fix(historian): respect gateway ceilings and history value contracts

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

@@ -227,7 +227,27 @@ internal static class GatewayAtTimeReader
                 {
                     if (!beforeKnown)
                     {
-                        before = await WalkBackAsync(ctx, qs, walkKnown, walkWidth).ConfigureAwait(false);
+                        // #696: prove the future stretch empty before jumping to now. Historian clocks or imports
+                        // can place real rows there; a full bounded probe seeds the original backward search.
+                        var searchEnd = qs;
+                        if (qs > now && walkKnown is null)
+                        {
+                            var future = await ReadTrustedAsync(ctx, MsFloor(now),
+                                Min(MsFloor(qs) + OneMs, horizon), ReadRows).ConfigureAwait(false);
+                            StoredSample? latest = null;
+                            foreach (var row in future.Rows)
+                                if (row.TimeUtc <= qs) latest = row;
+                            if (future.Complete)
+                            {
+                                before = latest;
+                                beforeKnown = latest is not null;
+                                searchEnd = now;
+                            }
+                            else if (latest is { } found)
+                                walkKnown = found.TimeUtc;
+                        }
+                        if (!beforeKnown)
+                            before = await WalkBackAsync(ctx, searchEnd, walkKnown, walkWidth).ConfigureAwait(false);
                         beforeKnown = true;
                     }
                     prev = before;

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

@@ -152,6 +152,7 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         catch (Exception ex)
         {
             RecordReadFailure(ex);
+            if (IsTransportFailure(ex)) throw new System.IO.IOException("HistorianGateway read transport is unavailable.", ex);
             throw;
         }
     }
@@ -239,7 +240,15 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         for (var attempt = 0; attempt < MaxCrowdedReReads && preWindow <= _maxPreWindowRows; attempt++)
         {
             var budget = (int)Math.Min(int.MaxValue, (long)cap + room + RawAggregateReader.PreWindowAllowance);
-            if (_ceiling.Learned is var known and > 0 && budget > known) break; // over a strict ceiling: refused anyway
+            // #695: doubling can leap over the ceiling while the cluster still fits. Spend the remaining room once.
+            if (_ceiling.Learned is var known and > 0 && budget > known)
+            {
+                var available = known - cap - RawAggregateReader.PreWindowAllowance;
+                if (available <= 0) break;
+                room = Math.Min(room, available);
+                budget = cap + room + RawAggregateReader.PreWindowAllowance;
+            }
+            if (budget <= pass.Budget) break; // no more room, never retry the same full read
             pass = await PassAsync(budget).ConfigureAwait(false);
             if (pass.Result.Complete) return pass.Result.Samples;
             preWindow = Math.Max(preWindow, pass.Result.PreWindowRows);
@@ -365,6 +374,7 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         catch (Exception ex)
         {
             RecordReadFailure(ex);
+            if (IsTransportFailure(ex)) throw new System.IO.IOException("HistorianGateway read transport is unavailable.", ex);
             throw;
         }
     }
@@ -408,6 +418,7 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         catch (Exception ex)
         {
             RecordReadFailure(ex);
+            if (IsTransportFailure(ex)) throw new System.IO.IOException("HistorianGateway read transport is unavailable.", ex);
             throw;
         }
     }
@@ -466,6 +477,7 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         catch (Exception ex)
         {
             RecordReadFailure(ex);
+            if (IsTransportFailure(ex)) throw new System.IO.IOException("HistorianGateway read transport is unavailable.", ex);
             throw;
         }
     }
@@ -486,6 +498,7 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         catch (Exception ex)
         {
             RecordReadFailure(ex);
+            if (IsTransportFailure(ex)) throw new System.IO.IOException("HistorianGateway read transport is unavailable.", ex);
             throw;
         }
     }
@@ -581,6 +594,7 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         catch (Exception ex)
         {
             RecordReadFailure(ex);
+            if (IsTransportFailure(ex)) throw new System.IO.IOException("HistorianGateway read transport is unavailable.", ex);
             throw;
         }
     }
@@ -655,6 +669,15 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         return result;
     }
 
+    /// <summary>Classifies actual gRPC/package transport failures without treating refusals as outages (#697).</summary>
+    private static bool IsTransportFailure(Exception ex) => ex switch
+    {
+        ZB.MOM.WW.HistorianGateway.Client.HistorianGatewayUnavailableException => true,
+        RpcException rpc => rpc.StatusCode is Grpc.Core.StatusCode.Unavailable or Grpc.Core.StatusCode.DeadlineExceeded,
+        TimeoutException => true,
+        _ => ex.InnerException is { } inner && IsTransportFailure(inner),
+    };
+
     /// <summary>
     ///     Records a failed read: bumps the health counters and logs a generic, redaction-safe
     ///     debug line (no tag, host, or value). The exception itself is rethrown by the caller.