temp

src/App/ViewModels/MainWindowViewModel.Sync.cs

@@ -221,6 +221,9 @@ public partial class MainWindowViewModel
                 _synchronizer.PreloadCaches(
                     _doc.Cache?.RawTxHex ?? [],
                     _doc.Cache?.VerifiedAt ?? [],
+                    _doc.Cache?.BlockHeaders,
+                    _doc.Cache?.NextReceiveIndex ?? 0,
+                    _doc.Cache?.NextChangeIndex ?? 0,
                     net);
                 _synchronizer.Progress += msg => Dispatcher.UIThread.Post(() => StatusMessage = msg);
             }
@@ -231,7 +234,7 @@ public partial class MainWindowViewModel
                 var result = await _synchronizer.SyncOnceAsync();
                 _lastTransactions = result.Transactions;
 
-                var (rawHex, verifiedAt) = _synchronizer.ExportCaches(
+                var (rawHex, verifiedAt, blockHeaders) = _synchronizer.ExportCaches(
                     PalladiumNetworks.For(_account.Profile.Kind));
                 _doc.Cache = new SyncCache
                 {
@@ -245,6 +248,7 @@ public partial class MainWindowViewModel
                     Addresses = [.. result.AddressRows],
                     RawTxHex = rawHex,
                     VerifiedAt = verifiedAt,
+                    BlockHeaders = blockHeaders,
                 };
                 WalletStore.Save(_doc, _walletPath!, _password);
                 ApplyCache(_doc.Cache);
@@ -334,11 +338,12 @@ public partial class MainWindowViewModel
         try
         {
             var net = PalladiumNetworks.For(_account.Profile.Kind);
-            var (rawHex, verifiedAt) = _synchronizer.ExportCaches(net);
+            var (rawHex, verifiedAt, blockHeaders) = _synchronizer.ExportCaches(net);
             if (rawHex.Count == 0 && verifiedAt.Count == 0)
                 return;
             (_doc.Cache ??= new SyncCache()).RawTxHex = rawHex;
             _doc.Cache.VerifiedAt = verifiedAt;
+            _doc.Cache.BlockHeaders = blockHeaders;
             WalletStore.Save(_doc, _walletPath, _password);
         }
         catch { /* non fatale: il prossimo salvataggio completo recupererà */ }

src/Cli/Program.cs

@@ -131,8 +131,17 @@ static async Task<int> Sync(string[] o)
 
     var sync = new WalletSynchronizer(account, client, doc.GapLimit);
     sync.Progress += msg => Console.WriteLine($"  {msg}");
+    var net = PalladiumNetworks.For(account.Profile.Kind);
+    sync.PreloadCaches(
+        doc.Cache?.RawTxHex ?? [],
+        doc.Cache?.VerifiedAt ?? [],
+        doc.Cache?.BlockHeaders,
+        doc.Cache?.NextReceiveIndex ?? 0,
+        doc.Cache?.NextChangeIndex ?? 0,
+        net);
     var result = await sync.SyncOnceAsync();
 
+    var (rawHex, verifiedAt, blockHeaders) = sync.ExportCaches(net);
     doc.Cache = new SyncCache
     {
         TipHeight = result.TipHeight,
@@ -143,6 +152,9 @@ static async Task<int> Sync(string[] o)
         History = [.. result.History],
         Utxos = [.. result.Utxos],
         Addresses = [.. result.AddressRows],
+        RawTxHex = rawHex,
+        VerifiedAt = verifiedAt,
+        BlockHeaders = blockHeaders,
     };
     WalletStore.Save(doc, path, Opt(o, "--password"));
 

src/Core/Net/ElectrumClient.cs

@@ -1,8 +1,10 @@
+using System.Buffers;
 using System.Collections.Concurrent;
+using System.IO.Pipelines;
 using System.Net.Security;
 using System.Net.Sockets;
-using System.Text;
 using System.Text.Json;
+using System.Threading.Channels;
 
 namespace PalladiumWallet.Core.Net;
 
