perf(runtime): gate the VT apply-boundary compile-cache clear on a changed expression set (02/P7)

archreview/plans/R2-03-vt-failure-quality-plan.md.tasks.json

@@ -71,7 +71,7 @@
     {
       "id": "R2-03-T10",
       "subject": "P7 fix: gate the OnApply ClearCompiledScripts on HashSet.SetEquals of desired vs _planByVtag expression sets (ordinal), with explanatory comment (turns T9 green)",
-      "status": "pending",
+      "status": "completed",
       "blockedBy": [
         "R2-03-T9"
       ]

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagHostActor.cs

@@ -85,11 +85,24 @@ public sealed class VirtualTagHostActor : ReceiveActor
 
     private void OnApply(ApplyVirtualTags msg)
     {
-        // Apply-boundary compile-cache drop: each deploy generation may carry edited script sources, and
-        // the production evaluator roots one collectible AssemblyLoadContext per compiled expression. Drop
-        // them here (mirroring ScriptedAlarmEngine's per-generation clear) so stale ALCs are reclaimed
-        // instead of accreting across edits. No-op for evaluators without a cache (NullVirtualTagEvaluator).
-        (_evaluator as IScriptCacheOwner)?.ClearCompiledScripts();
+        // Apply-boundary compile-cache drop (gated — 02/P7): the production evaluator roots one
+        // collectible AssemblyLoadContext per compiled expression, so a changed script set must drop the
+        // stale ALCs. But clearing on EVERY apply forces a full recompilation even on a byte-identical
+        // redeploy (and each unnecessary clear is an unnecessary 02/S12 race window). Clear only when the
+        // deployed EXPRESSION set actually changed. The cache is keyed by source only, so a rename /
+        // FolderPath move / Historize toggle (which respawns the child but keeps the source) needs no
+        // recompile; set semantics also mean two vtags sharing one expression keep it alive while either
+        // remains. _planByVtag still holds the PREVIOUS generation here (it is rebuilt to the desired set
+        // at the end of this method); on the first apply after (re)start it is empty, so the sets differ
+        // and the clear fires (harmless — converges the singleton evaluator's cache to this generation).
+        // NOTE: this gate reduces clear FREQUENCY only; the S12 retry-once in RoslynVirtualTagEvaluator —
+        // not this gate — is the correctness fix for a clear racing an in-flight evaluation.
+        var newExpressions = msg.Plans.Select(p => p.Expression).ToHashSet(StringComparer.Ordinal);
+        var prevExpressions = _planByVtag.Values.Select(p => p.Expression).ToHashSet(StringComparer.Ordinal);
+        if (!newExpressions.SetEquals(prevExpressions))
+        {
+            (_evaluator as IScriptCacheOwner)?.ClearCompiledScripts();
+        }
 
         var desired = new HashSet<string>(msg.Plans.Select(p => p.VirtualTagId), StringComparer.Ordinal);