6 +299,44 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         }
     }
 
+    /// <summary>
+    ///     A reverse (<c>start &gt; end</c>) native TimeAverage / Total read (#691.2), anchored at its later time as on the SQL
+    ///     backends (#677) — the gateway refuses <c>start &gt; end</c>, so it is served by forward native requests on the
+    ///     later-anchored grid. See <see cref="NativeReverseAggregatePlan"/>.
+    /// </summary>
+    private async Task<HistoryReadResult> ReadNativeReverseAsync(string fullReference, BucketPlan plan, RetrievalMode mode,
+        CancellationToken cancellationToken)
+    {
+        if (plan.BucketCount == 0) return new HistoryReadResult([], ContinuationPoint: null);
+        try
+        {
+            var byBucket = new Dictionary<long, DataValueSnapshot>();
+            foreach (var request in NativeReverseAggregatePlan.Requests(plan))
+            {
+                var buckets = new List<HistorianAggregateSample>();
+                await foreach (var bucket in _client
+                    .ReadAggregateAsync(fullReference, request.StartUtc, request.EndUtc, mode, request.Interval, cancellationToken)
+                    .ConfigureAwait(false))
+                {
+                    buckets.Add(bucket); // drained to completion, mapped afterwards
+                }
+                NativeReverseAggregatePlan.Place(plan, request, buckets, byBucket);
+            }
+
+            RecordOutcome(success: true, error: null);
+            return new HistoryReadResult(NativeReverseAggregatePlan.Assemble(plan, byBucket), ContinuationPoint: null);
+        }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            throw; // caller-requested, not a backend fault
+        }
+        catch (Exception ex)
+        {
+            RecordReadFailure(ex);
+            throw;
+        }
+    }
+
     /// <summary>Serves a per-sample aggregate from a raw read (#681); see <see cref="RawAggregateReader"/>.</summary>
     private async Task<HistoryReadResult> ReadProcessedFromRawAsync(
         string fullReference, DateTime startUtc, DateTime endUtc, TimeSpan interval,