[Client+Server] Deck version history with restore

- Fail the run when the connection drops while a sync is running, and clear
  the active conflict, so the queue is not sent on a dead client and a
  re-login as another account never receives this profile's uploads.
- Backfill last_modified from upload_time in the 37 -> 38 migration and drop
  the "last_modified is null" arm from the sync-check query, so rows that
  predate the column stop reporting as changed on every poll.
- Skip conflicts during auto runs and report how many were skipped, instead
  of opening the modal conflict dialog from a timer or a file-watcher event.

cockatrice/CMakeLists.txt

@@ -28,10 +28,12 @@ set(cockatrice_SOURCES
     src/interface/deck_loader/deck_loader.cpp
     src/interface/deck_sync/deck_sync_profile.cpp
     src/interface/deck_sync/deck_sync_sidecar.cpp
+    src/interface/deck_sync/deck_sync_manager.cpp
     src/interface/widgets/dialogs/dlg_connect.cpp
     src/interface/widgets/dialogs/dlg_convert_deck_to_cod_format.cpp
     src/interface/widgets/dialogs/dlg_create_game.cpp
     src/interface/widgets/dialogs/dlg_crash_report.cpp
+    src/interface/widgets/dialogs/dlg_deck_history.cpp
     src/interface/widgets/dialogs/dlg_default_tags_editor.cpp
     src/interface/widgets/dialogs/dlg_edit_avatar.cpp
     src/interface/widgets/dialogs/dlg_edit_password.cpp
@@ -57,6 +59,8 @@ set(cockatrice_SOURCES
     src/interface/widgets/dialogs/dlg_shared_decks_preview.cpp
     src/interface/widgets/dialogs/dlg_settings.cpp
     src/interface/widgets/dialogs/dlg_startup_card_check.cpp
+    src/interface/widgets/dialogs/dlg_sync_conflict.cpp
+    src/interface/widgets/dialogs/dlg_sync_onboarding.cpp
     src/interface/widgets/dialogs/dlg_tip_of_the_day.cpp
     src/interface/widgets/dialogs/dlg_update.cpp
     src/interface/widgets/dialogs/dlg_view_log.cpp

cockatrice/src/client/settings/cache_settings.cpp

@@ -25,6 +25,7 @@
 #include <libcockatrice/settings/commander_bracket_settings.h>
 #include <libcockatrice/settings/debug_settings.h>
 #include <libcockatrice/settings/deck_editor_settings.h>
+#include <libcockatrice/settings/deck_sync_settings.h>
 #include <libcockatrice/settings/download_settings.h>
 #include <libcockatrice/settings/game_filters_settings.h>
 #include <libcockatrice/settings/game_settings.h>
@@ -159,6 +160,7 @@ SettingsCache::SettingsCache()
     cardsDisplaySettings = new CardsDisplaySettings(settingsPath, this);
     interfaceSettings = new InterfaceSettings(settingsPath, this);
     deckEditorSettings = new DeckEditorSettings(settingsPath, this);
+    deckSyncSettings = new DeckSyncSettings(settingsPath, this);
     pathsSettings = new PathsSettings(settingsPath, this);
     visualDeckStorageSettings = new VisualDeckStorageSettings(settingsPath, this);
     appearanceSettings = new AppearanceSettings(settingsPath, this);
@@ -460,6 +462,11 @@ DeckEditorSettings &SettingsCache::deckEditor() const
     return *deckEditorSettings;
 }
 
+DeckSyncSettings &SettingsCache::deckSync() const
+{
+    return *deckSyncSettings;
+}
+
 VisualDeckStorageSettings &SettingsCache::visualDeckStorage() const
 {
     return *visualDeckStorageSettings;

cockatrice/src/client/settings/cache_settings.h

@@ -32,6 +32,7 @@ class ChatSettings;
 class CommanderBracketSettings;
 class DebugSettings;
 class DeckEditorSettings;
+class DeckSyncSettings;
 class DownloadSettings;
 class GameFiltersSettings;
 class GameSettings;
@@ -81,6 +82,7 @@ private:
     CardsDisplaySettings *cardsDisplaySettings = nullptr;
     InterfaceSettings *interfaceSettings = nullptr;
     DeckEditorSettings *deckEditorSettings = nullptr;
+    DeckSyncSettings *deckSyncSettings = nullptr;
     PathsSettings *pathsSettings = nullptr;
     VisualDeckStorageSettings *visualDeckStorageSettings = nullptr;
     AppearanceSettings *appearanceSettings = nullptr;
@@ -149,6 +151,7 @@ public:
     [[nodiscard]] CardsDisplaySettings &cardsDisplay() const;
     [[nodiscard]] InterfaceSettings &userInterface() const;
     [[nodiscard]] DeckEditorSettings &deckEditor() const;
+    [[nodiscard]] DeckSyncSettings &deckSync() const;
     [[nodiscard]] PathsSettings &paths() const;
     [[nodiscard]] VisualDeckStorageSettings &visualDeckStorage() const;
     [[nodiscard]] AppearanceSettings &appearance() const;

cockatrice/src/interface/deck_sync/deck_sync_conflict.h

@@ -0,0 +1,50 @@
+/**
+ * @file deck_sync_conflict.h
+ * @brief Conflict payload between the sync engine and the conflict dialog.
+ */
+
+#ifndef DECK_SYNC_CONFLICT_H
+#define DECK_SYNC_CONFLICT_H
+
+#include <QString>
+
+/** @brief How a both-sides-changed deck is resolved. */
+enum DeckSyncResolutionAction
+{
+    DeckSyncKeepMine,     ///< Upload the local deck, discarding the server version.
+    DeckSyncKeepServer,   ///< Download the server deck, discarding local changes.
+    DeckSyncKeepBoth,     ///< Keep the local deck and store the server version aside.
+    DeckSyncUseMerge,     ///< Upload the auto-merged deck (only when mergeable).
+    DeckSyncSkipConflict, ///< Leave both sides untouched for this run.
+};
+
+/** @brief Sync direction for one run. */
+enum class DeckSyncMode
+{
+    TwoWay,       ///< Mirror both sides (default).
+    UploadOnly,   ///< Push local content; never download or propagate deletes.
+    DownloadOnly, ///< Take server content; never upload or push deletes.
+    Separate,     ///< First-sync verdict only: keep this profile out of sync.
+};
+
+/** @brief One deck that changed both locally and on the server. */
+struct DeckSyncConflict
+{
+    QString relativePath;      ///< Local deck path relative to the deck folder.
+    QString localAbsolutePath; ///< Absolute local file path.
+    QString serverDeckString;  ///< Server deck content in native format.
+    QString baseDeckString;    ///< Last-synced content, empty when no snapshot exists.
+    bool hasBase = false;      ///< False when the base snapshot is missing.
+    qint64 localSecs = 0;      ///< Local modification time, seconds since epoch.
+    qint64 serverSecs = 0;     ///< Server modification time, seconds since epoch.
+};
+
+/** @brief The dialog verdict for one DeckSyncConflict. */
+struct DeckSyncResolution
+{
+    DeckSyncResolutionAction action = DeckSyncKeepMine;
+    bool rememberSession = false; ///< Reuse this verdict for later conflicts this run.
+    bool rememberAlways = false;  ///< Store this verdict as the conflict default setting.
+};
+
+#endif // DECK_SYNC_CONFLICT_H

cockatrice/src/interface/deck_sync/deck_sync_manager.h

@@ -0,0 +1,248 @@
+/**
+ * @file deck_sync_manager.h
+ * @brief One-shot two-way deck collection sync between local files and the server.
+ */
+
+#ifndef DECK_SYNC_MANAGER_H
+#define DECK_SYNC_MANAGER_H
+
+#include "deck_sync_conflict.h"
+#include "deck_sync_profile.h"
+
+#include <QDateTime>
+#include <QList>
+#include <QMap>
+#include <QObject>
+#include <QSet>
+#include <QString>
+#include <QVariant>
+#include <functional>
+#include <libcockatrice/settings/deck_sync_settings.h>
+
+class AbstractClient;
+class CommandContainer;
+class DeckList;
+class QFileSystemWatcher;
+class QTimer;
+class Response;
+class ServerInfo_DeckStorage_Folder;
+
+/**
+ * @class DeckSyncManager
+ * @brief Runs a manual whole-collection sync: local *.cod files vs server deck storage.
+ *
+ * Each syncNow() run loads the profile sidecar, scans the deck folder, fetches
+ * the server tree and mirrors both sides: unknown local decks upload, unknown
+ * server decks download, and decks changed on both sides resolve newer-wins
+ * (ties go to the server). A local file is matched to its server row by the
+ * sidecar's row id, then guid, then the server path, and finally by content, so
+ * renaming either side does not duplicate a deck. Before a download overwrites a
+ * locally modified file, the local content is copied to a .sync-backup file next
+ * to it.
+ *
+ * Ownership: fire-and-forget per run, driven by PendingCommand round trips.
+ * Both-sides-changed decks pause the queue for a verdict via conflictRequest /
+ * resolveConflict (or the conflict default setting); the first sync of an
+ * unknown profile pauses for an onboarding verdict via firstSyncDetected /
+ * continueFirstSync. No watcher and no auto triggers — those arrive later.
+ *
+ * Deletes are tombstones, never silent: a locally deleted deck is pushed with
+ * Command_DeckDel, a server-side delete is applied locally only after copying
+ * the file to a .sync-backup, and resurrection reuses the stored deck id.
+ */
+class DeckSyncManager : public QObject
+{
+    Q_OBJECT
+public:
+    /** @brief Computes the color identity string uploaded alongside a deck. May be empty. */
+    using ColorIdentityProvider = std::function<QString(const DeckList &)>;
+
+    /** @brief Outcome counts for a finished sync run, carried by syncFinished(). */
+    struct SyncSummary
+    {
+        int uploaded = 0;
+        int downloaded = 0;
+        int unchanged = 0;
+        int skipped = 0;
+    };
+
+    explicit DeckSyncManager(QObject *parent = nullptr);
+    DeckSyncManager(AbstractClient *client, ColorIdentityProvider provider, QObject *parent = nullptr);
+
+    void setServerIdentity(const QString &host, int port, const QString &username);
+    void setDeckPath(const QString &path);
+    void setConflictDefault(int conflictDefault);
+    void setAutoSyncEnabled(bool enabled);
+    void setSyncIntervalMinutes(int minutes);
+    [[nodiscard]] bool isSyncing() const;
+
+public slots:
+    void syncNow();
+    /** @brief Continues a paused first sync after the onboarding verdict. */
+    void continueFirstSync(DeckSyncMode mode);
+    /** @brief Aborts a paused first sync without writing any verdict. */
+    void cancelFirstSync();
+    /** @brief Applies the dialog verdict for the active conflict and resumes. */
+    void resolveConflict(const DeckSyncResolution &resolution);
+
+signals:
+    void syncStarted();
+    void statusMessage(const QString &message);
+    void syncFinished(const SyncSummary &summary, const QString &error);
+    /** @brief First sync with content on both sides: the tab must ask how to proceed. */
+    void firstSyncDetected(int localCount, int serverCount);
+    /** @brief A both-sides-changed deck needs a verdict; the queue pauses until resolveConflict. */
+    void conflictRequest(const DeckSyncConflict &conflict);
+
+private slots:
+    void onListFinished(const Response &response, const CommandContainer &commandContainer);
+    void onCheckFinished(const Response &response, const CommandContainer &commandContainer);
+    void
+    onEnsureDirFinished(const Response &response, const CommandContainer &commandContainer, const QVariant &extraData);
+    void
+    onUploadFinished(const Response &response, const CommandContainer &commandContainer, const QVariant &extraData);
+    void
+    onDownloadFinished(const Response &response, const CommandContainer &commandContainer, const QVariant &extraData);
+    void
+    onDeleteFinished(const Response &response, const CommandContainer &commandContainer, const QVariant &extraData);
+
+private:
+    struct ServerFile
+    {
+        int id = -1;
+        QString path; ///< Server-side address: folder prefix plus row name.
+        QString dir;  ///< Folder prefix, empty in the root.
+        QString name; ///< Row name exactly as the server stores it.
+        QString guid;
+        QString hash;
+        qint64 lastModifiedSecs = 0;
+    };
+    struct LocalFile
+    {
+        QString absolutePath;
+        QString hash;
+        QDateTime modified;
+    };
+    struct UploadOp
+    {
+        QString relativePath;
+        QString serverPath; ///< Target address of a brand-new row.
+        QString serverDir;  ///< Folder that must exist before the upload.
+        int deckId = -1;
+        QString guid;
+    };
+    struct DownloadOp
+    {
+        QString relativePath;
+        int deckId = -1;
+        QString guid;
+        QString hash;
+        qint64 lastModifiedSecs = 0;
+    };
+    struct ActiveConflict
+    {
+        bool active = false;
+        QString relativePath;
+        QString serverPath;
+        int deckId = -1;
+        QString serverGuid;
+        QString serverHash;
+        qint64 serverLastModifiedSecs = 0;
+        QString serverDeckString;
+    };
+    struct ConflictDownload
+    {
+        QString serverPath;
+        int deckId = -1;
+        QString guid;
+        QString hash;
+        qint64 lastModifiedSecs = 0;
+    };
+
+    void startSync(bool autoRun);
+    void failSync(const QString &error);
+    void finishSync();
+    [[nodiscard]] SyncSummary currentSummary() const;
+    void fetchFullList();
+    void sendSyncCheck(qint64 lastSyncSecs);
+    void autoSyncSoon();
+    [[nodiscard]] bool canAutoRun() const;
+    void onDebounceTimeout();
+    void onWatchedPathChanged(const QString &path);
+    void refreshWatcher();
+    void refreshInterval();
+    [[nodiscard]] DeckSyncProfile buildProfile() const;
+    void collectServerFiles(const ServerInfo_DeckStorage_Folder &folder,
+                            const QString &prefix,
+                            QMap<int, ServerFile> &files,
+                            QSet<QString> &dirs);
+    void processNext();
+    void processNextUpload();
+    void sendMkDirFor(const QString &dir);
+    void sendUpload(const UploadOp &op);
+    void sendDownload(const DownloadOp &op);
+    void sendDelete(const QString &relativePath, int deckId);
+    void snapshotLocalFile(const QString &relativePath, const QString &guid);
+    QString readBaseSnapshot(const QString &guid) const;
+    void classify(const QMap<int, ServerFile> &serverFiles);
+    void queueConflict(const QString &relativePath,
+                       const QString &serverPath,
+                       int deckId,
+                       const QString &serverGuid,
+                       const QString &serverHash,
+                       qint64 serverLastModifiedSecs);
+    void handleConflictDownload(const QString &relativePath, const QString &serverDeckString);
+    void applyDownloadedDeck(const QString &relativePath,
+                             int deckId,
+                             const QString &deckString,
+                             const QString &guid,
+                             const QString &hash,
+                             qint64 lastModifiedSecs,
+                             bool hasLastModified);
+    void applyUpload(const QString &relativePath, const QString &serverPath, int deckId, const QString &guid);
+    void applyDownload(const QString &relativePath,
+                       int deckId,
+                       const QString &guid,
+                       const QString &hash,
+                       qint64 lastModifiedSecs);
+    static QString parentDir(const QString &path);
+    QString uniqueLocalPath(const QString &relativePath) const;
+
+    AbstractClient *client = nullptr;
+    QString serverHost;
+    int serverPort = 0;
+    QString serverUsername;
+    QString deckPath;
+    ColorIdentityProvider colorIdentityProvider;
+    bool syncing = false;
+
+    QString sidecarPath;
+    QMap<QString, LocalFile> localFiles;
+    QSet<QString> ensuredDirs;
+    QList<UploadOp> pendingUploads;
+    QList<DownloadOp> pendingDownloads;
+    QList<QString> pendingDeletes;
+    QSet<QString> conflictPaths;
+    QMap<QString, ConflictDownload> pendingConflicts;
+    QMap<int, ServerFile> stashedServerFiles;
+    ActiveConflict activeConflict;
+    DeckSyncMode syncMode = DeckSyncMode::TwoWay;
+    QFileSystemWatcher *watcher = nullptr;
+    QTimer *debounceTimer = nullptr;
+    QTimer *intervalTimer = nullptr;
+    bool autoSyncEnabled = true;
+    int syncIntervalMinutes = 5;
+    bool offline = false;
+    qint64 pendingServerTime = -1;
+    int conflictDefault = DeckSyncConflictAsk;
+    int sessionDefault = -1;
+    bool firstSyncRun = false;
+    int uploadedCount = 0;
+    int downloadedCount = 0;
+    int unchangedCount = 0;
+    int skippedCount = 0;
+    int conflictCount = 0;
+    bool autoRun = false;
+};
+
+#endif // DECK_SYNC_MANAGER_H