@@ -18,10 +20,17 @@ public sealed class ElectrumClient : IAsyncDisposable
 
     private readonly TcpClient _tcp;
     private readonly Stream _stream;
-    private readonly SemaphoreSlim _writeLock = new(1, 1);
     private readonly ConcurrentDictionary<long, TaskCompletionSource<JsonElement>> _pending = new();
     private readonly CancellationTokenSource _cts = new();
     private readonly Task _readLoop;
+    private readonly Task _writeLoop;
+
+    // Channel single-reader: le task scrivono i payload senza lock;
+    // il write loop drena tutto in un unico WriteAsync+FlushAsync —
+    // identico al buffered writer asyncio di Electrum.
+    private readonly Channel<byte[]> _outgoing = Channel.CreateUnbounded<byte[]>(
+        new UnboundedChannelOptions { SingleReader = true, AllowSynchronousContinuations = false });
+
     private long _nextId;
 
     public string Host { get; }
@@ -29,10 +38,7 @@ public sealed class ElectrumClient : IAsyncDisposable
     public bool UseSsl { get; }
     public bool IsConnected => _tcp.Connected && !_cts.IsCancellationRequested;
 
-    /// <summary>(metodo, parametri) per le notifiche di subscription.</summary>
     public event Action<string, JsonElement>? NotificationReceived;
-
-    /// <summary>Scatta quando la connessione cade (errore di lettura o chiusura remota).</summary>
     public event Action<Exception?>? Disconnected;
 
     private ElectrumClient(TcpClient tcp, Stream stream, string host, int port, bool useSsl)
@@ -43,17 +49,13 @@ public sealed class ElectrumClient : IAsyncDisposable
         Port = port;
         UseSsl = useSsl;
         _readLoop  = Task.Run(ReadLoopAsync);
+        _writeLoop = Task.Run(WriteLoopAsync);
     }
 
-    /// <summary>
-    /// Connette al server. Con <paramref name="useSsl"/> la validazione del
-    /// certificato è TOFU tramite <paramref name="pins"/> (§9): primo contatto
-    /// salva, contatti successivi confrontano; mismatch ⇒ <see cref="CertificatePinMismatchException"/>.
-    /// </summary>
     public static async Task<ElectrumClient> ConnectAsync(string host, int port, bool useSsl,
         CertificatePinStore? pins = null, CancellationToken ct = default)
     {
-        var tcp = new TcpClient();
+        var tcp = new TcpClient { NoDelay = true };
         try
         {
             await tcp.ConnectAsync(host, port, ct);
@@ -64,10 +66,7 @@ public sealed class ElectrumClient : IAsyncDisposable
                 var ssl = new SslStream(stream, leaveInnerStreamOpen: false,
                     (_, cert, _, _) =>
                     {
-                        // I server Electrum sono tipicamente self-signed: la
-                        // fiducia è il pin TOFU, non la catena CA (§9).
-                        if (cert is null)
-                            return false;
+                        if (cert is null) return false;
                         pinOk = pins is null || pins.VerifyOrPin(host, port, cert);
                         return pinOk;
                     });
@@ -84,7 +83,6 @@ public sealed class ElectrumClient : IAsyncDisposable
             }
 
             var client = new ElectrumClient(tcp, stream, host, port, useSsl);
-            // Negoziazione obbligatoria prima di ogni altra richiesta.
             await client.RequestAsync("server.version", ct, ClientName, ProtocolVersion);
             return client;
         }
@@ -110,45 +108,133 @@ public sealed class ElectrumClient : IAsyncDisposable
             @params = parameters,
         });
 
-        await _writeLock.WaitAsync(ct);
-        try
-        {
-            await _stream.WriteAsync(payload, ct);
-            await _stream.WriteAsync("\n"u8.ToArray(), ct);
-            await _stream.FlushAsync(ct);
-        }
-        finally
-        {
-            _writeLock.Release();
-        }
+        _outgoing.Writer.TryWrite(payload);
 
-        await using var registration = ct.Register(() => tcs.TrySetCanceled(ct));
+        await using var registration = ct.Register(() =>
+        {
+            _pending.TryRemove(id, out _);
+            tcs.TrySetCanceled(ct);
+        });
         return await tcs.Task;
     }
 
