fix(historian): release the retention applock only when acquired (#667 F3)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer/SqlServerHistorianRetention.cs

@@ -39,15 +39,31 @@ internal sealed class SqlServerHistorianRetention : IHistorianRetention
     /// <summary>Test hook invoked after every delete batch.</summary>
     internal Action? AfterBatch { get; set; }
 
+    /// <summary>Test seam for <see cref="SqlConnection.ClearPool"/>.</summary>
+    internal Action<SqlConnection> ClearPool { get; set; } = SqlConnection.ClearPool;
+
     public async Task<IReadOnlyList<RetentionPurgeResult>> PurgeAsync(CancellationToken ct)
     {
         await _bootstrap.EnsureAsync(ct).ConfigureAwait(false);
         var cutoff = _time.GetUtcNow().UtcDateTime.AddDays(-_days);
 
         await using var conn = await _conn.OpenAsync(ct).ConfigureAwait(false);
+        var acquired = false;
         try
         {
-            if (!await TryLockAsync(conn, ct).ConfigureAwait(false))
+            try
+            {
+                acquired = await TryLockAsync(conn, ct).ConfigureAwait(false);
+            }
+            catch
+            {
+                // Ambiguous: the server may have granted the lock before the client saw the reply. We cannot
+                // release what we are not sure we hold, so close the session physically instead.
+                ClearPool(conn);
+                throw;
+            }
+
+            if (!acquired)
             {
                 _log.LogInformation("Historian retention purge skipped: another purger holds the applock.");
                 return [];
@@ -74,8 +90,9 @@ internal sealed class SqlServerHistorianRetention : IHistorianRetention
         }
         finally
         {
-            // Also runs when acquire threw: the server may have granted the lock before the client saw the reply.
-            await ReleaseAsync(conn).ConfigureAwait(false);
+            // Only when WE hold it: releasing an applock we were refused fails, which would log a false Warning
+            // and clear the shared pool on every skipped interval.
+            if (acquired) await ReleaseAsync(conn).ConfigureAwait(false);
         }
     }
 
@@ -121,7 +138,7 @@ internal sealed class SqlServerHistorianRetention : IHistorianRetention
             // applock); clearing the pool makes the dispose physically close it, which releases the lock.
             _log.LogWarning("sp_releaseapplock failed ({Exception}); discarding the connection pool so the session " +
                 "holding the retention applock is closed rather than pooled.", ex.GetType().Name);
-            SqlConnection.ClearPool(conn);
+            ClearPool(conn);
         }
     }
 }