cockatrice/src/interface/deck_sync/deck_sync_sidecar.cpp

@@ -16,9 +16,16 @@ DeckSyncSidecar::DeckSyncSidecar(const QString &_filePath) : filePath(_filePath)
 {
 }
 
+QString DeckSyncSidecar::pathForDeckPath(const QString &deckPath, const QString &profileKey)
+{
+    return QDir(deckPath).filePath(".sync/" + profileKey + ".json");
+}
+
 bool DeckSyncSidecar::load()
 {
     recordMap.clear();
+    mode = MODE_LINKED;
+    lastSyncSecs = 0;
 
     QFile file(filePath);
     if (!file.exists()) {
@@ -31,6 +38,9 @@ bool DeckSyncSidecar::load()
     if (!document.isObject() || !document.object().value("records").isObject()) {
         return false;
     }
+    const QJsonValue modeValue = document.object().value("mode");
+    mode = modeValue.isString() ? modeValue.toString() : MODE_LINKED;
+    lastSyncSecs = static_cast<qint64>(document.object().value("lastSync").toDouble());
     const QJsonObject records = document.object().value("records").toObject();
     for (auto it = records.begin(); it != records.end(); ++it) {
         if (!it.value().isObject()) {
@@ -62,6 +72,8 @@ bool DeckSyncSidecar::save() const
     }
     QJsonObject root;
     root.insert("version", SIDECAR_FORMAT_VERSION);
+    root.insert("mode", mode);
+    root.insert("lastSync", static_cast<double>(lastSyncSecs));
     root.insert("records", records);
 
     QDir().mkpath(QFileInfo(filePath).absolutePath());

cockatrice/src/interface/deck_sync/deck_sync_sidecar.h

@@ -30,10 +30,18 @@ struct DeckSyncRecord
 class DeckSyncSidecar
 {
 public:
-    QString filePath; ///< Full path of the sidecar JSON file.
+    static constexpr const char *MODE_LINKED = "linked";
+    static constexpr const char *MODE_SEPARATE = "separate";
+
+    QString filePath;           ///< Full path of the sidecar JSON file.
+    QString mode = MODE_LINKED; ///< Linked normally; separate keeps this profile out of sync.
+    qint64 lastSyncSecs = 0;    ///< Last completed sync, seconds since epoch; 0 means never.
 
     explicit DeckSyncSidecar(const QString &_filePath);
 
+    /** @brief Sidecar file path for a deck folder and profile key. */
+    static QString pathForDeckPath(const QString &deckPath, const QString &profileKey);
+
     /** @brief Loads records; a missing file loads as empty and returns true. */
     bool load();
     /** @brief Stores records, creating parent folders as needed. */

cockatrice/src/interface/widgets/dialogs/dlg_deck_history.cpp

@@ -0,0 +1,176 @@
+#include "dlg_deck_history.h"
+
+#include <QDateTime>
+#include <QDialogButtonBox>
+#include <QLabel>
+#include <QListWidget>
+#include <QLocale>
+#include <QMessageBox>
+#include <QPushButton>
+#include <QVBoxLayout>
+#include <libcockatrice/deck_list/deck_diff.h>
+#include <libcockatrice/deck_list/deck_list.h>
+#include <libcockatrice/network/client/abstract/abstract_client.h>
+#include <libcockatrice/protocol/pb/command_deck_download.pb.h>
+#include <libcockatrice/protocol/pb/command_deck_history_list.pb.h>
+#include <libcockatrice/protocol/pb/response.pb.h>
+#include <libcockatrice/protocol/pb/response_deck_download.pb.h>
+#include <libcockatrice/protocol/pb/response_deck_history_list.pb.h>
+#include <libcockatrice/protocol/pending_command.h>
+
+namespace
+{
+QString formatVersionTime(qint64 secsSinceEpoch)
+{
+    return QLocale().toString(QDateTime::fromSecsSinceEpoch(secsSinceEpoch), QLocale::ShortFormat);
+}
+} // namespace
+
+DlgDeckHistory::DlgDeckHistory(int _deckId,
+                               const QString &_deckName,
+                               int _limit,
+                               AbstractClient *_client,
+                               QWidget *parent)
+    : QDialog(parent), deckId(_deckId), deckName(_deckName), limit(_limit), client(_client)
+{
+    auto *layout = new QVBoxLayout(this);
+
+    detailLabel = new QLabel(this);
+    detailLabel->setWordWrap(true);
+    versionsList = new QListWidget(this);
+    previewList = new QListWidget(this);
+    restoreButton = new QPushButton(this);
+    restoreButton->setEnabled(false);
+
+    layout->addWidget(detailLabel);
+    layout->addWidget(versionsList);
+    layout->addWidget(previewList);
+    layout->addWidget(restoreButton);
+
+    auto *buttons = new QDialogButtonBox(QDialogButtonBox::Close, this);
+    connect(buttons, &QDialogButtonBox::rejected, this, &QDialog::reject);
+    layout->addWidget(buttons);
+
+    connect(versionsList, &QListWidget::currentRowChanged, this, &DlgDeckHistory::onSelectionChanged);
+    connect(restoreButton, &QPushButton::clicked, this, &DlgDeckHistory::onRestoreClicked);
+
+    Command_DeckHistoryList listCmd;
+    listCmd.set_deck_id(deckId);
+    listCmd.set_limit(static_cast<google::protobuf::uint32>(limit));
+    PendingCommand *listPend = AbstractClient::prepareSessionCommand(listCmd);
+    connect(listPend, &PendingCommand::finished, this, &DlgDeckHistory::onHistoryFinished);
+    client->sendCommand(listPend);
+
+    Command_DeckDownload downloadCmd;
+    downloadCmd.set_deck_id(deckId);
+    PendingCommand *downloadPend = AbstractClient::prepareSessionCommand(downloadCmd);
+    connect(downloadPend, &PendingCommand::finished, this, &DlgDeckHistory::onCurrentFinished);
+    client->sendCommand(downloadPend);
+
+    retranslateUi();
+}
+
+void DlgDeckHistory::retranslateUi()
+{
+    setWindowTitle(tr("Deck history"));
+    detailLabel->setText(
+        tr("Stored versions of %1 (newest first). Restoring uploads the version as new content.").arg(deckName));
+    restoreButton->setText(tr("Restore selected version"));
+    renderVersions();
+    renderPreview();
+}
+
+void DlgDeckHistory::onHistoryFinished(const Response &response, const CommandContainer & /*commandContainer*/)
+{
+    if (response.response_code() != Response::RespOk) {
+        detailLabel->setText(tr("Could not load version history."));
+        return;
+    }
+    const Response_DeckHistoryList &list = response.GetExtension(Response_DeckHistoryList::ext);
+    versions.clear();
+    for (const DeckVersionInfo &version : list.versions()) {
+        DeckHistoryVersion entry;
+        entry.versionNo = version.version_no();
+        entry.createdAtSecs = static_cast<qint64>(version.created_at());
+        entry.createdByClientId = QString::fromStdString(version.created_by_clientid());
+        entry.hash = QString::fromStdString(version.hash());
+        entry.content = QString::fromStdString(version.content());
+        versions.append(entry);
+    }
+    historyArrived = true;
+    renderVersions();
+    renderPreview();
+}
+
+void DlgDeckHistory::onCurrentFinished(const Response &response, const CommandContainer & /*commandContainer*/)
+{
+    if (response.response_code() != Response::RespOk) {
+        detailLabel->setText(tr("Could not load the current deck."));
+        return;
+    }
+    const Response_DeckDownload &download = response.GetExtension(Response_DeckDownload::ext);
+    currentContent = QString::fromStdString(download.deck());
+    currentArrived = true;
+    renderPreview();
+}
+
+void DlgDeckHistory::renderVersions()
+{
+    const int row = versionsList->currentRow();
+    versionsList->clear();
+    for (const DeckHistoryVersion &version : versions) {
+        QString label = tr("Version %1 — %2").arg(version.versionNo).arg(formatVersionTime(version.createdAtSecs));
+        if (!version.createdByClientId.isEmpty()) {
+            label += tr(" — %1").arg(version.createdByClientId);
+        }
+        versionsList->addItem(label);
+    }
+    if (row >= 0 && row < versionsList->count()) {
+        versionsList->setCurrentRow(row);
+    } else if (versionsList->count() > 0) {
+        versionsList->setCurrentRow(0);
+    }
+    onSelectionChanged();
+}
+
+void DlgDeckHistory::renderPreview()
+{
+    previewList->clear();
+    restoreButton->setEnabled(false);
+    const int row = versionsList->currentRow();
+    if (!historyArrived || !currentArrived || row < 0 || row >= versions.size()) {
+        return;
+    }
+    DeckList currentDeck;
+    DeckList selectedDeck;
+    if (!currentDeck.loadFromString_Native(currentContent) ||
+        !selectedDeck.loadFromString_Native(versions.at(row).content)) {
+        return;
+    }
+    const DeckDiffResult shown = DeckDiff::diff(currentDeck, selectedDeck);
+    for (const DeckCardChange &change : shown.changes) {
+        previewList->addItem(
+            tr("%1: %2 %3 → %4").arg(change.zone).arg(change.cardName).arg(change.oldCount).arg(change.newCount));
+    }
+    restoreButton->setEnabled(true);
+}
+
+void DlgDeckHistory::onSelectionChanged()
+{
+    renderPreview();
+}
+
+void DlgDeckHistory::onRestoreClicked()
+{
+    const int row = versionsList->currentRow();
+    if (row < 0 || row >= versions.size()) {
+        return;
+    }
+    if (QMessageBox::question(this, tr("Restore version"),
+                              tr("Restore version %1 as the current deck content? This becomes a new version.")
+                                  .arg(versions.at(row).versionNo)) != QMessageBox::Yes) {
+        return;
+    }
+    emit restoreRequested(deckId, versions.at(row).content);
+    accept();
+}

