feat(batch14): complete group2 lifecycle checks and wave2 tests

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/ImplBacklog/ConcurrencyTests2.Impltests.cs

@@ -1,4 +1,5 @@
 using System.Collections.Concurrent;
+using System.Reflection;
 using Shouldly;
 using ZB.MOM.NatsNet.Server;
 
@@ -59,6 +60,38 @@ public sealed partial class ConcurrencyTests2
         }
     }
 
+    [Fact] // T:2476
+    public void NoRaceFilestoreBinaryStreamSnapshotEncodingLargeGaps_ShouldSucceed()
+    {
+        WithStore((fs, _) =>
+        {
+            const int numMsgs = 5000;
+            var payload = new byte[128];
+
+            fs.StoreMsg("zzz", null, payload, 0).Seq.ShouldBe(1UL);
+            for (var i = 2; i < numMsgs; i++)
+            {
+                var (seq, _) = fs.StoreMsg("zzz", null, null, 0);
+                seq.ShouldBeGreaterThan(1UL);
+                fs.RemoveMsg(seq).Removed.ShouldBeTrue();
+            }
+
+            fs.StoreMsg("zzz", null, payload, 0).Seq.ShouldBe((ulong)numMsgs);
+            Should.NotThrow(() => InvokePrivate(fs, "SyncBlocks"));
+
+            var (snapshot, err) = fs.EncodedStreamState(0);
+            err.ShouldBeNull();
+            StoreParity.IsEncodedStreamState(snapshot).ShouldBeTrue();
+            snapshot.Length.ShouldBeLessThan(2048);
+
+            var state = fs.State();
+            state.FirstSeq.ShouldBe(1UL);
+            state.LastSeq.ShouldBe((ulong)numMsgs);
+            state.Msgs.ShouldBe(2UL);
+            state.NumDeleted.ShouldBe(numMsgs - 2);
+        });
+    }
+
     private static void WithStore(Action<JetStreamFileStore, string> action, StreamConfig? cfg = null)
     {
         var root = NewRoot();
@@ -94,5 +127,12 @@ public sealed partial class ConcurrencyTests2
         };
     }
 
+    private static void InvokePrivate(object target, string methodName, params object[] args)
+    {
+        var method = target.GetType().GetMethod(methodName, BindingFlags.Instance | BindingFlags.NonPublic);
+        method.ShouldNotBeNull();
+        method!.Invoke(target, args);
+    }
+
     private static string NewRoot() => Path.Combine(Path.GetTempPath(), $"impl-fs-c2-{Guid.NewGuid():N}");
 }