Sync: mirror a pulled page in one write and read the outbox once per page, not twice per change

src/DodoSSH.Client.Sync/SyncEngine.cs

@@ -154,9 +154,10 @@ public sealed class SyncEngine
                 continue;
             }
 
+            await ApplyPageAsync(vaultId, response.Changes, report, cancellationToken).ConfigureAwait(false);
+
             foreach (var change in response.Changes)
             {
-                await ApplyAsync(vaultId, change, report, cancellationToken).ConfigureAwait(false);
                 report.Pulled++;
 
                 // Counted apart for the same reason the pushed ones are: a machine reads back the log
@@ -310,6 +311,61 @@ public sealed class SyncEngine
     private static string PulledTypes { get; } =
         string.Join(',', ItemKinds.SyncedTypes.Select(type => (int)type).Order());
 
+    /// <summary>Applies one page of pulled changes, in order, with as few writes as that allows.</summary>
+    /// <remarks>
+    /// <para>
+    /// Each change used to cost a write to mirror it and a read to look for a local edit of the same item —
+    /// two or more SQLite transactions apiece, which on a first sync of a large keychain was most of the time
+    /// the sync took. The outbox is read once per page instead, and every run of changes with no local edit
+    /// waiting is mirrored in a single write.
+    /// </para>
+    /// <para>
+    /// ◆ A change that does have an edit waiting goes through <see cref="ApplyAsync"/> exactly as before, with
+    /// the run in front of it written first: the reconciler reads the mirror and the outbox as they stand at
+    /// that change, and both are what the one-by-one loop would have left them as. The one-read snapshot only
+    /// decides which path a change takes; the path that matters re-reads the outbox for itself.
+    /// </para>
+    /// </remarks>
+    private async Task ApplyPageAsync(
+        Guid vaultId,
+        IReadOnlyList<SyncChange> changes,
+        SyncReportBuilder report,
+        CancellationToken cancellationToken)
+    {
+        if (changes.Count == 0)
+        {
+            return;
+        }
+
+        var waiting = (await outbox.ListAllAsync(vaultId, cancellationToken).ConfigureAwait(false))
+            .Select(pending => (pending.EntityType, pending.EntityId))
+            .ToHashSet();
+
+        var run = new List<SyncChange>();
+
+        foreach (var change in changes)
+        {
+            if (!reconcilers.ContainsKey(change.EntityType))
+            {
+                // See ApplyAsync: a type this build cannot sync is neither mirrored nor reconciled.
+                continue;
+            }
+
+            if (!waiting.Contains((change.EntityType, change.EntityId)))
+            {
+                run.Add(change);
+                continue;
+            }
+
+            await ItemMirror.WriteManyAsync(items, vaultId, run, cancellationToken).ConfigureAwait(false);
+            run.Clear();
+
+            await ApplyAsync(vaultId, change, report, cancellationToken).ConfigureAwait(false);
+        }
+
+        await ItemMirror.WriteManyAsync(items, vaultId, run, cancellationToken).ConfigureAwait(false);
+    }
+
     private async Task ApplyAsync(
         Guid vaultId,
         SyncChange change,