cockatrice/src/interface/widgets/dialogs/dlg_deck_history.h

@@ -0,0 +1,73 @@
+/**
+ * @file dlg_deck_history.h
+ * @brief Browser for the stored versions of one server deck.
+ */
+
+#ifndef DLG_DECK_HISTORY_H
+#define DLG_DECK_HISTORY_H
+
+#include <QDialog>
+#include <QList>
+#include <QString>
+
+class AbstractClient;
+class CommandContainer;
+class QLabel;
+class QListWidget;
+class QPushButton;
+class Response;
+
+/** @brief One stored deck version for display. */
+struct DeckHistoryVersion
+{
+    unsigned int versionNo = 0;
+    qint64 createdAtSecs = 0;
+    QString createdByClientId;
+    QString hash;
+    QString content;
+};
+
+/**
+ * @class DlgDeckHistory
+ * @brief Lists stored versions with a diff preview against the current deck.
+ *
+ * Restoring uploads the selected version as the new current content through
+ * the regular deck-upload path, so it becomes a version itself instead of
+ * rewriting history. Rejected dialogs restore nothing.
+ */
+class DlgDeckHistory : public QDialog
+{
+    Q_OBJECT
+public:
+    DlgDeckHistory(int deckId, const QString &deckName, int limit, AbstractClient *client, QWidget *parent = nullptr);
+    void retranslateUi();
+
+signals:
+    void restoreRequested(int deckId, const QString &content);
+
+private slots:
+    void onHistoryFinished(const Response &response, const CommandContainer &commandContainer);
+    void onCurrentFinished(const Response &response, const CommandContainer &commandContainer);
+    void onSelectionChanged();
+    void onRestoreClicked();
+
+private:
+    void renderVersions();
+    void renderPreview();
+
+    int deckId = -1;
+    QString deckName;
+    int limit = 20;
+    AbstractClient *client = nullptr;
+    QList<DeckHistoryVersion> versions;
+    QString currentContent;
+    bool historyArrived = false;
+    bool currentArrived = false;
+
+    QLabel *detailLabel;
+    QListWidget *versionsList;
+    QListWidget *previewList;
+    QPushButton *restoreButton;
+};
+
+#endif // DLG_DECK_HISTORY_H

cockatrice/src/interface/widgets/dialogs/dlg_sync_conflict.cpp

@@ -0,0 +1,178 @@
+#include "dlg_sync_conflict.h"
+
+#include <QCheckBox>
+#include <QDateTime>
+#include <QDialogButtonBox>
+#include <QFile>
+#include <QLabel>
+#include <QListWidget>
+#include <QLocale>
+#include <QRadioButton>
+#include <QVBoxLayout>
+#include <libcockatrice/deck_list/deck_diff.h>
+#include <libcockatrice/deck_list/deck_list.h>
+#include <libcockatrice/deck_list/tree/deck_list_card_node.h>
+#include <libcockatrice/deck_list/tree/inner_deck_list_node.h>
+
+namespace
+{
+QString formatTime(qint64 secsSinceEpoch)
+{
+    return QLocale().toString(QDateTime::fromSecsSinceEpoch(secsSinceEpoch), QLocale::ShortFormat);
+}
+
+int zoneCardCount(const DeckList &deck, const QString &zone)
+{
+    int count = 0;
+    deck.forEachCard([&count, &zone](InnerDecklistNode *zoneNode, DecklistCardNode *card) {
+        if (zoneNode->getName() == zone) {
+            count += card->getNumber();
+        }
+    });
+    return count;
+}
+} // namespace
+
+DlgSyncConflict::DlgSyncConflict(const DeckSyncConflict &_conflict, QWidget *parent)
+    : QDialog(parent), conflict(_conflict)
+{
+    auto *layout = new QVBoxLayout(this);
+
+    titleLabel = new QLabel(this);
+    detailLabel = new QLabel(this);
+    detailLabel->setWordWrap(true);
+    keepMineRadio = new QRadioButton(this);
+    keepServerRadio = new QRadioButton(this);
+    keepBothRadio = new QRadioButton(this);
+    mergeRadio = new QRadioButton(this);
+    changesList = new QListWidget(this);
+    mergeSummaryLabel = new QLabel(this);
+    mergeSummaryLabel->setWordWrap(true);
+    rememberSessionBox = new QCheckBox(this);
+    rememberAlwaysBox = new QCheckBox(this);
+
+    layout->addWidget(titleLabel);
+    layout->addWidget(detailLabel);
+    layout->addWidget(keepMineRadio);
+    layout->addWidget(keepServerRadio);
+    layout->addWidget(keepBothRadio);
+    layout->addWidget(mergeRadio);
+    layout->addWidget(changesList);
+    layout->addWidget(mergeSummaryLabel);
+    layout->addWidget(rememberSessionBox);
+    layout->addWidget(rememberAlwaysBox);
+
+    auto *buttons = new QDialogButtonBox(QDialogButtonBox::Ok | QDialogButtonBox::Cancel, this);
+    connect(buttons, &QDialogButtonBox::accepted, this, &QDialog::accept);
+    connect(buttons, &QDialogButtonBox::rejected, this, &QDialog::reject);
+    layout->addWidget(buttons);
+
+    DeckList localDeck;
+    DeckList serverDeck;
+    QFile localFile(conflict.localAbsolutePath);
+    decksParsed = localFile.open(QIODevice::ReadOnly) && localDeck.loadFromFile_Native(&localFile) &&
+                  serverDeck.loadFromString_Native(conflict.serverDeckString);
+    localIsNewer = conflict.localSecs >= conflict.serverSecs;
+
+    if (decksParsed) {
+        shownChanges = DeckDiff::diff(localDeck, serverDeck);
+        if (conflict.hasBase) {
+            DeckList baseDeck;
+            if (baseDeck.loadFromString_Native(conflict.baseDeckString)) {
+                const DeckDiffResult localChanges = DeckDiff::diff(baseDeck, localDeck);
+                const DeckDiffResult remoteChanges = DeckDiff::diff(baseDeck, serverDeck);
+                mergeable = DeckDiff::canAutoMerge(localChanges, remoteChanges);
+                if (mergeable) {
+                    const DeckList merged = DeckDiff::merge(baseDeck, localChanges, remoteChanges);
+                    mergedMainCount = zoneCardCount(merged, "main");
+                    mergedSideCount = zoneCardCount(merged, "side");
+                }
+            }
+        }
+    }
+
+    if (localIsNewer) {
+        keepMineRadio->setChecked(true);
+    } else {
+        keepServerRadio->setChecked(true);
+    }
+    mergeRadio->setEnabled(mergeable);
+
+    connect(keepMineRadio, &QRadioButton::toggled, this, &DlgSyncConflict::updateRememberBoxes);
+    connect(keepServerRadio, &QRadioButton::toggled, this, &DlgSyncConflict::updateRememberBoxes);
+    connect(keepBothRadio, &QRadioButton::toggled, this, &DlgSyncConflict::updateRememberBoxes);
+    connect(mergeRadio, &QRadioButton::toggled, this, &DlgSyncConflict::updateRememberBoxes);
+    updateRememberBoxes();
+    retranslateUi();
+}
+
+void DlgSyncConflict::retranslateUi()
+{
+    setWindowTitle(tr("Sync conflict"));
+    titleLabel->setText(conflict.relativePath);
+    if (!decksParsed) {
+        detailLabel->setText(tr("This deck changed on both sides, but one version could not be read."));
+    } else {
+        detailLabel->setText(tr("This deck changed both on this device and on the server."));
+    }
+
+    keepMineRadio->setText(
+        tr("Keep my deck (%1%2)").arg(formatTime(conflict.localSecs), localIsNewer ? tr(", newest") : QString()));
+    keepServerRadio->setText(tr("Keep the server deck (%1%2)")
+                                 .arg(formatTime(conflict.serverSecs), localIsNewer ? QString() : tr(", newest")));
+    keepBothRadio->setText(tr("Keep both (store the server version aside)"));
+    mergeRadio->setText(mergeable ? tr("Use the auto-merge") : tr("Use the auto-merge (not available)"));
+
+    changesList->clear();
+    for (const DeckCardChange &change : shownChanges.changes) {
+        changesList->addItem(
+            tr("%1: %2 %3 → %4").arg(change.zone).arg(change.cardName).arg(change.oldCount).arg(change.newCount));
+    }
+
+    if (mergeable) {
+        mergeSummaryLabel->setText(tr("Auto-merge proposal: union of both sides (%1 main, %2 side).")
+                                       .arg(mergedMainCount)
+                                       .arg(mergedSideCount));
+    } else if (!conflict.hasBase) {
+        mergeSummaryLabel->setText(tr("No base snapshot exists, so the versions cannot be merged automatically."));
+    } else {
+        mergeSummaryLabel->setText(tr("The changes overlap, so they cannot be merged automatically."));
+    }
+
+    rememberSessionBox->setText(tr("Remember for the rest of this sync"));
+    rememberAlwaysBox->setText(tr("Always use this rule"));
+}
+
+void DlgSyncConflict::updateRememberBoxes()
+{
+    const bool single = keepMineRadio->isChecked() || keepServerRadio->isChecked();
+    rememberSessionBox->setEnabled(single);
+    rememberAlwaysBox->setEnabled(single);
+    if (!single) {
+        rememberSessionBox->setChecked(false);
+        rememberAlwaysBox->setChecked(false);
+    }
+}
+
+DeckSyncResolutionAction DlgSyncConflict::selectedAction() const
+{
+    if (keepServerRadio->isChecked()) {
+        return DeckSyncKeepServer;
+    }
+    if (keepBothRadio->isChecked()) {
+        return DeckSyncKeepBoth;
+    }
+    if (mergeRadio->isChecked()) {
+        return DeckSyncUseMerge;
+    }
+    return DeckSyncKeepMine;
+}
+
+DeckSyncResolution DlgSyncConflict::resolution() const
+{
+    DeckSyncResolution result;
+    result.action = selectedAction();
+    result.rememberSession = rememberSessionBox->isEnabled() && rememberSessionBox->isChecked();
+    result.rememberAlways = rememberAlwaysBox->isEnabled() && rememberAlwaysBox->isChecked();
+    return result;
+}

cockatrice/src/interface/widgets/dialogs/dlg_sync_conflict.h

@@ -0,0 +1,63 @@
+/**
+ * @file dlg_sync_conflict.h
+ * @brief Dialog for resolving a deck that changed both locally and on the server.
+ */
+
+#ifndef DLG_SYNC_CONFLICT_H
+#define DLG_SYNC_CONFLICT_H
+
+#include "../../deck_sync/deck_sync_conflict.h"
+
+#include <QDialog>
+#include <QString>
+#include <libcockatrice/deck_list/deck_diff.h>
+
+class QCheckBox;
+class QLabel;
+class QListWidget;
+class QRadioButton;
+
+/**
+ * @class DlgSyncConflict
+ * @brief Shows both versions of a conflicted deck and returns the verdict.
+ *
+ * Radio choice between Keep mine / Keep server (newest preselected), Keep both
+ * and Use merge (enabled only when the changes touch disjoint cards). The
+ * change list shows card-count deltas between the two versions; the merge
+ * summary describes the auto-merge proposal. Rejected dialogs resolve as Skip.
+ */
+class DlgSyncConflict : public QDialog
+{
+    Q_OBJECT
+public:
+    explicit DlgSyncConflict(const DeckSyncConflict &conflict, QWidget *parent = nullptr);
+    [[nodiscard]] DeckSyncResolution resolution() const;
+    void retranslateUi();
+
+private slots:
+    void updateRememberBoxes();
+
+private:
+    DeckSyncResolutionAction selectedAction() const;
+
+    DeckSyncConflict conflict;
+    bool decksParsed = false;
+    bool localIsNewer = true;
+    DeckDiffResult shownChanges;
+    bool mergeable = false;
+    int mergedMainCount = 0;
+    int mergedSideCount = 0;
+
+    QLabel *titleLabel;
+    QLabel *detailLabel;
+    QRadioButton *keepMineRadio;
+    QRadioButton *keepServerRadio;
+    QRadioButton *keepBothRadio;
+    QRadioButton *mergeRadio;
+    QListWidget *changesList;
+    QLabel *mergeSummaryLabel;
+    QCheckBox *rememberSessionBox;
+    QCheckBox *rememberAlwaysBox;
+};
+
+#endif // DLG_SYNC_CONFLICT_H

cockatrice/src/interface/widgets/dialogs/dlg_sync_onboarding.cpp

@@ -0,0 +1,59 @@
+#include "dlg_sync_onboarding.h"
+
+#include <QDialogButtonBox>
+#include <QLabel>
+#include <QRadioButton>
+#include <QVBoxLayout>
+
+DlgSyncOnboarding::DlgSyncOnboarding(int _localCount, int _serverCount, QWidget *parent)
+    : QDialog(parent), localCount(_localCount), serverCount(_serverCount)
+{
+    auto *layout = new QVBoxLayout(this);
+
+    detailLabel = new QLabel(this);
+    detailLabel->setWordWrap(true);
+    mergeRadio = new QRadioButton(this);
+    uploadRadio = new QRadioButton(this);
+    downloadRadio = new QRadioButton(this);
+    separateRadio = new QRadioButton(this);
+    mergeRadio->setChecked(true);
+
+    layout->addWidget(detailLabel);
+    layout->addWidget(mergeRadio);
+    layout->addWidget(uploadRadio);
+    layout->addWidget(downloadRadio);
+    layout->addWidget(separateRadio);
+
+    auto *buttons = new QDialogButtonBox(QDialogButtonBox::Ok | QDialogButtonBox::Cancel, this);
+    connect(buttons, &QDialogButtonBox::accepted, this, &QDialog::accept);
+    connect(buttons, &QDialogButtonBox::rejected, this, &QDialog::reject);
+    layout->addWidget(buttons);
+
+    retranslateUi();
+}
+
+void DlgSyncOnboarding::retranslateUi()
+{
+    setWindowTitle(tr("Sync decks to this server"));
+    detailLabel->setText(tr("You have %1 local decks and %2 decks stored on this server. How should they be combined?")
+                             .arg(localCount)
+                             .arg(serverCount));
+    mergeRadio->setText(tr("Merge: keep everything, sync both ways (recommended)"));
+    uploadRadio->setText(tr("Upload mine: push local decks to the server"));
+    downloadRadio->setText(tr("Download server: take the server decks"));
+    separateRadio->setText(tr("Keep separate: do not sync with this server"));
+}
+
+DeckSyncMode DlgSyncOnboarding::selectedMode() const
+{
+    if (uploadRadio->isChecked()) {
+        return DeckSyncMode::UploadOnly;
+    }
+    if (downloadRadio->isChecked()) {
+        return DeckSyncMode::DownloadOnly;
+    }
+    if (separateRadio->isChecked()) {
+        return DeckSyncMode::Separate;
+    }
+    return DeckSyncMode::TwoWay;
+}