+    /// <summary>
+    /// Drain loop: svuota il channel in un unico buffer → un solo WriteAsync+FlushAsync
+    /// per tutti i messaggi in coda. Quando N richieste sono in coda, vengono
+    /// trasmesse in un singolo segmento TCP invece di N flush seriali.
+    /// </summary>
+    private async Task WriteLoopAsync()
+    {
+        try
+        {
+            while (await _outgoing.Reader.WaitToReadAsync(_cts.Token))
+            {
+                using var ms = new MemoryStream(512);
+                while (_outgoing.Reader.TryRead(out var data))
+                {
+                    ms.Write(data);
+                    ms.WriteByte((byte)'\n');
+                }
+                await _stream.WriteAsync(ms.GetBuffer().AsMemory(0, (int)ms.Length), _cts.Token);
+                await _stream.FlushAsync(_cts.Token);
+            }
+        }
+        catch (OperationCanceledException) { }
+        catch (Exception ex)
+        {
+            foreach (var (_, pending) in _pending)
+                pending.TrySetException(ex);
+        }
+    }
+
+    /// <summary>
+    /// Read loop con PipeReader: buffer pooled, zero allocazioni per-risposta
+    /// (nessuna stringa intermedia), parsing JSON direttamente da byte span.
+    /// </summary>
     private async Task ReadLoopAsync()
     {
         Exception? failure = null;
+        var pipe = PipeReader.Create(_stream, new StreamPipeReaderOptions(leaveOpen: true));
         try
         {
-            using var reader = new StreamReader(_stream, Encoding.UTF8, leaveOpen: true);
-            while (!_cts.IsCancellationRequested)
+            while (true)
             {
-                var line = await reader.ReadLineAsync(_cts.Token);
-                if (line is null)
-                    break; // chiusura remota
-                if (string.IsNullOrWhiteSpace(line))
-                    continue;
+                ReadResult result;
+                try { result = await pipe.ReadAsync(_cts.Token); }
+                catch (OperationCanceledException) { break; }
 
-                using var doc = JsonDocument.Parse(line);
+                var buffer = result.Buffer;
+                try
+                {
+                    while (TrySliceLine(ref buffer, out var line))
+                        if (!line.IsEmpty)
+                            DispatchLine(line);
+                }
+                finally
+                {
+                    // consumed = tutto ciò che abbiamo consumato (fino all'ultimo \n)
+                    // examined = tutto ciò che abbiamo guardato (fino alla fine del buffer)
+                    pipe.AdvanceTo(buffer.Start, buffer.End);
+                }
+
+                if (result.IsCompleted) break;
+            }
+        }
+        catch (Exception ex)
+        {
+            failure = ex;
+        }
+        finally
+        {
+            await pipe.CompleteAsync();
+            foreach (var (_, tcs) in _pending)
+                tcs.TrySetException(failure ?? new IOException("Connessione al server chiusa."));
+            _pending.Clear();
+            Disconnected?.Invoke(failure);
+        }
+    }
+
+    private static bool TrySliceLine(ref ReadOnlySequence<byte> buffer,
+        out ReadOnlySequence<byte> line)
+    {
+        var pos = buffer.PositionOf((byte)'\n');
+        if (pos is null) { line = default; return false; }
+        line   = buffer.Slice(0, pos.Value);
+        buffer = buffer.Slice(buffer.GetPosition(1, pos.Value));
+        return true;
+    }
+
+    private void DispatchLine(ReadOnlySequence<byte> line)
+    {
+        if (line.IsSingleSegment)
+        {
+            DispatchSpan(line.FirstSpan);
+            return;
+        }
+        // Multi-segmento (risposta molto lunga): copia su ArrayPool poi parsa.
+        var len = (int)line.Length;
+        var buf = ArrayPool<byte>.Shared.Rent(len);
+        try
+        {
+            line.CopyTo(buf);
+            DispatchSpan(buf.AsSpan(0, len));
+        }
+        finally { ArrayPool<byte>.Shared.Return(buf); }
+    }
+
+    private void DispatchSpan(ReadOnlySpan<byte> utf8)
+    {
+        // JsonDocument.Parse via Utf8JsonReader: nessuna stringa intermedia,
+        // parsing direttamente dallo span pooled.
+        var reader = new Utf8JsonReader(utf8);
+        using var doc = JsonDocument.ParseValue(ref reader);
         var root = doc.RootElement;
 
