fix(store-and-forward): resolve StoreAndForward-006,007,008,009 — transactional parked reads, PipeTo, fault-isolated activity events; 002/011/012 deferred

src/ScadaLink.StoreAndForward/StoreAndForwardStorage.cs

@@ -8,6 +8,19 @@ namespace ScadaLink.StoreAndForward;
 /// WP-9: SQLite persistence layer for store-and-forward messages.
 /// Uses direct Microsoft.Data.Sqlite (not EF Core) for lightweight site-side storage.
 /// No max buffer size per design decision.
+///
+/// StoreAndForward-008: every method opens a fresh <see cref="SqliteConnection"/> for
+/// the duration of the call rather than holding a long-lived connection. This is a
+/// deliberate trade-off, not an oversight: Microsoft.Data.Sqlite maintains an internal
+/// connection pool keyed on the connection string, so <c>OpenAsync</c> on a previously
+/// used connection string reuses a pooled handle instead of performing a real file
+/// open. The retry sweep therefore relies on that pool for acceptable performance —
+/// it calls <see cref="RemoveMessageAsync"/> / <see cref="UpdateMessageIfStatusAsync"/>
+/// once per due message, and with no max buffer size (by design) the buffer can grow
+/// large. The connection-per-call style keeps each method self-contained and
+/// transaction-scoped; if profiling ever shows the pooled open to be a bottleneck on
+/// the hot retry path, the remedy is a batched sweep API that opens one connection (and
+/// one transaction) per sweep.
 /// </summary>
 public class StoreAndForwardStorage
 {
@@ -222,6 +235,12 @@ public class StoreAndForwardStorage
 
     /// <summary>
     /// WP-12: Gets all parked messages, optionally filtered by category, with pagination.
+    ///
+    /// StoreAndForward-006: the COUNT(*) and the paged SELECT run inside a single
+    /// transaction so they observe one consistent snapshot. Without it, a concurrent
+    /// enqueue/park/discard arriving between the two statements yields a TotalCount
+    /// inconsistent with the returned page (flickering totals / off-by-one page math
+    /// in the paginated UI).
     /// </summary>
     public async Task<(List<StoreAndForwardMessage> Messages, int TotalCount)> GetParkedMessagesAsync(
         StoreAndForwardCategory? category = null,
@@ -231,8 +250,11 @@ public class StoreAndForwardStorage
         await using var connection = new SqliteConnection(_connectionString);
         await connection.OpenAsync();
 
+        await using var transaction = (SqliteTransaction)await connection.BeginTransactionAsync();
+
         // Count
         await using var countCmd = connection.CreateCommand();
+        countCmd.Transaction = transaction;
         countCmd.CommandText = category.HasValue
             ? "SELECT COUNT(*) FROM sf_messages WHERE status = @parked AND category = @category"
             : "SELECT COUNT(*) FROM sf_messages WHERE status = @parked";
@@ -242,6 +264,7 @@ public class StoreAndForwardStorage
 
         // Page
         await using var pageCmd = connection.CreateCommand();
+        pageCmd.Transaction = transaction;
         var categoryFilter = category.HasValue ? " AND category = @category" : "";
         pageCmd.CommandText = $@"
             SELECT id, category, target, payload_json, retry_count, max_retries,
@@ -257,6 +280,8 @@ public class StoreAndForwardStorage
         pageCmd.Parameters.AddWithValue("@offset", (pageNumber - 1) * pageSize);
 
         var messages = await ReadMessagesAsync(pageCmd);
+
+        await transaction.CommitAsync();
         return (messages, totalCount);
     }