cockatrice/src/interface/widgets/dialogs/dlg_sync_onboarding.h

@@ -0,0 +1,43 @@
+/**
+ * @file dlg_sync_onboarding.h
+ * @brief First-sync choice when local decks and server decks meet.
+ */
+
+#ifndef DLG_SYNC_ONBOARDING_H
+#define DLG_SYNC_ONBOARDING_H
+
+#include "../../deck_sync/deck_sync_conflict.h"
+
+#include <QDialog>
+
+class QLabel;
+class QRadioButton;
+
+/**
+ * @class DlgSyncOnboarding
+ * @brief Asks how to combine a non-empty local collection with a non-empty server collection.
+ *
+ * Merge (two-way) is the default. Upload and download restrict the run to one
+ * direction; keeping separate records the profile as sync-disabled.
+ * Rejected dialogs cancel the sync without recording anything.
+ */
+class DlgSyncOnboarding : public QDialog
+{
+    Q_OBJECT
+public:
+    DlgSyncOnboarding(int _localCount, int _serverCount, QWidget *parent = nullptr);
+    [[nodiscard]] DeckSyncMode selectedMode() const;
+    void retranslateUi();
+
+private:
+    int localCount = 0;
+    int serverCount = 0;
+
+    QLabel *detailLabel;
+    QRadioButton *mergeRadio;
+    QRadioButton *uploadRadio;
+    QRadioButton *downloadRadio;
+    QRadioButton *separateRadio;
+};
+
+#endif // DLG_SYNC_ONBOARDING_H

cockatrice/src/interface/widgets/settings_page/general_settings_page.cpp

@@ -27,6 +27,7 @@
 #include <QtGlobal>
 #include <libcockatrice/card/card_localization.h>
 #include <libcockatrice/settings/cards_display_settings.h>
+#include <libcockatrice/settings/deck_sync_settings.h>
 #include <libcockatrice/settings/download_settings.h>
 #include <libcockatrice/settings/paths_settings.h>
 #include <libcockatrice/settings/personal_settings.h>
@@ -359,6 +360,46 @@ GeneralSettingsPage::GeneralSettingsPage()
     pathsGroupBox = new QGroupBox;
     pathsGroupBox->setLayout(pathsGrid);
 
+    // deck sync settings
+    conflictDefaultSelector.addItem("", DeckSyncConflictAsk);
+    conflictDefaultSelector.addItem("", DeckSyncConflictKeepMine);
+    conflictDefaultSelector.addItem("", DeckSyncConflictKeepServer);
+    conflictDefaultSelector.addItem("", DeckSyncConflictKeepNewer);
+    int conflictIndex = conflictDefaultSelector.findData(settings.deckSync().getConflictDefault());
+    if (conflictIndex >= 0) {
+        conflictDefaultSelector.setCurrentIndex(conflictIndex);
+    }
+    connect(&conflictDefaultSelector, qOverload<int>(&QComboBox::currentIndexChanged), this, [this](int index) {
+        SettingsCache::instance().deckSync().setConflictDefault(conflictDefaultSelector.itemData(index).toInt());
+    });
+
+    autoSyncCheckBox.setChecked(settings.deckSync().getAutoSync());
+    connect(&autoSyncCheckBox, &QCheckBox::toggled, &SettingsCache::instance().deckSync(),
+            &DeckSyncSettings::setAutoSync);
+
+    syncIntervalEdit.setMinimum(1);
+    syncIntervalEdit.setMaximum(60);
+    syncIntervalEdit.setValue(settings.deckSync().getSyncIntervalMinutes());
+    connect(&syncIntervalEdit, qOverload<int>(&QSpinBox::valueChanged), &SettingsCache::instance().deckSync(),
+            &DeckSyncSettings::setSyncIntervalMinutes);
+
+    historyLimitEdit.setMinimum(1);
+    historyLimitEdit.setMaximum(100);
+    historyLimitEdit.setValue(settings.deckSync().getHistoryLimit());
+    connect(&historyLimitEdit, qOverload<int>(&QSpinBox::valueChanged), &SettingsCache::instance().deckSync(),
+            &DeckSyncSettings::setHistoryLimit);
+
+    auto *deckSyncGrid = new QGridLayout;
+    deckSyncGrid->addWidget(&conflictDefaultLabel, 0, 0);
+    deckSyncGrid->addWidget(&conflictDefaultSelector, 0, 1);
+    deckSyncGrid->addWidget(&autoSyncCheckBox, 1, 0, 1, 2);
+    deckSyncGrid->addWidget(&syncIntervalLabel, 2, 0);
+    deckSyncGrid->addWidget(&syncIntervalEdit, 2, 1);
+    deckSyncGrid->addWidget(&historyLimitLabel, 3, 0);
+    deckSyncGrid->addWidget(&historyLimitEdit, 3, 1);
+    deckSyncGroupBox = new QGroupBox;
+    deckSyncGroupBox->setLayout(deckSyncGrid);
+
     auto *mainLayout = new QVBoxLayout;
     mainLayout->addWidget(languageGroupBox);
     mainLayout->addWidget(versionGroupBox);
@@ -366,6 +407,7 @@ GeneralSettingsPage::GeneralSettingsPage()
     mainLayout->addWidget(cardDatabaseGroupBox);
     mainLayout->addWidget(startupGroupBox);
     mainLayout->addWidget(pathsGroupBox);
+    mainLayout->addWidget(deckSyncGroupBox);
     mainLayout->addStretch();
 
     GeneralSettingsPage::retranslateUi();
@@ -647,5 +689,17 @@ void GeneralSettingsPage::retranslateUi()
     cardDatabasePathLabel.setText(tr("Card database:"));
     customCardDatabasePathLabel.setText(tr("Custom database directory:"));
     tokenDatabasePathLabel.setText(tr("Token database:"));
+    deckSyncGroupBox->setTitle(tr("Deck sync"));
+    conflictDefaultLabel.setText(tr("When a deck changed on both sides:"));
+    conflictDefaultSelector.setItemText(conflictDefaultSelector.findData(DeckSyncConflictAsk), tr("Ask me every time"));
+    conflictDefaultSelector.setItemText(conflictDefaultSelector.findData(DeckSyncConflictKeepMine), tr("Keep my deck"));
+    conflictDefaultSelector.setItemText(conflictDefaultSelector.findData(DeckSyncConflictKeepServer),
+                                        tr("Keep the server deck"));
+    conflictDefaultSelector.setItemText(conflictDefaultSelector.findData(DeckSyncConflictKeepNewer),
+                                        tr("Keep the newer deck"));
+    autoSyncCheckBox.setText(tr("Sync decks automatically"));
+    syncIntervalLabel.setText(tr("Check every:"));
+    syncIntervalEdit.setSuffix(" " + tr("min"));
+    historyLimitLabel.setText(tr("Kept versions per deck:"));
     resetAllPathsButton->setText(tr("Reset all paths"));
 }

cockatrice/src/interface/widgets/settings_page/general_settings_page.h

@@ -52,6 +52,7 @@ private:
     QGroupBox *cardDatabaseGroupBox;
     QGroupBox *startupGroupBox;
     QGroupBox *pathsGroupBox;
+    QGroupBox *deckSyncGroupBox;
 
     QLabel languageLabel;
     QComboBox languageBox;
@@ -104,6 +105,14 @@ private:
     QLineEdit *tokenDatabasePathEdit;
     QPushButton *resetAllPathsButton;
     QLabel *allPathsResetLabel;
+
+    QLabel conflictDefaultLabel;
+    QComboBox conflictDefaultSelector;
+    QCheckBox autoSyncCheckBox;
+    QLabel syncIntervalLabel;
+    QSpinBox syncIntervalEdit;
+    QLabel historyLimitLabel;
+    QSpinBox historyLimitEdit;
 };
 
 #endif // COCKATRICE_GENERAL_SETTINGS_PAGE_H

cockatrice/src/interface/widgets/tabs/tab_deck_storage.cpp

@@ -2,10 +2,15 @@
 
 #include "../../../client/settings/cache_settings.h"
 #include "../../deck_loader/deck_loader.h"
+#include "../../deck_sync/deck_sync_conflict.h"
+#include "../../deck_sync/deck_sync_manager.h"
 #include "../../pixel_map_generator.h"
 #include "../cards/additional_info/deck_color_identity.h"
 #include "../deck_share/deck_share_utils.h"
 #include "../deck_share/share_bar_widget.h"
+#include "../dialogs/dlg_deck_history.h"
+#include "../dialogs/dlg_sync_conflict.h"
+#include "../dialogs/dlg_sync_onboarding.h"
 #include "../interface/widgets/server/remote/remote_decklist_tree_widget.h"
 #include "../interface/widgets/utility/get_text_with_max.h"
 #include "abstract_client.h"
@@ -29,6 +34,7 @@
 #include <QInputDialog>
 #include <QItemSelection>
 #include <QItemSelectionModel>
+#include <QLabel>
 #include <QLineEdit>
 #include <QMessageBox>
 #include <QModelIndex>
@@ -61,6 +67,7 @@
 #include <libcockatrice/protocol/pb/response_deck_download.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_upload.pb.h>
 #include <libcockatrice/protocol/pending_command.h>
+#include <libcockatrice/settings/deck_sync_settings.h>
 #include <libcockatrice/settings/network_settings.h>
 #include <libcockatrice/settings/paths_settings.h>
 #include <libcockatrice/utility/qt_utils.h>
@@ -177,6 +184,9 @@ TabDeckStorage::TabDeckStorage(TabSupervisor *_tabSupervisor,
     rightVbox->addWidget(shareBar);
     rightVbox->addWidget(serverDirView);
     rightVbox->addLayout(rightToolBarLayout);
+    syncStatusLabel = new QLabel(this);
+    syncStatusLabel->setWordWrap(true);
+    rightVbox->addWidget(syncStatusLabel);
     rightGroupBox = new QGroupBox;
     rightGroupBox->setLayout(rightVbox);
 
@@ -228,6 +238,14 @@ TabDeckStorage::TabDeckStorage(TabSupervisor *_tabSupervisor,
     aPublishDeck->setIcon(QPixmap("theme:icons/lock"));
     connect(aPublishDeck, &QAction::triggered, this, &TabDeckStorage::actPublishDeck);
 
+    aSyncNow = new QAction(this);
+    aSyncNow->setIcon(qApp->style()->standardIcon(QStyle::SP_BrowserReload));
+    connect(aSyncNow, &QAction::triggered, this, &TabDeckStorage::actSyncNow);
+
+    aDeckHistory = new QAction(this);
+    aDeckHistory->setIcon(qApp->style()->standardIcon(QStyle::SP_FileDialogDetailedView));
+    connect(aDeckHistory, &QAction::triggered, this, &TabDeckStorage::actDeckHistory);
+
     // Add actions to toolbars
     leftToolBar->addAction(aOpenLocalDeck);
     leftToolBar->addAction(aRenameLocal);
@@ -239,11 +257,40 @@ TabDeckStorage::TabDeckStorage(TabSupervisor *_tabSupervisor,
 
     rightToolBar->addAction(aOpenRemoteDeck);
     rightToolBar->addAction(aDownload);
+    rightToolBar->addAction(aSyncNow);
+    rightToolBar->addAction(aDeckHistory);
     rightToolBar->addAction(aShareDecks);
     rightToolBar->addAction(aPublishDeck);
     rightToolBar->addAction(aNewFolder);
     rightToolBar->addAction(aDeleteRemoteDeck);
 
+    syncManager = new DeckSyncManager(
+        client, [](const DeckList &deck) { return getDeckColorIdentity(deck, CardDatabaseManager::query()); }, this);
+    syncManager->setDeckPath(SettingsCache::instance().paths().getDeckPath());
+    connect(&SettingsCache::instance().paths(), &PathsSettings::deckPathChanged, this, [this] {
+        const QString deckPath = SettingsCache::instance().paths().getDeckPath();
+        localDirModel->setRootPath(deckPath);
+        localDirView->setRootIndex(localDirModel->index(localDirModel->rootPath(), 0));
+        syncManager->setDeckPath(deckPath);
+    });
+    syncManager->setConflictDefault(SettingsCache::instance().deckSync().getConflictDefault());
+    syncManager->setAutoSyncEnabled(SettingsCache::instance().deckSync().getAutoSync());
+    syncManager->setSyncIntervalMinutes(SettingsCache::instance().deckSync().getSyncIntervalMinutes());
+    connect(&SettingsCache::instance().deckSync(), &DeckSyncSettings::conflictDefaultChanged, syncManager,
+            &DeckSyncManager::setConflictDefault);
+    connect(&SettingsCache::instance().deckSync(), &DeckSyncSettings::autoSyncChanged, syncManager,
+            &DeckSyncManager::setAutoSyncEnabled);
+    connect(&SettingsCache::instance().deckSync(), &DeckSyncSettings::syncIntervalMinutesChanged, syncManager,
+            &DeckSyncManager::setSyncIntervalMinutes);
+    connect(syncManager, &DeckSyncManager::statusMessage, this, &TabDeckStorage::onSyncStatusMessage);
+    connect(syncManager, &DeckSyncManager::syncFinished, this, &TabDeckStorage::onSyncFinished);
+    connect(syncManager, &DeckSyncManager::firstSyncDetected, this, &TabDeckStorage::onFirstSyncDetected);
+    connect(syncManager, &DeckSyncManager::conflictRequest, this, &TabDeckStorage::onConflictRequest);
+    if (currentUserInfo != nullptr) {
+        syncUsername = QString::fromStdString(currentUserInfo->name());
+    }
+    syncManager->setServerIdentity(client->serverName(), client->serverPort(), syncUsername);
+
     retranslateUi();
 