         if (root.TryGetProperty("id", out var idEl) && idEl.ValueKind == JsonValueKind.Number)
         {
-                    if (!_pending.TryRemove(idEl.GetInt64(), out var tcs))
-                        continue;
-                    if (root.TryGetProperty("error", out var error) && error.ValueKind != JsonValueKind.Null)
-                        tcs.TrySetException(new ElectrumServerException(error.ToString()));
+            if (!_pending.TryRemove(idEl.GetInt64(), out var tcs)) return;
+            if (root.TryGetProperty("error", out var err) && err.ValueKind != JsonValueKind.Null)
+                tcs.TrySetException(new ElectrumServerException(err.ToString()));
             else
                 tcs.TrySetResult(root.GetProperty("result").Clone());
         }
@@ -159,28 +245,15 @@ public sealed class ElectrumClient : IAsyncDisposable
                 root.GetProperty("params").Clone());
         }
     }
-        }
-        catch (OperationCanceledException) { }
-        catch (Exception ex)
-        {
-            failure = ex;
-        }
-        finally
-        {
-            foreach (var (_, tcs) in _pending)
-                tcs.TrySetException(failure ?? new IOException("Connessione al server chiusa."));
-            _pending.Clear();
-            Disconnected?.Invoke(failure);
-        }
-    }
 
     public async ValueTask DisposeAsync()
     {
         await _cts.CancelAsync();
+        _outgoing.Writer.Complete();
         _tcp.Close();
-        try { await _readLoop; } catch { /* in chiusura */ }
+        try { await _readLoop;  } catch { }
+        try { await _writeLoop; } catch { }
         _cts.Dispose();
-        _writeLock.Dispose();
     }
 }
 

src/Core/Spv/WalletSynchronizer.cs

@@ -31,46 +31,45 @@ public sealed class SyncResult
 }
 
 /// <summary>
-/// Sincronizzazione del wallet (blueprint §7.4): per ogni indirizzo calcola lo
-/// scripthash e si sottoscrive; scarica storico e transazioni; verifica ogni tx
-/// confermata con la prova di Merkle contro l'header del suo blocco (le risposte
-/// del server non sono fidate, §17); ricostruisce localmente UTXO e saldo;
-/// estende la scansione fino al gap limit (§5).
+/// Sincronizzazione del wallet (blueprint §7.4).
 /// </summary>
 public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient client, int gapLimit = 20)
 {
     /// <summary>Avanzamento leggibile (per CLI e barra di stato GUI).</summary>
     public event Action<string>? Progress;
 
-    // Richieste contemporanee verso il server. Troppo alte → -102 "server busy";
-    // troppo basse → throughput scarso su storie grandi.
-    private const int MaxConcurrent = 20;
-
-    // Cache tra le passate (stesso synchronizer per tutta la vita della
-    // connessione): le tx già scaricate e le prove di Merkle già verificate a
-    // una data altezza non si rifanno — le risincronizzazioni da notifica
-    // costano solo ciò che è cambiato (modello Electrum).
-    private readonly Dictionary<string, Transaction> _txCache = [];
+    private readonly ConcurrentDictionary<string, Transaction> _txCache = new();
     private readonly Dictionary<string, int> _verifiedAtHeight = [];
-
-    // Header grezzi per altezza: una Task<string> per altezza, condivisa tra
-    // tutte le tx dello stesso blocco → ogni blocco viene scaricato una sola
-    // volta anche con centinaia di tx confermate nello stesso blocco.
     private readonly ConcurrentDictionary<int, Task<string>> _headerFetches = new();
 
+    // Indici noti dal sync precedente: usati da ScanChainAsync per la discovery
+    // incrementale — gli indirizzi già usati vengono fetchati in un unico burst
+    // invece di batches sequenziali, riducendo i round-trip da O(used/gapLimit) a O(1).
+    private int _knownReceiveIndex;
+    private int _knownChangeIndex;
+
     /// <summary>
-    /// Pre-popola le cache interne da dati salvati su disco (SyncCache).
+    /// Pre-popola le cache interne da dati salvati su disco.
     /// Chiamare prima di SyncOnceAsync per evitare di riscaricale le tx già note.
     /// </summary>
-    public void PreloadCaches(Dictionary<string, string> rawTxHex,
-        Dictionary<string, int> verifiedAt, Network network)
+    public void PreloadCaches(
+        Dictionary<string, string> rawTxHex,
+        Dictionary<string, int> verifiedAt,
+        Dictionary<int, string>? blockHeaders,
+        int knownReceiveIndex,
+        int knownChangeIndex,
+        Network network)
     {
         foreach (var (txid, hex) in rawTxHex)
-            if (!_txCache.ContainsKey(txid))
-                _txCache[txid] = Transaction.Parse(hex, network);
+            _txCache.TryAdd(txid, Transaction.Parse(hex, network));
         foreach (var (txid, height) in verifiedAt)
             if (!_verifiedAtHeight.ContainsKey(txid))
                 _verifiedAtHeight[txid] = height;
+        if (blockHeaders is not null)
+            foreach (var (height, hex) in blockHeaders)
+                _headerFetches.TryAdd(height, Task.FromResult(hex));
+        _knownReceiveIndex = knownReceiveIndex;
+        _knownChangeIndex  = knownChangeIndex;
     }
 
