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

src/DodoSSH.Client.Storage/ItemStore.cs

@@ -118,6 +118,65 @@ public sealed class ItemStore(
         await context.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
     }
 
+    /// <summary>
+    /// Writes the server's version of many items at once, in one transaction, in the order given.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <see cref="SaveAsync"/> for a page of pulled changes. A pull used to write each change on its own —
+    /// a context, a read, a commit — so a first sync of a keychain with a few thousand items was a few
+    /// thousand SQLite transactions, each one a sync to disk. This is one read for the rows that exist
+    /// already and one commit for the lot.
+    /// </para>
+    /// <para>
+    /// The same blind overwrite, and the same answer when one item appears more than once: they are applied
+    /// in order to the same row, so the last one given is the one kept — exactly what writing them one by one
+    /// would have left.
+    /// </para>
+    /// </remarks>
+    public async Task SaveManyAsync(IReadOnlyCollection<StoredItem> items, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(items);
+
+        if (items.Count == 0)
+        {
+            return;
+        }
+
+        var vaultIds = items.Select(item => item.VaultId).Distinct().ToList();
+        var entityIds = items.Select(item => item.EntityId).Distinct().ToList();
+
+        var context = contexts.CreateDbContext();
+        await using var scope = context.ConfigureAwait(false);
+
+        var existing = await context.Set<CachedItemRow>()
+            .Where(r => vaultIds.Contains(r.VaultId) && entityIds.Contains(r.EntityId))
+            .ToListAsync(cancellationToken)
+            .ConfigureAwait(false);
+
+        var rows = existing.ToDictionary(row => (row.VaultId, row.EntityType, row.EntityId));
+
+        foreach (var item in items)
+        {
+            if (!rows.TryGetValue((item.VaultId, item.EntityType, item.EntityId), out var row))
+            {
+                row = new CachedItemRow
+                {
+                    VaultId = item.VaultId,
+                    EntityType = item.EntityType,
+                    EntityId = item.EntityId,
+                };
+
+                context.Add(row);
+                rows.Add((item.VaultId, item.EntityType, item.EntityId), row);
+            }
+
+            Apply(row, item);
+        }
+
+        await context.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
+    }
+
     /// <summary>Removes a tombstone whose change has been seen by everything that needed it.</summary>
     /// <remarks>
     /// Only ever called for a row that is already a tombstone. Collecting a live item here would make