     QWidget *mainWidget = new QWidget(this);
@@ -272,6 +319,9 @@ void TabDeckStorage::retranslateUi()
     aShareDecks->setText(tr("Share decks"));
     aPublishDeck->setText(tr("Publish/unpublish deck"));
     aOpenDecksFolder->setText(tr("Open decks folder"));
+    aSyncNow->setText(tr("Sync now"));
+    aDeckHistory->setText(tr("Version history"));
+    renderSyncStatus();
     shareBar->retranslateUi();
     if (shareBar->isVisible()) {
         onServerSelectionChanged();
@@ -299,6 +349,8 @@ QString TabDeckStorage::getTargetPath() const
 
 void TabDeckStorage::handleConnected(const ServerInfo_User &userInfo)
 {
+    syncUsername = QString::fromStdString(userInfo.name());
+    syncManager->setServerIdentity(client->serverName(), client->serverPort(), syncUsername);
     setRemoteEnabled(userInfo.user_level() & ServerInfo_User::IsRegistered);
 }
 
@@ -316,6 +368,7 @@ void TabDeckStorage::handleConnectionChanged(ClientStatus status)
 
 void TabDeckStorage::setRemoteEnabled(bool enabled)
 {
+    remoteEnabled = enabled;
     aUpload->setEnabled(enabled);
     aOpenRemoteDeck->setEnabled(enabled);
     aDownload->setEnabled(enabled);
@@ -323,6 +376,8 @@ void TabDeckStorage::setRemoteEnabled(bool enabled)
     aPublishDeck->setEnabled(enabled);
     aNewFolder->setEnabled(enabled);
     aDeleteRemoteDeck->setEnabled(enabled);
+    aDeckHistory->setEnabled(enabled);
+    aSyncNow->setEnabled(enabled && !syncManager->isSyncing());
 
     if (enabled) {
         serverDirView->refreshTree();
@@ -526,6 +581,139 @@ void TabDeckStorage::actOpenDecksFolder()
     QtUtils::openPathInFileManager(dir);
 }
 
+void TabDeckStorage::actSyncNow()
+{
+    syncManager->setDeckPath(SettingsCache::instance().paths().getDeckPath());
+    aSyncNow->setEnabled(false);
+    syncManager->syncNow();
+}
+
+void TabDeckStorage::onFirstSyncDetected(int localCount, int serverCount)
+{
+    DlgSyncOnboarding dialog(localCount, serverCount, this);
+    if (dialog.exec() == QDialog::Accepted) {
+        syncManager->continueFirstSync(dialog.selectedMode());
+    } else {
+        syncManager->cancelFirstSync();
+    }
+}
+
+void TabDeckStorage::onConflictRequest(const DeckSyncConflict &conflict)
+{
+    DlgSyncConflict dialog(conflict, this);
+    if (dialog.exec() != QDialog::Accepted) {
+        DeckSyncResolution skip;
+        skip.action = DeckSyncSkipConflict;
+        syncManager->resolveConflict(skip);
+        return;
+    }
+    const DeckSyncResolution resolution = dialog.resolution();
+    if (resolution.rememberAlways &&
+        (resolution.action == DeckSyncKeepMine || resolution.action == DeckSyncKeepServer)) {
+        SettingsCache::instance().deckSync().setConflictDefault(
+            resolution.action == DeckSyncKeepMine ? DeckSyncConflictKeepMine : DeckSyncConflictKeepServer);
+    }
+    syncManager->resolveConflict(resolution);
+}
+
+void TabDeckStorage::onSyncStatusMessage(const QString &message)
+{
+    syncStatusType = SyncStatusType::Working;
+    syncStatusDetail = message;
+    renderSyncStatus();
+}
+
+void TabDeckStorage::onSyncFinished(const SyncSummary &summary, const QString &error)
+{
+    aSyncNow->setEnabled(remoteEnabled);
+    if (!error.isEmpty()) {
+        syncStatusType = SyncStatusType::Failed;
+        syncStatusDetail = error;
+    } else {
+        syncStatusType = SyncStatusType::Finished;
+        syncStatusDetail.clear();
+        lastSyncSummary = summary;
+    }
+    renderSyncStatus();
+    if (error.isEmpty()) {
+        serverDirView->refreshTree();
+    }
+}
+
+void TabDeckStorage::renderSyncStatus()
+{
+    switch (syncStatusType) {
+        case SyncStatusType::Working:
+            syncStatusLabel->setText(syncStatusDetail);
+            break;
+        case SyncStatusType::Failed:
+            syncStatusLabel->setText(tr("Sync failed: %1").arg(syncStatusDetail));
+            break;
+        case SyncStatusType::Finished:
+            syncStatusLabel->setText(tr("Sync complete: %1 uploaded, %2 downloaded, %3 unchanged, %4 skipped.")
+                                         .arg(lastSyncSummary.uploaded)
+                                         .arg(lastSyncSummary.downloaded)
+                                         .arg(lastSyncSummary.unchanged)
+                                         .arg(lastSyncSummary.skipped));
+            break;
+        case SyncStatusType::Idle:
+            syncStatusLabel->setText(tr("Not synced yet"));
+            break;
+    }
+}
+
+void TabDeckStorage::actDeckHistory()
+{
+    QList<RemoteDeckList_TreeModel::FileNode *> files;
+    for (const auto curRight : serverDirView->getCurrentSelection()) {
+        if (auto *file = dynamic_cast<RemoteDeckList_TreeModel::FileNode *>(curRight)) {
+            files.append(file);
+        }
+    }
+    if (files.size() != 1) {
+        QMessageBox::information(this, tr("Version history"), tr("Select a single server deck to view its history."));
+        return;
+    }
+
+    auto *dialog = new DlgDeckHistory(files.first()->getId(), files.first()->getName(),
+                                      SettingsCache::instance().deckSync().getHistoryLimit(), client, this);
+    connect(dialog, &DlgDeckHistory::restoreRequested, this, &TabDeckStorage::onHistoryRestoreRequested);
+    dialog->setAttribute(Qt::WA_DeleteOnClose);
+    dialog->show();
+}
+
+void TabDeckStorage::onHistoryRestoreRequested(int deckId, const QString &content)
+{
+    DeckList deck;
+    if (!deck.loadFromString_Native(content)) {
+        return;
+    }
+    const QString deckString = deck.writeToString_Native();
+    if (deckString.length() > MAX_FILE_LENGTH) {
+        QMessageBox::critical(this, tr("Error"), tr("Restored deck is too large"));
+        return;
+    }
+
+    Command_DeckUpload cmd;
+    cmd.set_deck_id(deckId);
+    cmd.set_deck_list(deckString.toStdString());
+    cmd.set_color_identity(getDeckColorIdentity(deck, CardDatabaseManager::query()).toStdString());
+
+    PendingCommand *pend = client->prepareSessionCommand(cmd);
+    connect(pend, &PendingCommand::finished, this, &TabDeckStorage::onHistoryRestoreFinished);
+    client->sendCommand(pend);
+}
+
+void TabDeckStorage::onHistoryRestoreFinished(const Response &response, const CommandContainer & /*commandContainer*/)
+{
+    if (response.response_code() != Response::RespOk) {
+        QMessageBox::critical(this, tr("Error"), tr("Failed to restore deck version"));
+        return;
+    }
+    onSyncStatusMessage(tr("Version restored."));
+    serverDirView->refreshTree();
+}
+
 void TabDeckStorage::actRemoteDoubleClick(const QModelIndex &curRight)
 {
     if (dynamic_cast<RemoteDeckList_TreeModel::FileNode *>(serverDirView->getNode(curRight))) {

cockatrice/src/interface/widgets/tabs/tab_deck_storage.h

@@ -8,7 +8,8 @@
 #ifndef TAB_DECK_STORAGE_H
 #define TAB_DECK_STORAGE_H
 
-#include "../interface/widgets/server/remote/remote_decklist_tree_widget.h"
+#include "../../deck_sync/deck_sync_manager.h"
+#include "../server/remote/remote_decklist_tree_widget.h"
 #include "tab.h"
 
 #include <QList>
@@ -19,12 +20,15 @@
 #include <qtmetamacros.h>
 
 struct LoadedDeck;
+struct DeckSyncConflict;
+struct DeckSyncResolution;
 class ServerInfo_User;
 class QTreeView;
 class QFileSystemModel;
 class QToolBar;
 class QGroupBox;
 class QTimer;
+class QLabel;
 class CommandContainer;
 class Response;
 class ShareBarWidget;
@@ -50,6 +54,24 @@ private:
     QAction *aOpenLocalDeck, *aRenameLocal, *aUpload, *aNewLocalFolder, *aDeleteLocalDeck;
     QAction *aOpenDecksFolder;
     QAction *aOpenRemoteDeck, *aDownload, *aShareDecks, *aPublishDeck, *aNewFolder, *aDeleteRemoteDeck;
+    QAction *aSyncNow;
+    QAction *aDeckHistory;
+    QLabel *syncStatusLabel;
+    DeckSyncManager *syncManager;
+    QString syncUsername;
+    bool remoteEnabled = false;
+    enum class SyncStatusType
+    {
+        Idle,
+        Working,
+        Finished,
+        Failed
+    };
+    using SyncSummary = DeckSyncManager::SyncSummary;
+    SyncStatusType syncStatusType = SyncStatusType::Idle;
+    QString syncStatusDetail;
+    SyncSummary lastSyncSummary;
+    void renderSyncStatus();
     bool visibilityRefreshStarted = false;
     QTimer *visibilityRefreshTimer;
     QStringList visibilityFailures;
@@ -108,6 +130,15 @@ private slots:
     void deleteFolderFinished(const Response &response, const CommandContainer &commandContainer);
     void deleteDeckFinished(const Response &response, const CommandContainer &commandContainer);
 
+    void actSyncNow();
+    void actDeckHistory();
+    void onHistoryRestoreRequested(int deckId, const QString &content);
+    void onHistoryRestoreFinished(const Response &response, const CommandContainer &commandContainer);
+    void onFirstSyncDetected(int localCount, int serverCount);
+    void onConflictRequest(const DeckSyncConflict &conflict);
+    void onSyncStatusMessage(const QString &message);
+    void onSyncFinished(const SyncSummary &summary, const QString &error);
+
 public:
     TabDeckStorage(TabSupervisor *_tabSupervisor, AbstractClient *_client, const ServerInfo_User *currentUserInfo);
     void retranslateUi() override;

libcockatrice_deck_list/libcockatrice/deck_list/deck_list.cpp

@@ -9,6 +9,7 @@
 #include "tree/inner_deck_list_node.h"
 
 #include <QAnyStringView>
+#include <QCryptographicHash>
 #include <QDebug>
 #include <QSet>
 #include <QSharedPointer>
@@ -453,6 +454,16 @@ void DeckList::refreshDeckHash()
     cachedDeckHash = QString();
 }
 
+QString DeckList::contentHash(const QString &nativeDeck)
+{
+    return QString::fromLatin1(QCryptographicHash::hash(nativeDeck.toUtf8(), QCryptographicHash::Sha1).toHex());
+}
+
+QString DeckList::getContentHash() const
+{
+    return contentHash(writeToString_Native());
+}
+
 /**
  * Calls a given function on each card in the deck.
  */

libcockatrice_deck_list/libcockatrice/deck_list/deck_list.h

@@ -289,6 +289,16 @@ public:
     /** @name Deck identity */
     ///@{
     QString getDeckHash() const;
+    /**
+     * @brief SHA-1 of a native deck string, covering everything the card hash
+     * ignores (name, printings, banner, tokens, comments, sideboard plans).
+     *
+     * Deck-collection sync hashes the exact string it sends or receives, so both
+     * peers compare the same bytes.
+     */
+    static QString contentHash(const QString &nativeDeck);
+    /** @brief contentHash() of this deck's native serialisation. */
+    QString getContentHash() const;
     void refreshDeckHash();
     ///@}
 

libcockatrice_protocol/libcockatrice/protocol/pb/CMakeLists.txt

@@ -16,6 +16,7 @@ set(PROTO_FILES
     command_deck_del_dir.proto
     command_deck_download.proto
     command_deck_download_public.proto
+    command_deck_history_list.proto
     command_deck_list.proto
     command_deck_list_other_user.proto
     command_deck_new_dir.proto
@@ -26,6 +27,7 @@ set(PROTO_FILES
     command_deck_share_list.proto
     command_deck_share_list_mine.proto
     command_deck_share_remove.proto
+    command_deck_sync_check.proto
     command_deck_upload.proto
     command_del_counter.proto
     command_delete_arrow.proto
@@ -146,11 +148,13 @@ set(PROTO_FILES
     response_ban_history.proto
     response_card_art_rule_entry.proto
     response_deck_download.proto
+    response_deck_history_list.proto
     response_deck_list.proto
     response_deck_share_create.proto
     response_deck_share_download.proto
     response_deck_share_list.proto
     response_deck_share_list_mine.proto
+    response_deck_sync_status.proto
     response_deck_upload.proto
     response_dump_zone.proto
     response_forgotpasswordrequest.proto

libcockatrice_protocol/libcockatrice/protocol/pb/command_deck_history_list.proto

@@ -0,0 +1,12 @@
+syntax = "proto2";
+import "session_commands.proto";
+
+message Command_DeckHistoryList {
+    extend SessionCommand {
+        optional Command_DeckHistoryList ext = 1035;
+    }
+    optional sint32 deck_id = 1 [default = -1];
+    // Maximum versions to return, newest first. Capped server-side by
+    // deck_sync/max_versions.
+    optional uint32 limit = 2 [default = 20];
+}

libcockatrice_protocol/libcockatrice/protocol/pb/command_deck_sync_check.proto

@@ -0,0 +1,11 @@
+syntax = "proto2";
+import "session_commands.proto";
+
+message Command_DeckSyncCheck {
+    extend SessionCommand {
+        optional Command_DeckSyncCheck ext = 1034;
+    }
+    // Seconds since Unix epoch of the client's last completed sync.
+    // 0 means "never synced": the server reports every stored row.
+    optional uint64 last_sync_time = 1 [default = 0];
+}

libcockatrice_protocol/libcockatrice/protocol/pb/response.proto

@@ -85,6 +85,8 @@ message Response {
         DECK_SHARE_LIST = 1104;         // Response listing shared decks
         DECK_SHARE_DOWNLOAD = 1105;     // Response for shared deck download
         DECK_SHARE_LIST_MINE = 1106;    // Response listing the caller's own shares
+        DECK_SYNC_STATUS = 1107;        // Response with changed decks since a timestamp
+        DECK_HISTORY_LIST = 1108;       // Response listing deck versions
         CARD_ART_RULE_LIST = 1200;      // Response containing a list of card art rules
     }
 

libcockatrice_protocol/libcockatrice/protocol/pb/response_deck_history_list.proto