     /// <summary>
@@ -78,15 +77,23 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
     /// Solo le tx confermate (height > 0) vengono incluse: le non confermate
     /// possono cambiare (RBF) e vanno sempre riscaricate.
     /// </summary>
-    public (Dictionary<string, string> RawTxHex, Dictionary<string, int> VerifiedAt)
+    public (Dictionary<string, string> RawTxHex,
+            Dictionary<string, int> VerifiedAt,
+            Dictionary<int, string> BlockHeaders)
         ExportCaches(Network network)
     {
-        // Includi solo le tx associate a una prova di Merkle verificata
-        // (cioè confermate e verificate): sono le uniche immutabili.
         var rawHex = _verifiedAtHeight.Keys
             .Where(_txCache.ContainsKey)
             .ToDictionary(txid => txid, txid => _txCache[txid].ToHex());
-        return (rawHex, new Dictionary<string, int>(_verifiedAtHeight));
+
+        // Solo gli header già completati: Task<string> non ancora completate
+        // non vengono persistite (verranno rifetchate al prossimo sync se necessario).
+        var headers = new Dictionary<int, string>();
+        foreach (var (height, task) in _headerFetches)
+            if (task.IsCompletedSuccessfully)
+                headers[height] = task.Result;
+
+        return (rawHex, new Dictionary<string, int>(_verifiedAtHeight), headers);
     }
 
     public async Task<SyncResult> SyncOnceAsync(CancellationToken ct = default)
@@ -101,38 +108,41 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
 
         if (account.FixedAddresses is { } fixedAddresses)
         {
-            // Importati WIF: lista fissa, nessun gap limit.
-            // Pochi indirizzi → subscribe diretto per notifiche push.
             foreach (var (addr, isChange, idx) in fixedAddresses)
                 tracked.Add(new TrackedAddress(addr, Scripthash.FromAddress(addr), isChange, idx));
             nextReceive = tracked.Count(t => !t.IsChange);
             nextChange  = 0;
 
-            var histories = await Task.WhenAll(
-                tracked.Select(t => client.GetHistoryAsync(t.ScriptHash, ct)));
-            for (var i = 0; i < tracked.Count; i++)
+            await Task.WhenAll(tracked.Select(t => RetryOnBusyAsync(async () =>
             {
-                if (histories[i].Count > 0)
-                    historyByAddress[tracked[i].ScriptHash] = histories[i];
-            }
-            // Subscribe a tutti (pochi): notifiche push per ogni indirizzo importato.
-            await Task.WhenAll(tracked.Select(t => client.SubscribeScripthashAsync(t.ScriptHash, ct)));
+                var h = await client.GetHistoryAsync(t.ScriptHash, ct);
+                if (h.Count > 0) historyByAddress[t.ScriptHash] = h;
+            }, ct)).Concat(tracked.Select(t =>
+                RetryOnBusyAsync(() => client.SubscribeScripthashAsync(t.ScriptHash, ct), ct))));
         }
         else
         {
-            // HD: discovery con GetHistoryAsync (senza subscription → no -101 su wallet grandi);
-            // subscribe solo al gap window per ricevere notifiche push di nuove tx.
-            nextReceive = await ScanChainAsync(isChange: false, tracked, historyByAddress, ct);
-            nextChange  = await ScanChainAsync(isChange: true,  tracked, historyByAddress, ct);
+            // Receive e change chain in parallelo (indipendenti per definizione).
+            // ScanChainAsync usa _knownReceiveIndex/_knownChangeIndex per la discovery
+            // incrementale: gli indirizzi già usati vengono fetchati in un burst unico.
+            var receiveTask = ScanChainAsync(isChange: false, _knownReceiveIndex, ct);
+            var changeTask  = ScanChainAsync(isChange: true,  _knownChangeIndex,  ct);
+            var rxScan = await receiveTask;
+            var chScan = await changeTask;
+
+            tracked.AddRange(rxScan.Tracked);
+            tracked.AddRange(chScan.Tracked);
+            foreach (var (k, v) in rxScan.History) historyByAddress[k] = v;
+            foreach (var (k, v) in chScan.History) historyByAddress[k] = v;
+            nextReceive = rxScan.NextIndex;
+            nextChange  = chScan.NextIndex;
 
