fix(historian): #691.4 walk reverse events reads back on forward-only backends (Gateway)

A reverse or end-only capped events read on a backend without SupportsReverseEvents (Gateway, Null) was refused BadHistoryOperationUnsupported (explicit reverse) or answered with the OLDEST N ascending (end-only, #675). It is now served newest-first by the same ReverseWalk as raw reads: page-sized ms-aligned segments, never answered from a read the source flagged truncated, ties sorted by EventId (the gateway orders by event time only - the fake shuffles them per call), one extra event walked to signal 'more', uncapped = one forward read reversed, and the width + proven-empty stretch carried in the events cursor. The oversized-cluster over-fetch keeps exactly the events at T in the same order. A tie cluster over the budget is BadTooManyOperations. Docs note the gateway's EventReadMaxRows must be unlimited (it truncates silently).
2026-10-12 16:48:05 -04:00
parent 186b7d5ea5
commit d277f34a96
9 changed files with 251 additions and 82 deletions
@@ -35,7 +35,7 @@ public sealed class GatewayEventBoundsTests
    {
        var fake = new FakeHistorianGatewayClient();
        IHistorianDataSource ds = new GatewayHistorianDataSource(fake, NullLogger<GatewayHistorianDataSource>.Instance);
-        ds.SupportsReverseEvents.ShouldBeFalse("the server must keep end-only reads forward on the default backend");
+        ds.SupportsReverseEvents.ShouldBeFalse("the server must walk reverse reads back from forward reads on the default backend (#691.4)");

        await Should.ThrowAsync<NotSupportedException>(() =>
            ds.ReadEventsAsync("Pump1", DateTime.UtcNow, DateTime.UtcNow.AddHours(-1), 10, reverse: true, Ct));