@@ -0,0 +1,19 @@
+syntax = "proto2";
+import "response.proto";
+
+message DeckVersionInfo {
+    optional uint32 version_no = 1;
+    optional uint64 created_at = 2;
+    optional string created_by_clientid = 3;
+    optional string hash = 4;
+    // Full deck content in native format, so the client can diff and
+    // restore without further round trips.
+    optional string content = 5;
+}
+
+message Response_DeckHistoryList {
+    extend Response {
+        optional Response_DeckHistoryList ext = 1108;
+    }
+    repeated DeckVersionInfo versions = 1;
+}

libcockatrice_protocol/libcockatrice/protocol/pb/response_deck_sync_status.proto

@@ -0,0 +1,21 @@
+syntax = "proto2";
+import "response.proto";
+
+message DeckSyncChangedFile {
+    // Server-relative folder path without file extension, e.g. "Modern/Affinity".
+    optional string path = 1;
+    optional uint32 deck_id = 2;
+    optional string guid = 3;
+    optional string hash = 4;
+    optional uint64 last_modified = 5;
+    optional bool deleted = 6 [default = false];
+}
+
+message Response_DeckSyncStatus {
+    extend Response {
+        optional Response_DeckSyncStatus ext = 1107;
+    }
+    repeated DeckSyncChangedFile changed_files = 1;
+    // Server clock at query time; the client adopts it as its sync baseline.
+    optional uint64 server_time = 2;
+}

libcockatrice_protocol/libcockatrice/protocol/pb/session_commands.proto

@@ -37,6 +37,8 @@ message SessionCommand {
         DECK_SHARE_LIST_MINE = 1032;
         DECK_SHARE_REMOVE = 1033;
         SUBMIT_PASSWORD_VERIFIER = 1034;
+        DECK_SYNC_CHECK = 1035;
+        DECK_HISTORY_LIST = 1036;
         REPLAY_LIST = 1100;
         REPLAY_DOWNLOAD = 1101;
         REPLAY_MODIFY_MATCH = 1102;

libcockatrice_settings/CMakeLists.txt

@@ -12,6 +12,7 @@ set(HEADERS
     libcockatrice/settings/commander_bracket_settings.h
     libcockatrice/settings/debug_settings.h
     libcockatrice/settings/deck_editor_settings.h
+    libcockatrice/settings/deck_sync_settings.h
     libcockatrice/settings/download_settings.h
     libcockatrice/settings/game_filters_settings.h
     libcockatrice/settings/game_settings.h
@@ -43,6 +44,7 @@ add_library(
   libcockatrice/settings/chat_settings.cpp
   libcockatrice/settings/debug_settings.cpp
   libcockatrice/settings/deck_editor_settings.cpp
+  libcockatrice/settings/deck_sync_settings.cpp
   libcockatrice/settings/download_settings.cpp
   libcockatrice/settings/game_filters_settings.cpp
   libcockatrice/settings/game_settings.cpp

libcockatrice_settings/libcockatrice/settings/deck_sync_settings.cpp

@@ -0,0 +1,55 @@
+#include "deck_sync_settings.h"
+
+#include "libcockatrice/settings/settings_manager.h"
+
+#include <QVariant>
+#include <QtGlobal>
+
+DeckSyncSettings::DeckSyncSettings(const QString &settingPath, QObject *parent)
+    : SettingsManager(settingPath + "deck_sync.ini", "decksync", QString(), parent)
+{
+}
+
+int DeckSyncSettings::getConflictDefault() const
+{
+    return getValue("conflictDefault", QString(), QString(), DeckSyncConflictAsk).toInt();
+}
+
+void DeckSyncSettings::setConflictDefault(int _conflictDefault)
+{
+    setValue(_conflictDefault, "conflictDefault");
+    emit conflictDefaultChanged(_conflictDefault);
+}
+
+bool DeckSyncSettings::getAutoSync() const
+{
+    return getValue("autoSync", QString(), QString(), true).toBool();
+}
+
+void DeckSyncSettings::setAutoSync(bool _autoSync)
+{
+    setValue(_autoSync, "autoSync");
+    emit autoSyncChanged(_autoSync);
+}
+
+int DeckSyncSettings::getSyncIntervalMinutes() const
+{
+    return qBound(1, getValue("syncIntervalMinutes", QString(), QString(), 5).toInt(), 60);
+}
+
+void DeckSyncSettings::setSyncIntervalMinutes(int _syncIntervalMinutes)
+{
+    setValue(_syncIntervalMinutes, "syncIntervalMinutes");
+    emit syncIntervalMinutesChanged(_syncIntervalMinutes);
+}
+
+int DeckSyncSettings::getHistoryLimit() const
+{
+    return qBound(1, getValue("historyLimit", QString(), QString(), 20).toInt(), 100);
+}
+
+void DeckSyncSettings::setHistoryLimit(int _historyLimit)
+{
+    setValue(_historyLimit, "historyLimit");
+    emit historyLimitChanged(_historyLimit);
+}

libcockatrice_settings/libcockatrice/settings/deck_sync_settings.h

@@ -0,0 +1,46 @@
+#ifndef DECK_SYNC_SETTINGS_H
+#define DECK_SYNC_SETTINGS_H
+
+#include "settings_manager.h"
+
+#include <QString>
+#include <qtmetamacros.h>
+
+class QObject;
+
+enum DeckSyncConflictDefault
+{
+    DeckSyncConflictAsk = 0,    ///< Ask every time a deck changed on both sides.
+    DeckSyncConflictKeepMine,   ///< Silently keep the local deck.
+    DeckSyncConflictKeepServer, ///< Silently keep the server deck.
+    DeckSyncConflictKeepNewer,  ///< Silently keep whichever side changed last.
+};
+
+class DeckSyncSettings : public SettingsManager
+{
+    Q_OBJECT
+    friend class SettingsCache;
+
+public:
+    explicit DeckSyncSettings(const QString &settingPath, QObject *parent = nullptr);
+
+    [[nodiscard]] int getConflictDefault() const;
+    void setConflictDefault(int _conflictDefault);
+    [[nodiscard]] bool getAutoSync() const;
+    void setAutoSync(bool _autoSync);
+    [[nodiscard]] int getSyncIntervalMinutes() const;
+    void setSyncIntervalMinutes(int _syncIntervalMinutes);
+    [[nodiscard]] int getHistoryLimit() const;
+    void setHistoryLimit(int _historyLimit);
+
+signals:
+    void conflictDefaultChanged(int conflictDefault);
+    void autoSyncChanged(bool autoSync);
+    void syncIntervalMinutesChanged(int syncIntervalMinutes);
+    void historyLimitChanged(int historyLimit);
+
+private:
+    DeckSyncSettings(const DeckSyncSettings & /*other*/);
+};
+
+#endif // DECK_SYNC_SETTINGS_H

libcockatrice_settings/libcockatrice/settings/paths_settings.cpp

@@ -71,6 +71,7 @@ QString PathsSettings::getRedirectCachePath() const
 void PathsSettings::setDeckPath(const QString &_deckPath)
 {
     setValue(_deckPath, "decks");
+    emit deckPathChanged();
 }
 
 void PathsSettings::setFiltersPath(const QString &_filtersPath)

libcockatrice_settings/libcockatrice/settings/paths_settings.h

@@ -59,6 +59,7 @@ public:
     void setAppDirWarningAcknowledged(bool _appDirWarningAcknowledged);
 
 signals:
+    void deckPathChanged();
     void cardDatabasePathChanged();
     void picsPathChanged();
     void themeChanged();

servatrice/migrations/servatrice_0040_to_0041.sql

@@ -16,6 +16,12 @@ ALTER TABLE `cockatrice_decklist_files`
   ADD COLUMN `last_modified` datetime NULL AFTER `hash`,
   ADD COLUMN `deleted` tinyint(1) NOT NULL DEFAULT 0 AFTER `last_modified`;
 
+-- Rows created before this migration only get a last_modified when a client
+-- re-uploads them, so every sync check would keep reporting them as changed.
+-- Backfill from the upload time once, here, instead of carrying a
+-- "last_modified is null" branch in the poll query forever.
+UPDATE `cockatrice_decklist_files` SET `last_modified` = `upload_time` WHERE `last_modified` IS NULL;
+
 CREATE TABLE IF NOT EXISTS `cockatrice_deck_versions` (
   `id` int(7) unsigned zerofill NOT NULL auto_increment,
   `deck_id` int(7) unsigned zerofill NOT NULL,

servatrice/servatrice.ini.example

@@ -598,3 +598,16 @@ max_shares_per_day=50
 ; Minimum enforced: 1.
 ; Default: 20
 max_versions=20
+
+; How many days a deleted deck stays restorable as a tombstone before the
+; purge clock removes it (with its versions) for good. Re-uploading the
+; same deck within this window revives the tombstone instead of creating a copy.
+; A value of 0 purges tombstones on the next purge run.
+; Default: 30
+tombstone_days=30
+
+; How often, in minutes, the tombstone purge runs. This is independent of
+; [deck_share] cleanup_interval, so disabling share cleanup does not disable
+; purging. A value of 0 disables purging entirely.
+; Default: 60
+purge_interval=60

servatrice/src/servatrice.cpp

@@ -632,6 +632,16 @@ bool Servatrice::initServer()
     connect(legacyBackupSweepClock, &QTimer::timeout, this, &Servatrice::sweepLegacyBackups);
     legacyBackupSweepClock->start(kLegacyBackupSweepIntervalMs);
 
+    // Tombstone purging runs on its own clock: turning share cleanup off must not
+    // silently disable it as well.
+    deckTombstoneClock = new QTimer(this);
+    connect(deckTombstoneClock, SIGNAL(timeout()), this, SLOT(purgeDeckTombstones()));
+    const int deckSyncPurgeInterval = getDeckSyncPurgeInterval();
+    if (deckSyncPurgeInterval > 0) {
+        qDebug() << "Starting deck tombstone purge clock, interval" << deckSyncPurgeInterval << "ms";
+        deckTombstoneClock->start(deckSyncPurgeInterval);
+    }
+
     // SOCKET SERVER
     if (getNumberOfTCPPools() > 0) {
         gameServer =
@@ -856,6 +866,11 @@ void Servatrice::cleanupExpiredDeckShares()
     servatriceDatabaseInterface->cleanupExpiredDeckShares();
 }
 
+void Servatrice::purgeDeckTombstones()
+{
+    servatriceDatabaseInterface->purgeDeckTombstones(getDeckSyncTombstoneDays());
+}
+
 void Servatrice::statusUpdate()
 {
     if (!servatriceDatabaseInterface->checkSql()) {
@@ -1401,6 +1416,17 @@ int Servatrice::getDeckSyncMaxVersions() const
     return qMax(1, settingsCache->value("deck_sync/max_versions", 20).toInt());
 }
 
+int Servatrice::getDeckSyncTombstoneDays() const
+{
+    return qMax(0, settingsCache->value("deck_sync/tombstone_days", 30).toInt());
+}
+
+int Servatrice::getDeckSyncPurgeInterval() const
+{
+    // default: every 60 minutes, independent of deck_share/cleanup_interval
+    return qMax(0, settingsCache->value("deck_sync/purge_interval", 60).toInt()) * 60000;
+}
+
 int Servatrice::getNumberOfTCPPools() const
 {
     return settingsCache->value("server/number_pools", 1).toInt();

servatrice/src/servatrice.h

@@ -166,6 +166,7 @@ private slots:
     void sweepLegacyBackups();
     void shutdownTimeout();
     void cleanupExpiredDeckShares();
+    void purgeDeckTombstones();
 
 protected:
     void doSendIslMessage(const IslMessage &msg, int _serverId) override;
@@ -181,6 +182,7 @@ private:
     QTimer *pingClock, *statusUpdateClock;
     QTimer *deckShareCleanupClock;
     QTimer *legacyBackupSweepClock;
+    QTimer *deckTombstoneClock;
     Servatrice_GameServer *gameServer;
     Servatrice_WebsocketGameServer *websocketGameServer;
     Servatrice_IslServer *islServer;
@@ -321,6 +323,8 @@ public:
     int getDeckShareMaxDecksPerShare() const;
     int getDeckShareMaxSharesPerDay() const;
     int getDeckSyncMaxVersions() const;
+    int getDeckSyncTombstoneDays() const;
+    int getDeckSyncPurgeInterval() const;
     int getMaxUsersPerAddress() const;
     /// Raw security/trusted_sources list; match addresses against it with isTrustedSource().
     QString getTrustedSources() const;

servatrice/src/servatrice_database_interface.cpp

@@ -1370,8 +1370,8 @@ DeckList *Servatrice_DatabaseInterface::getDeckFromDatabase(int deckId, int user
 {
     checkSql();
 
