Commit resync writes in one transaction

Each insert, delete and rescore update was its own commit to disk, so a
resync of about a thousand QSOs froze the window thread. LogStore has a
new Batch() scope that commits its writes together. ContestSession uses
it in AddRangeFromNetwork, ClearFromNetwork and Rescore.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Nonemm.Session/ContestSession.cs

@@ -128,10 +128,13 @@ public sealed class ContestSession
                 && q.TimestampUtc >= fromUtc
                 && q.TimestampUtc < fromUtc + length)
             .ToList();
+        using (store.Batch())
+        {
             foreach (Qso qso in gone)
             {
                 store.Delete(qso.Id);
             }
+        }
         if (gone.Count > 0)
         {
             Log.RemoveRange([.. gone.Select(q => q.Id)]);
@@ -152,11 +155,14 @@ public sealed class ContestSession
     {
         Log.Rescore();
         int written = 0;
+        using (store.Batch())
+        {
             foreach (Qso qso in Log.Qsos.Where(q => q.TimestampUtc >= fromUtc))
             {
                 store.Update(qso);
                 written++;
             }
+        }
         Changed?.Invoke(this, EventArgs.Empty);
         return written;
     }
@@ -216,6 +222,8 @@ public sealed class ContestSession
     {
         HashSet<string> held = [.. Log.Qsos.Select(q => q.Id)];
         List<Qso> stored = [];
+        using (store.Batch())
+        {
             foreach (Qso qso in arriving)
             {
                 if (held.Add(qso.Id))
@@ -223,6 +231,7 @@ public sealed class ContestSession
                     stored.Add(store.Add(FromNetwork(qso)));
                 }
             }
+        }
         if (stored.Count == 0)
         {
             return 0;

src/Nonemm.Storage/LogStore.cs

@@ -28,4 +28,9 @@ public interface LogStore : IDisposable
     void Update(Qso qso);
 
     void Delete(string id);
+
+    /// Writes made until the returned scope is disposed are committed
+    /// together. A write on its own is a commit to disk, which makes a burst
+    /// of a thousand contacts take seconds.
+    IDisposable Batch();
 }

src/Nonemm.Storage/SqliteLogStore.cs

@@ -188,6 +188,18 @@ public sealed class SqliteLogStore : LogStore
 
     public void Dispose() => connection.Dispose();
 
+    public IDisposable Batch() => new CommitOnDispose(connection.BeginTransaction());
+
+    /// Disposing a `SqliteTransaction` rolls it back, so this commits first.
+    private sealed class CommitOnDispose(SqliteTransaction transaction) : IDisposable
+    {
+        public void Dispose()
+        {
+            transaction.Commit();
+            transaction.Dispose();
+        }
+    }
+
     private void Insert(Qso qso)
     {
         using SqliteCommand command = connection.CreateCommand();

tests/Nonemm.Session.Tests/FakeLogStore.cs

@@ -57,7 +57,16 @@ public sealed class FakeLogStore : LogStore
 
     public void Delete(string id) => qsos.RemoveAll(q => q.Id == id);
 
+    public IDisposable Batch() => new NoBatch();
+
     public void Dispose()
     {
     }
+
+    private sealed class NoBatch : IDisposable
+    {
+        public void Dispose()
+        {
+        }
+    }
 }