-            // Iscriviti al gap window (prossimi indirizzi attesi) per notifiche push.
-            // In questo modo il numero di subscription è sempre ≤ 2×gapLimit, indipendentemente
-            // dalla dimensione dello storico — nessun rischio di -101.
             var gapAddresses = tracked.Where(t =>
                 (!t.IsChange && t.Index >= nextReceive && t.Index < nextReceive + gapLimit) ||
                 ( t.IsChange && t.Index >= nextChange  && t.Index < nextChange  + gapLimit)).ToList();
             if (gapAddresses.Count > 0)
-                await Task.WhenAll(gapAddresses.Select(t => client.SubscribeScripthashAsync(t.ScriptHash, ct)));
+                await Task.WhenAll(gapAddresses.Select(t =>
+                    RetryOnBusyAsync(() => client.SubscribeScripthashAsync(t.ScriptHash, ct), ct)));
         }
 
         // 3. Storico unico (txid → altezza massima riportata).
@@ -140,49 +150,32 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
         foreach (var item in historyByAddress.Values.SelectMany(h => h))
             txHeights[item.TxHash] = item.Height;
 
-        // 4. Scarica le transazioni nuove: semaforo MaxConcurrent per non saturare
-        //    il server, con aggiornamento progresso in tempo reale.
+        // 4+5. Download tx mancanti e verifica Merkle in parallelo senza semaforo.
         var network  = PalladiumNetworks.For(account.Profile.Kind);
         var missing  = txHeights.Keys.Where(txid => !_txCache.ContainsKey(txid)).ToList();
-        if (missing.Count > 0)
-        {
-            var dlSem = new SemaphoreSlim(MaxConcurrent, MaxConcurrent);
-            var dlDone = 0;
-            Progress?.Invoke($"scarico 0/{missing.Count} transazioni…");
-            await Task.WhenAll(missing.Select(async txid =>
-            {
-                await dlSem.WaitAsync(ct);
-                try
-                {
-                    var raw = await client.GetTransactionAsync(txid, ct);
-                    _txCache[txid] = Transaction.Parse(raw, network);
-                    var n = Interlocked.Increment(ref dlDone);
-                    Progress?.Invoke($"scarico {n}/{missing.Count} transazioni…");
-                }
-                finally { dlSem.Release(); }
-            }));
-        }
-        var transactions = txHeights.Keys.ToDictionary(txid => txid, txid => _txCache[txid]);
-
-        // 5. Verifica Merkle delle confermate (§7.4 punto 4).
-        //    Gli header per altezza sono condivisi via _headerFetches: se 500 tx
-        //    stanno nello stesso blocco, l'header viene scaricato una sola volta.
         var toVerify = txHeights
             .Where(kv => kv.Value > 0
                 && (!_verifiedAtHeight.TryGetValue(kv.Key, out var h) || h != kv.Value))
             .ToList();
-        if (toVerify.Count > 0)
+
+        if (missing.Count > 0 || toVerify.Count > 0)
         {
-            var merkSem = new SemaphoreSlim(MaxConcurrent, MaxConcurrent);
+            Progress?.Invoke($"scarico {missing.Count} tx, verifico {toVerify.Count} prove…");
+            var dlDone   = 0;
             var merkDone = 0;
-            Progress?.Invoke($"verifico 0/{toVerify.Count} prove di Merkle…");
-            await Task.WhenAll(toVerify.Select(async kv =>
+
+            var dlTasks = missing.Select(txid => RetryOnBusyAsync(async () =>
             {
-                await merkSem.WaitAsync(ct);
-                try
+                var raw = await client.GetTransactionAsync(txid, ct);
+                _txCache[txid] = Transaction.Parse(raw, network);
+                var n = Interlocked.Increment(ref dlDone);
+                if (n % 50 == 0 || n == missing.Count)
+                    Progress?.Invoke($"tx {n}/{missing.Count}, prove {merkDone}/{toVerify.Count}…");
+            }, ct));
+
+            var merkTasks = toVerify.Select(kv => RetryOnBusyAsync(async () =>
             {
                 var (txid, height) = kv;
-                    // Proof e header in parallelo; l'header è condiviso per altezza.
                 var proofTask  = client.GetMerkleAsync(txid, height, ct);
                 var headerTask = _headerFetches.GetOrAdd(height,
                     h => client.GetBlockHeaderAsync(h, ct));
@@ -194,17 +187,19 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
                     throw new SpvVerificationException(
                         $"Prova di Merkle non valida per {txid} (blocco {height}): server non affidabile.");
                 var n = Interlocked.Increment(ref merkDone);
-                    Progress?.Invoke($"verifico {n}/{toVerify.Count} prove di Merkle…");
-                }
-                finally { merkSem.Release(); }
-            }));
+                if (n % 50 == 0 || n == toVerify.Count)
+                    Progress?.Invoke($"tx {dlDone}/{missing.Count}, prove {n}/{toVerify.Count}…");
+            }, ct));
+
+            await Task.WhenAll(dlTasks.Concat(merkTasks));
             foreach (var (txid, height) in toVerify)
                 _verifiedAtHeight[txid] = height;
         }
+
+        var transactions = txHeights.Keys.ToDictionary(txid => txid, txid => _txCache[txid]);
         var verified     = txHeights.ToDictionary(kv => kv.Key, kv => kv.Value > 0);
 