-    QSqlQuery *query =
-        prepareQuery("select content from {prefix}_decklist_files where id = :id and id_user = :id_user");
+    QSqlQuery *query = prepareQuery(
+        "select content from {prefix}_decklist_files where id = :id and id_user = :id_user and deleted = 0");
     query->bindValue(":id", deckId);
     query->bindValue(":id_user", userId);
     execSqlQuery(query);
@@ -1522,6 +1522,19 @@ void Servatrice_DatabaseInterface::cleanupExpiredDeckShares()
     execSqlQuery(query);
 }
 
+void Servatrice_DatabaseInterface::purgeDeckTombstones(int olderThanDays)
+{
+    checkSql();
+
+    // DB-native clock: tombstones are stamped with NOW() on this same
+    // database, so the grace window is immune to app/db clock skew.
+    // The null arm only catches manually flagged rows.
+    QSqlQuery *query = prepareQuery("delete from {prefix}_decklist_files where deleted = 1 and (last_modified < "
+                                    "DATE_SUB(NOW(), INTERVAL :days DAY) or last_modified is null)");
+    query->bindValue(":days", qMax(0, olderThanDays));
+    execSqlQuery(query);
+}
+
 bool Servatrice_DatabaseInterface::getDeckSharesForUser(int userId, QList<DeckShareSummaryRecord> &shares)
 {
     checkSql();

servatrice/src/servatrice_database_interface.h

@@ -152,6 +152,11 @@ public:
     bool getDeckShareItem(const QString &token, int itemId, QString &content);
     /** @brief Deletes all expired share bundles (cascades to their items). */
     void cleanupExpiredDeckShares();
+    /**
+     * @brief Hard-deletes tombstoned decks older than the grace period (cascades to versions).
+     * @param olderThanDays Tombstones older than this many days are purged; 0 purges all tombstones.
+     */
+    void purgeDeckTombstones(int olderThanDays);
 
     int getNextGameId() override;
     int getNextReplayId() override;

servatrice/src/serversocketinterface.cpp

@@ -89,6 +89,7 @@
 #include <libcockatrice/protocol/pb/command_deck_del_dir.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_download.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_download_public.pb.h>
+#include <libcockatrice/protocol/pb/command_deck_history_list.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_list.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_list_other_user.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_new_dir.pb.h>
@@ -98,6 +99,7 @@
 #include <libcockatrice/protocol/pb/command_deck_share_list.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_share_list_mine.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_share_remove.pb.h>
+#include <libcockatrice/protocol/pb/command_deck_sync_check.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_upload.pb.h>
 #include <libcockatrice/protocol/pb/command_get_server_stats.pb.h>
 #include <libcockatrice/protocol/pb/command_replay_delete_match.pb.h>
@@ -129,11 +131,13 @@
 #include <libcockatrice/protocol/pb/response_card_art_rule_entry.pb.h>
 #include <libcockatrice/protocol/pb/response_clear_blacklisted_banners.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_download.pb.h>
+#include <libcockatrice/protocol/pb/response_deck_history_list.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_list.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_share_create.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_share_download.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_share_list.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_share_list_mine.pb.h>
+#include <libcockatrice/protocol/pb/response_deck_sync_status.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_upload.pb.h>
 #include <libcockatrice/protocol/pb/response_forgotpasswordrequest.pb.h>
 #include <libcockatrice/protocol/pb/response_get_admin_notes.pb.h>
@@ -371,6 +375,10 @@ Response::ResponseCode AbstractServerSocketInterface::processExtendedSessionComm
             return cmdDeckUpload(cmd.GetExtension(Command_DeckUpload::ext), rc);
         case SessionCommand::DECK_DOWNLOAD:
             return cmdDeckDownload(cmd.GetExtension(Command_DeckDownload::ext), rc);
+        case SessionCommand::DECK_SYNC_CHECK:
+            return cmdDeckSyncCheck(cmd.GetExtension(Command_DeckSyncCheck::ext), rc);
+        case SessionCommand::DECK_HISTORY_LIST:
+            return cmdDeckHistoryList(cmd.GetExtension(Command_DeckHistoryList::ext), rc);
         case SessionCommand::REPLAY_LIST:
             return cmdReplayList(cmd.GetExtension(Command_ReplayList::ext), rc);
         case SessionCommand::REPLAY_DOWNLOAD:
@@ -709,7 +717,8 @@ bool AbstractServerSocketInterface::deckListHelper(int folderId,
 
     query = sqlInterface->prepareQuery("select id, name, upload_time, is_public, banner_card_name, "
                                        "banner_card_provider, color_identity, tags, guid, hash, last_modified from "
-                                       "{prefix}_decklist_files where id_folder = :id_folder and id_user = :id_user");
+                                       "{prefix}_decklist_files where id_folder = :id_folder and id_user = :id_user "
+                                       "and deleted = 0");
     query->bindValue(":id_folder", folderId);
     query->bindValue(":id_user", userId);
     if (!sqlInterface->execSqlQuery(query)) {
@@ -854,8 +863,8 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckSetVisibility(const
     sqlInterface->checkSql();
 
     if (cmd.has_deck_id()) {
-        QSqlQuery *query =
-            sqlInterface->prepareQuery("select 1 from {prefix}_decklist_files where id = :id and id_user = :id_user");
+        QSqlQuery *query = sqlInterface->prepareQuery(
+            "select 1 from {prefix}_decklist_files where id = :id and id_user = :id_user and deleted = 0");
         query->bindValue(":id", cmd.deck_id());
         query->bindValue(":id_user", userInfo->id());
         sqlInterface->execSqlQuery(query);
@@ -893,6 +902,105 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckSetVisibility(const
     return Response::RespOk;
 }
 
+bool AbstractServerSocketInterface::syncCheckHelper(int userId, qint64 sinceSecs, Response_DeckSyncStatus *status)
+{
+    // Single query over the user's files: folder rows are hard-deleted while
+    // their files linger as tombstones, so walking the live folder tree would
+    // miss deletes pointing at nonexistent folders. The client only checks
+    // changed_files_size(), so folder paths are not reconstructed here.
+    // last_modified is backfilled by the 37 -> 38 migration, so a plain range
+    // check is enough.
+    QSqlQuery *query = sqlInterface->prepareQuery("select id, guid, hash, last_modified, deleted from "
+                                                  "{prefix}_decklist_files where id_user = :id_user "
+                                                  "and last_modified >= FROM_UNIXTIME(:since)");
+    query->bindValue(":id_user", userId);
+    query->bindValue(":since", sinceSecs);
+    if (!sqlInterface->execSqlQuery(query)) {
+        return false;
+    }
+    while (query->next()) {
+        DeckSyncChangedFile *changed = status->add_changed_files();
+        changed->set_deck_id(query->value(0).toUInt());
+        changed->set_guid(query->value(1).toString().toStdString());
+        changed->set_hash(query->value(2).toString().toStdString());
+        const QDateTime lastModified = query->value(3).toDateTime();
+        if (lastModified.isValid()) {
+            changed->set_last_modified(lastModified.toSecsSinceEpoch());
+        }
+        changed->set_deleted(query->value(4).toBool());
+    }
+    return true;
+}
+
+Response::ResponseCode AbstractServerSocketInterface::cmdDeckSyncCheck(const Command_DeckSyncCheck &cmd,
+                                                                       ResponseContainer &rc)
+{
+    if (authState != PasswordRight) {
+        return Response::RespFunctionNotAllowed;
+    }
+
+    sqlInterface->checkSql();
+
+    Response_DeckSyncStatus *re = new Response_DeckSyncStatus;
+    if (!syncCheckHelper(userInfo->id(), static_cast<qint64>(cmd.last_sync_time()), re)) {
+        delete re;
+        return Response::RespContextError;
+    }
+
+    QSqlQuery *clockQuery = sqlInterface->prepareQuery("select unix_timestamp(now())");
+    if (!sqlInterface->execSqlQuery(clockQuery) || !clockQuery->next()) {
+        delete re;
+        return Response::RespContextError;
+    }
+    re->set_server_time(clockQuery->value(0).toULongLong());
+    rc.setResponseExtension(re);
+    return Response::RespOk;
+}
+
+Response::ResponseCode AbstractServerSocketInterface::cmdDeckHistoryList(const Command_DeckHistoryList &cmd,
+                                                                         ResponseContainer &rc)
+{
+    if (authState != PasswordRight) {
+        return Response::RespFunctionNotAllowed;
+    }
+
+    sqlInterface->checkSql();
+
+    QSqlQuery *ownerQuery =
+        sqlInterface->prepareQuery("select 1 from {prefix}_decklist_files where id = :id and id_user = :id_user");
+    ownerQuery->bindValue(":id", cmd.deck_id());
+    ownerQuery->bindValue(":id_user", userInfo->id());
+    if (!sqlInterface->execSqlQuery(ownerQuery)) {
+        return Response::RespContextError;
+    }
+    if (!ownerQuery->next()) {
+        return Response::RespNameNotFound;
+    }
+
+    const int limit = qMin(cmd.has_limit() ? static_cast<int>(cmd.limit()) : 20, servatrice->getDeckSyncMaxVersions());
+    QSqlQuery *query =
+        sqlInterface->prepareQuery("select version_no, unix_timestamp(created_at), created_by_clientid, hash, "
+                                   "content from {prefix}_deck_versions where deck_id = :deck_id order by version_no "
+                                   "desc limit :limit");
+    query->bindValue(":deck_id", cmd.deck_id());
+    query->bindValue(":limit", qMax(1, limit));
+    if (!sqlInterface->execSqlQuery(query)) {
+        return Response::RespContextError;
+    }
+
+    Response_DeckHistoryList *re = new Response_DeckHistoryList;
+    while (query->next()) {
+        DeckVersionInfo *version = re->add_versions();
+        version->set_version_no(query->value(0).toUInt());
+        version->set_created_at(query->value(1).toULongLong());
+        version->set_created_by_clientid(query->value(2).toString().toStdString());
+        version->set_hash(query->value(3).toString().toStdString());
+        version->set_content(query->value(4).toString().toStdString());
+    }
+    rc.setResponseExtension(re);
+    return Response::RespOk;
+}
+
 Response::ResponseCode AbstractServerSocketInterface::cmdDeckDownloadPublic(const Command_DeckDownloadPublic &cmd,
                                                                             ResponseContainer &rc)
 {
@@ -965,7 +1073,8 @@ void AbstractServerSocketInterface::deckDelDirHelper(int basePathId)
         deckDelDirHelper(query->value(0).toInt());
     }
 
-    query = sqlInterface->prepareQuery("delete from {prefix}_decklist_files where id_folder = :id_folder");
+    query = sqlInterface->prepareQuery("update {prefix}_decklist_files set deleted = 1, last_modified = NOW() "
+                                       "where id_folder = :id_folder and deleted = 0");
     query->bindValue(":id_folder", basePathId);
     sqlInterface->execSqlQuery(query);
 
@@ -1016,18 +1125,17 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckDel(const Command_D
 
     sqlInterface->checkSql();
     QSqlQuery *query =
-        sqlInterface->prepareQuery("select id from {prefix}_decklist_files where id = :id and id_user = :id_user");
+        sqlInterface->prepareQuery("update {prefix}_decklist_files set deleted = 1, last_modified = NOW() where id = "
+                                   ":id and id_user = :id_user and deleted = 0");
     query->bindValue(":id", cmd.deck_id());
     query->bindValue(":id_user", userInfo->id());
-    sqlInterface->execSqlQuery(query);
-    if (!query->next()) {
+    if (!sqlInterface->execSqlQuery(query)) {
+        return Response::RespContextError;
+    }
+    if (query->numRowsAffected() == 0) {
         return Response::RespNameNotFound;
     }
 
-    query = sqlInterface->prepareQuery("delete from {prefix}_decklist_files where id = :id");
-    query->bindValue(":id", cmd.deck_id());
-    sqlInterface->execSqlQuery(query);
-
     return Response::RespOk;
 }
 
@@ -1114,7 +1222,9 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckUpload(const Comman
     const QString bannerCardProvider = deck.getBannerCard().providerId.left(32);
     const QString tagsJson = serializeDeckTags(deck.getTags());
     const QString colorIdentity = sanitizeColorIdentity(nameFromStdString(cmd.color_identity()));
-    const QString deckHash = deck.getDeckHash();
+    // Hash the exact bytes the client sent: both peers must compare the same
+    // string, not each side's re-serialisation of it.
+    const QString deckHash = DeckList::contentHash(deckStr);
     QString guid = cmd.has_guid() ? nameFromStdString(cmd.guid()).left(36) : QString();
     if (QUuid::fromString(guid).isNull()) {
         guid = QUuid::createUuid().toString(QUuid::WithoutBraces);
@@ -1126,31 +1236,71 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckUpload(const Comman
             return Response::RespNameNotFound;
         }
 
