fix(mtconnect): resetTriggered only resets data sets; empty set is Good {}; re-baseline test (#650 review)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDtos.cs

@@ -178,6 +178,12 @@ public sealed record MTConnectStreamsResult(
 ///     <c>true</c> when the observation carried a non-empty <c>resetTriggered</c> attribute — the Agent's statement that
 ///     the set was reset, so this observation <b>replaces</b> the accumulated set rather than merging into it.
 /// </param>
+/// <param name="IsDataSet">
+///     <c>true</c> when the element is data-set shaped: it carries the <c>count</c> attribute every DATA_SET / TABLE
+///     observation has (and no SAMPLE or ordinary EVENT does) or has <c>&lt;Entry&gt;</c> children. This — not
+///     <see cref="ResetTriggered"/> — is what lets the index serve an entry-less set as <c>{}</c> rather than as
+///     an absent value; <c>resetTriggered</c> also appears on ordinary accumulating SAMPLEs and must not matter there.
+/// </param>
 /// <param name="Condition">
 ///     The CONDITION-only detail (#626) — set exactly when the observation came from a
 ///     <c>&lt;Condition&gt;</c> container, <c>null</c> for every Sample/Event. Its presence is what marks an
@@ -192,7 +198,8 @@ public sealed record MTConnectObservation(
     double? SampleRate = null,
     MTConnectConditionDetail? Condition = null,
     IReadOnlyList<MTConnectDataSetEntry>? Entries = null,
-    bool ResetTriggered = false);
+    bool ResetTriggered = false,
+    bool IsDataSet = false);
 
 /// <summary>One <c>&lt;Cell key="…"&gt;</c> of a <c>TABLE</c> row (#650).</summary>
 public sealed record MTConnectDataSetCell(string Key, string Value);

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectObservationIndex.cs

@@ -121,6 +121,7 @@ internal sealed class MTConnectObservationIndex
     // Apply, but the lock keeps a concurrent one-shot read-through from corrupting a shared index.
     private readonly Dictionary<string, SortedDictionary<string, SetEntry>> _sets = new(StringComparer.Ordinal);
     private readonly object _setsLock = new();
+    private bool _anySets;
 
     private sealed record SetEntry(string? Value, SortedDictionary<string, string>? Cells);
 
@@ -264,18 +265,37 @@ internal sealed class MTConnectObservationIndex
 
         if (observation.Entries is not { Count: > 0 } entries)
         {
-            if (observation.ResetTriggered && !observation.IsStructured)
+            var unavailable = string.IsNullOrWhiteSpace(observation.Value) && !observation.IsDataSet ||
+                              string.Equals(observation.Value, UnavailableSentinel, StringComparison.Ordinal);
+
+            if (unavailable)
             {
-                // resetTriggered with no entries: an empty set, which is a legitimate value rather than an absence.
-                lock (_setsLock) _sets[id] = new SortedDictionary<string, SetEntry>(StringComparer.Ordinal);
-                seenSets.Add(id);
-                return observation with { Value = "{}", ResetTriggered = false };
+                // Hot path for every ordinary UNAVAILABLE observation: skip the lock until a set exists at all.
+                if (Volatile.Read(ref _anySets)) lock (_setsLock) _sets.Remove(id);
+
+                return observation;
             }
 
-            if (string.IsNullOrWhiteSpace(observation.Value) ||
-                string.Equals(observation.Value, UnavailableSentinel, StringComparison.Ordinal))
+            if (observation.IsDataSet && !observation.IsStructured)
             {
-                lock (_setsLock) _sets.Remove(id);
+                // An entry-less data-set-shaped observation (count="0", or resetTriggered) is the EMPTY set - a
+                // legitimate value, Good "{}", not an absence. A reset or a /current baseline discards what
+                // was accumulated; a bare empty delta leaves the set as it is.
+                string emptyJson;
+                lock (_setsLock)
+                {
+                    var first = seenSets.Add(id);
+                    if (!_sets.TryGetValue(id, out var set) || observation.ResetTriggered || (baseline && first))
+                    {
+                        set = new SortedDictionary<string, SetEntry>(StringComparer.Ordinal);
+                        _sets[id] = set;
+                        Volatile.Write(ref _anySets, true);
+                    }
+
+                    emptyJson = RenderSet(set);
+                }
+
+                return observation with { Value = emptyJson, ResetTriggered = false, IsDataSet = false };
             }
 
             return observation;
@@ -289,6 +309,7 @@ internal sealed class MTConnectObservationIndex
             {
                 set = new SortedDictionary<string, SetEntry>(StringComparer.Ordinal);
                 _sets[id] = set;
+                Volatile.Write(ref _anySets, true);
             }
 
             foreach (var entry in entries)
@@ -312,7 +333,7 @@ internal sealed class MTConnectObservationIndex
             json = RenderSet(set);
         }
 
-        return observation with { Value = json, IsStructured = false, Entries = null, ResetTriggered = false };
+        return observation with { Value = json, IsStructured = false, Entries = null, ResetTriggered = false, IsDataSet = false };
     }
 
     /// <summary>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectStreamsParser.cs

@@ -382,6 +382,7 @@ internal static class MTConnectStreamsParser
         int? sampleCount = null;
         double? sampleRate = null;
         var resetTriggered = false;
+        var hasCount = false;
         List<MTConnectDataSetEntry>? entries = null;
         (string? NativeCode, string? NativeSeverity, string? Qualifier, string? Type) conditionAttributes = default;
 
@@ -410,6 +411,7 @@ internal static class MTConnectStreamsParser
             if (!isCondition)
             {
                 resetTriggered = !string.IsNullOrWhiteSpace(MTConnectXml.OptionalAttribute(reader, "resetTriggered"));
+                hasCount = MTConnectXml.OptionalAttribute(reader, "count") is not null;
                 entries = [];
                 sampleCount = int.TryParse(
                     MTConnectXml.OptionalAttribute(reader, "sampleCount"),
@@ -483,7 +485,11 @@ internal static class MTConnectStreamsParser
             SampleRate: sampleRate,
             Condition: condition,
             Entries: entries is { Count: > 0 } ? entries : null,
-            ResetTriggered: resetTriggered);
+
+            // resetTriggered only means "replace the set" on a data-set-shaped observation; on an ordinary
+            // accumulating SAMPLE/EVENT (PartCount resetTriggered="DAILY") it is not our concern.
+            ResetTriggered: resetTriggered && (hasCount || entries is { Count: > 0 }),
+            IsDataSet: !isCondition && (hasCount || entries is { Count: > 0 }));
     }
 
     /// <summary>