-        // 6. Ricostruzione locale degli UTXO: accrediti = output verso nostri
-        //    script; spesi = outpoint consumati da una qualunque tx del wallet.
+        // 6. Ricostruzione locale degli UTXO.
         var byScript = tracked.ToDictionary(t => t.ScriptPubKey, t => t);
         var spent = transactions.Values
             .SelectMany(tx => tx.Inputs)
@@ -234,7 +229,7 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
             }
         }
 
-        // 7. Delta per voce di storico (entrate - uscite del wallet).
+        // 7. Delta per voce di storico.
         var history = new List<CachedTx>();
         foreach (var (txid, tx) in transactions)
         {
@@ -255,13 +250,11 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
         }
         history.Sort((a, b) =>
         {
-            // Non confermate (height<=0) in cima, poi per altezza decrescente.
             var ha = a.Height <= 0 ? int.MaxValue : a.Height;
             var hb = b.Height <= 0 ? int.MaxValue : b.Height;
             return hb.CompareTo(ha);
         });
 
-        // Saldo e numero di transazioni per singolo indirizzo (vista indirizzi).
         var balanceByAddress = utxos
             .GroupBy(u => u.Address)
             .ToDictionary(g => g.Key, g => g.Sum(u => u.ValueSats));
@@ -293,38 +286,63 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
     }
 
     /// <summary>
-    /// Scansiona una catena (receiving o change) finché trova gapLimit indirizzi
-    /// vuoti consecutivi (§5), procedendo a batch paralleli di gapLimit per volta.
-    /// Usa GetHistoryAsync per la discovery — senza subscription → nessun rischio di
-    /// -101 "excessive resource usage" su wallet con molti indirizzi storici.
-    /// Le subscription per notifiche push vengono gestite dal chiamante (solo gap window).
-    /// Ritorna il primo indice non usato.
+    /// Scansiona una catena (receiving o change).
+    ///
+    /// Phase 1 — indirizzi noti (0..fromIndex-1): tutti i GetHistoryAsync partono
+    /// in un unico burst parallelo, senza batching sequenziale. Per un wallet con
+    /// 100 indirizzi usati → 1 RTT invece di 5 round sequenziali di gapLimit.
+    ///
+    /// Phase 2 — discovery dal fromIndex in poi: batching con gap limit come prima,
+    /// necessario per sapere dove fermarsi.
     /// </summary>
-    private async Task<int> ScanChainAsync(bool isChange, List<TrackedAddress> tracked,
-        Dictionary<string, IReadOnlyList<HistoryItem>> historyByAddress, CancellationToken ct)
+    private async Task<(int NextIndex,
+                         List<TrackedAddress> Tracked,
+                         Dictionary<string, IReadOnlyList<HistoryItem>> History)>
+        ScanChainAsync(bool isChange, int fromIndex, CancellationToken ct)
     {
+        var tracked = new List<TrackedAddress>();
+        var history = new Dictionary<string, IReadOnlyList<HistoryItem>>();
+
+        // Phase 1: burst unico per tutti gli indirizzi già noti.
+        if (fromIndex > 0)
+        {
+            var known = Enumerable.Range(0, fromIndex).Select(i =>
+            {
+                var addr = account.GetAddress(isChange, i);
+                return new TrackedAddress(addr, Scripthash.FromAddress(addr), isChange, i);
+            }).ToList();
+            tracked.AddRange(known);
+
+            var knownHistories = await Task.WhenAll(
+                known.Select(t => RetryOnBusyAsync(
+                    () => client.GetHistoryAsync(t.ScriptHash, ct), ct)));
+            for (var i = 0; i < known.Count; i++)
+                if (knownHistories[i].Count > 0)
+                    history[known[i].ScriptHash] = knownHistories[i];
+        }
+
+        // Phase 2: discovery gap-limit dal fromIndex in poi.
         var consecutiveEmpty = 0;
-        var index = 0;
-        var firstUnused = 0;
+        var index      = fromIndex;
+        var firstUnused = fromIndex;
+
         while (consecutiveEmpty < gapLimit)
         {
             var batch = Enumerable.Range(index, gapLimit).Select(i =>
             {
-                var address = account.GetAddress(isChange, i);
-                return new TrackedAddress(address, Scripthash.FromAddress(address), isChange, i);
+                var addr = account.GetAddress(isChange, i);
+                return new TrackedAddress(addr, Scripthash.FromAddress(addr), isChange, i);
             }).ToList();
             index += batch.Count;
             tracked.AddRange(batch);
 
-            // GetHistoryAsync per discovery: risposta vuota [] se inutilizzato,
-            // lista di tx se usato — un solo round-trip per indirizzo.
             var histories = await Task.WhenAll(
-                batch.Select(t => client.GetHistoryAsync(t.ScriptHash, ct)));
+                batch.Select(t => RetryOnBusyAsync(
+                    () => client.GetHistoryAsync(t.ScriptHash, ct), ct)));
 
             for (var i = 0; i < batch.Count && consecutiveEmpty < gapLimit; i++)
             {
-                var history = histories[i];
-                if (history.Count == 0)
+                if (histories[i].Count == 0)
                 {
                     consecutiveEmpty++;
                 }
@@ -332,12 +350,47 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
                 {
                     consecutiveEmpty = 0;
                     firstUnused = batch[i].Index + 1;
-                    historyByAddress[batch[i].ScriptHash] = history;
+                    history[batch[i].ScriptHash] = histories[i];
                 }
             }
         }
-        return firstUnused;
+
+        return (firstUnused, tracked, history);
     }
+
+    private static async Task RetryOnBusyAsync(Func<Task> op, CancellationToken ct)
+    {
+        var delay = 200;
+        for (var attempt = 0; ; attempt++)
+        {
+            try   { await op(); return; }
+            catch (ElectrumServerException ex)
+                when (IsBusy(ex) && attempt < 7)
+            {
+                await Task.Delay(delay, ct);
+                delay = Math.Min(delay * 2, 5_000);
+            }
+        }
+    }
+
+    private static async Task<T> RetryOnBusyAsync<T>(Func<Task<T>> op, CancellationToken ct)
+    {
+        var delay = 200;
+        for (var attempt = 0; ; attempt++)
+        {
+            try   { return await op(); }
+            catch (ElectrumServerException ex)
+                when (IsBusy(ex) && attempt < 7)
+            {
+                await Task.Delay(delay, ct);
+                delay = Math.Min(delay * 2, 5_000);
+            }
+        }
+    }
+
+    private static bool IsBusy(ElectrumServerException ex) =>
+        ex.Message.Contains("-102") ||
+        ex.Message.Contains("server busy", StringComparison.OrdinalIgnoreCase);
 }
 
 /// <summary>La verifica SPV è fallita: i dati del server contraddicono le prove (§17).</summary>

src/Core/Storage/WalletDocument.cs

@@ -113,6 +113,13 @@ public sealed class SyncCache
     /// riverificare le stesse prove ad ogni avvio: le conferme sono immutabili.
     /// </summary>
     public Dictionary<string, int>? VerifiedAt { get; set; }
+
+    /// <summary>
+    /// Header grezzi per altezza (altezza → hex). Immutabili: non vengono mai
+    /// rifetchati una volta salvati. Elimina i GetBlockHeader sulle prove Merkle
+    /// già verificate nei sync successivi.
+    /// </summary>
+    public Dictionary<int, string>? BlockHeaders { get; set; }
 }
 
 /// <summary>Indirizzo scansionato con saldo proprio e numero di transazioni (vista indirizzi).</summary>