-        QSqlQuery *query =
-            sqlInterface->prepareQuery("insert into {prefix}_decklist_files (id_folder, id_user, name, upload_time, "
-                                       "content, is_public, banner_card_name, banner_card_provider, color_identity, "
-                                       "tags, guid, hash, last_modified) values(:id_folder, :id_user, :name, NOW(), "
-                                       ":content, :is_public, :banner_card_name, :banner_card_provider, "
-                                       ":color_identity, :tags, :guid, :hash, NOW())");
-        query->bindValue(":id_folder", folderId);
-        query->bindValue(":id_user", userInfo->id());
-        query->bindValue(":name", deckName);
-        query->bindValue(":content", deckStr);
-        query->bindValue(":is_public", cmd.has_is_public() && cmd.is_public() ? 1 : 0);
-        query->bindValue(":banner_card_name", bannerCardName);
-        query->bindValue(":banner_card_provider", bannerCardProvider);
-        query->bindValue(":color_identity", colorIdentity);
-        query->bindValue(":tags", tagsJson);
-        query->bindValue(":guid", guid);
-        query->bindValue(":hash", deckHash);
-        if (!sqlInterface->execSqlQuery(query)) {
+        // A deck that was deleted elsewhere leaves a tombstone behind. Revive it
+        // instead of inserting a second row with the same guid, which would split
+        // the deck's version history and leave the tombstone to be purged.
+        int deckId = -1;
+        bool inserted = false;
+        QSqlQuery *tombstoneQuery =
+            sqlInterface->prepareQuery("select id from {prefix}_decklist_files where id_user = :id_user and guid = "
+                                       ":guid and deleted = 1 order by last_modified desc limit 1");
+        tombstoneQuery->bindValue(":id_user", userInfo->id());
+        tombstoneQuery->bindValue(":guid", guid);
+        if (!sqlInterface->execSqlQuery(tombstoneQuery)) {
             return Response::RespContextError;
         }
-        const int deckId = query->lastInsertId().toInt();
+        if (tombstoneQuery->next()) {
+            deckId = tombstoneQuery->value(0).toInt();
+            QSqlQuery *reviveQuery = sqlInterface->prepareQuery(
+                "update {prefix}_decklist_files set id_folder=:id_folder, name=:name, upload_time=NOW(), "
+                "content=:content, is_public=:is_public, banner_card_name=:banner_card_name, "
+                "banner_card_provider=:banner_card_provider, color_identity=:color_identity, tags=:tags, "
+                "hash=:hash, last_modified=NOW(), deleted=0 where id=:id and id_user=:id_user");
+            reviveQuery->bindValue(":id_folder", folderId);
+            reviveQuery->bindValue(":id_user", userInfo->id());
+            reviveQuery->bindValue(":id", deckId);
+            reviveQuery->bindValue(":name", deckName);
+            reviveQuery->bindValue(":content", deckStr);
+            reviveQuery->bindValue(":is_public", cmd.has_is_public() && cmd.is_public() ? 1 : 0);
+            reviveQuery->bindValue(":banner_card_name", bannerCardName);
+            reviveQuery->bindValue(":banner_card_provider", bannerCardProvider);
+            reviveQuery->bindValue(":color_identity", colorIdentity);
+            reviveQuery->bindValue(":tags", tagsJson);
+            reviveQuery->bindValue(":hash", deckHash);
+            if (!sqlInterface->execSqlQuery(reviveQuery)) {
+                return Response::RespContextError;
+            }
+        } else {
+            QSqlQuery *query = sqlInterface->prepareQuery(
+                "insert into {prefix}_decklist_files (id_folder, id_user, name, upload_time, "
+                "content, is_public, banner_card_name, banner_card_provider, color_identity, "
+                "tags, guid, hash, last_modified) values(:id_folder, :id_user, :name, NOW(), "
+                ":content, :is_public, :banner_card_name, :banner_card_provider, "
+                ":color_identity, :tags, :guid, :hash, NOW())");
+            query->bindValue(":id_folder", folderId);
+            query->bindValue(":id_user", userInfo->id());
+            query->bindValue(":name", deckName);
+            query->bindValue(":content", deckStr);
+            query->bindValue(":is_public", cmd.has_is_public() && cmd.is_public() ? 1 : 0);
+            query->bindValue(":banner_card_name", bannerCardName);
+            query->bindValue(":banner_card_provider", bannerCardProvider);
+            query->bindValue(":color_identity", colorIdentity);
+            query->bindValue(":tags", tagsJson);
+            query->bindValue(":guid", guid);
+            query->bindValue(":hash", deckHash);
+            if (!sqlInterface->execSqlQuery(query)) {
+                return Response::RespContextError;
+            }
+            deckId = query->lastInsertId().toInt();
+            inserted = true;
+        }
         if (!storeDeckVersion(deckId, deckStr, deckHash)) {
-            QSqlQuery *cleanupQuery = sqlInterface->prepareQuery("delete from {prefix}_decklist_files where id = :id");
-            cleanupQuery->bindValue(":id", deckId);
-            sqlInterface->execSqlQuery(cleanupQuery);
+            if (inserted) {
+                QSqlQuery *cleanupQuery =
+                    sqlInterface->prepareQuery("delete from {prefix}_decklist_files where id = :id");
+                cleanupQuery->bindValue(":id", deckId);
+                sqlInterface->execSqlQuery(cleanupQuery);
+            }
             return Response::RespContextError;
         }
 
@@ -1160,9 +1310,12 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckUpload(const Comman
         stampQuery->bindValue(":id", deckId);
         stampQuery->bindValue(":id_user", userInfo->id());
         if (!sqlInterface->execSqlQuery(stampQuery) || !stampQuery->next()) {
-            QSqlQuery *cleanupQuery = sqlInterface->prepareQuery("delete from {prefix}_decklist_files where id = :id");
-            cleanupQuery->bindValue(":id", deckId);
-            sqlInterface->execSqlQuery(cleanupQuery);
+            if (inserted) {
+                QSqlQuery *cleanupQuery =
+                    sqlInterface->prepareQuery("delete from {prefix}_decklist_files where id = :id");
+                cleanupQuery->bindValue(":id", deckId);
+                sqlInterface->execSqlQuery(cleanupQuery);
+            }
             return Response::RespContextError;
         }
 
@@ -1195,9 +1348,11 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckUpload(const Comman
         }
         const QString oldHash = currentQuery->value(1).toString();
 
-        QString updateQuery = "update {prefix}_decklist_files set name=:name, upload_time=NOW(), content=:content, "
-                              "banner_card_name=:banner_card_name, banner_card_provider=:banner_card_provider, "
-                              "color_identity=:color_identity, tags=:tags, guid=:guid, hash=:hash, last_modified=NOW()";
+        QString updateQuery =
+            "update {prefix}_decklist_files set name=:name, upload_time=NOW(), content=:content, "
+            "banner_card_name=:banner_card_name, banner_card_provider=:banner_card_provider, "
+            "color_identity=:color_identity, tags=:tags, guid=:guid, hash=:hash, last_modified=NOW(), "
+            "deleted=0";
         if (cmd.has_is_public()) {
             updateQuery += ", is_public=:is_public";
         }
@@ -1399,7 +1554,7 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckShareCreate(const C
         // issues its own query on the same cached statement set.
         QSqlQuery *query =
             sqlInterface->prepareQuery("select id, color_identity from {prefix}_decklist_files where id_folder = "
-                                       ":id_folder and id_user = :id_user");
+                                       ":id_folder and id_user = :id_user and deleted = 0");
         query->bindValue(":id_folder", folderId);
         query->bindValue(":id_user", userInfo->id());
         if (!sqlInterface->execSqlQuery(query)) {

servatrice/src/serversocketinterface.h

@@ -56,6 +56,8 @@ class Command_DeckShareList;
 class Command_DeckShareListMine;
 class Command_DeckShareRemove;
 class Command_DeckShareDownload;
+class Command_DeckHistoryList;
+class Command_DeckSyncCheck;
 class Command_ReplayList;
 class Command_ReplayDownload;
 class Command_ReplayModifyMatch;
@@ -111,6 +113,8 @@ class Command_ViewLogHistory;
 class Command_WarnUser;
 class QObject;
 class ResponseContainer;
+class Response_DeckHistoryList;
+class Response_DeckSyncStatus;
 
 class AbstractServerSocketInterface : public Server_ProtocolHandler
 {
@@ -163,6 +167,9 @@ private:
     bool storeDeckVersion(int deckId, const QString &content, const QString &hash);
     DeckList *getDeckFromDatabase(int deckId);
     Response::ResponseCode cmdDeckDownload(const Command_DeckDownload &cmd, ResponseContainer &rc);
+    bool syncCheckHelper(int userId, qint64 sinceSecs, Response_DeckSyncStatus *status);
+    Response::ResponseCode cmdDeckSyncCheck(const Command_DeckSyncCheck &cmd, ResponseContainer &rc);
+    Response::ResponseCode cmdDeckHistoryList(const Command_DeckHistoryList &cmd, ResponseContainer &rc);
     Response::ResponseCode cmdDeckDownloadPublic(const Command_DeckDownloadPublic &cmd, ResponseContainer &rc);
     Response::ResponseCode cmdDeckShareCreate(const Command_DeckShareCreate &cmd, ResponseContainer &rc);
     Response::ResponseCode cmdDeckShareList(const Command_DeckShareList &cmd, ResponseContainer &rc);

tests/CMakeLists.txt

@@ -416,6 +416,15 @@ cockatrice_add_test(
   libcockatrice_deck_list
   libcockatrice_utility
 )
+cockatrice_add_test(
+  NAME
+  deck_content_hash_test
+  SOURCES
+  deck_content_hash_test.cpp
+  LIBS
+  libcockatrice_deck_list
+  libcockatrice_utility
+)
 cockatrice_add_test(
   NAME
   deck_sync_profile_test

tests/deck_content_hash_test.cpp

@@ -0,0 +1,51 @@
+#include "gtest/gtest.h"
+#include <QString>
+#include <libcockatrice/deck_list/deck_list.h>
+
+namespace
+{
+QString deckWithName(const QString &name)
+{
+    return QStringLiteral(R"(<?xml version="1.0"?><cockatrice_deck version="1">)"
+                          R"(<deckname>%1</deckname><comments></comments>)"
+                          R"(<zone name="main"><card number="4" name="Lightning Bolt"/></zone>)"
+                          R"(<zone name="side"><card number="2" name="Pyroblast"/></zone>)"
+                          R"(</cockatrice_deck>)")
+        .arg(name);
+}
+} // namespace
+
+TEST(DeckContentHashTest, MatchesNativeSerialisation)
+{
+    DeckList deck;
+    ASSERT_TRUE(deck.loadFromString_Native(deckWithName(QStringLiteral("Burn"))));
+    EXPECT_EQ(deck.getContentHash(), DeckList::contentHash(deck.writeToString_Native()));
+}
+
+TEST(DeckContentHashTest, IsStableAcrossLoads)
+{
+    DeckList first;
+    DeckList second;
+    ASSERT_TRUE(first.loadFromString_Native(deckWithName(QStringLiteral("Burn"))));
+    ASSERT_TRUE(second.loadFromString_Native(deckWithName(QStringLiteral("Burn"))));
+    EXPECT_EQ(first.getContentHash(), second.getContentHash());
+}
+
+TEST(DeckContentHashTest, CoversDeckName)
+{
+    DeckList burn;
+    DeckList renamed;
+    ASSERT_TRUE(burn.loadFromString_Native(deckWithName(QStringLiteral("Burn"))));
+    ASSERT_TRUE(renamed.loadFromString_Native(deckWithName(QStringLiteral("Burn v2"))));
+
+    // The card hash only looks at card names and counts, so it cannot tell the
+    // two decks apart; the content hash must.
+    EXPECT_EQ(burn.getDeckHash(), renamed.getDeckHash());
+    EXPECT_NE(burn.getContentHash(), renamed.getContentHash());
+}
+
+int main(int argc, char **argv)
+{
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}

tests/deck_sync_sidecar_test.cpp

@@ -119,6 +119,29 @@ TEST(DeckSyncSidecarTest, RemoveDropsRecord)
     EXPECT_FALSE(reloaded.contains("a.cod"));
 }
 
+TEST(DeckSyncSidecarTest, ModeRoundTripsAndDefaultsToLinked)
+{
+    QTemporaryDir dir;
+    ASSERT_TRUE(dir.isValid());
+    DeckSyncSidecar sidecar(sidecarPath(dir));
+    EXPECT_EQ(sidecar.mode, DeckSyncSidecar::MODE_LINKED);
+    EXPECT_EQ(sidecar.lastSyncSecs, 0);
+    sidecar.mode = DeckSyncSidecar::MODE_SEPARATE;
+    sidecar.lastSyncSecs = 1791739200;
+    EXPECT_TRUE(sidecar.save());
+
+    DeckSyncSidecar reloaded(sidecarPath(dir));
+    EXPECT_TRUE(reloaded.load());
+    EXPECT_EQ(reloaded.mode, DeckSyncSidecar::MODE_SEPARATE);
+    EXPECT_EQ(reloaded.lastSyncSecs, 1791739200);
+}
+
+TEST(DeckSyncSidecarTest, PathForDeckPathBuildsSidecarPath)
+{
+    QString path = DeckSyncSidecar::pathForDeckPath("/decks", "abc123");
+    EXPECT_EQ(path, QString("/decks/.sync/abc123.json"));
+}
+
 int main(int argc, char **argv)
 {
     ::testing::InitGoogleTest(&argc, argv);

tests/settings/settings_defaults_test.cpp

@@ -13,6 +13,7 @@
 #include <libcockatrice/settings/cards_display_settings.h>
 #include <libcockatrice/settings/chat_settings.h>
 #include <libcockatrice/settings/deck_editor_settings.h>
+#include <libcockatrice/settings/deck_sync_settings.h>
 #include <libcockatrice/settings/download_settings.h>
 #include <libcockatrice/settings/game_settings.h>
 #include <libcockatrice/settings/interface_settings.h>
@@ -586,6 +587,64 @@ TEST_F(SettingsDefaultsTest, DeckEditor_VdeStartupTab_SetAndGet)
     ASSERT_EQ(s.getVdeStartupTab(), VdeStartupTabContext);
 }
 
+// --- DeckSyncSettings ---
+
+TEST_F(SettingsDefaultsTest, DeckSync_ConflictDefault_Default)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    ASSERT_EQ(s.getConflictDefault(), DeckSyncConflictAsk);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_ConflictDefault_SetAndGet)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    s.setConflictDefault(DeckSyncConflictKeepNewer);
+    ASSERT_EQ(s.getConflictDefault(), DeckSyncConflictKeepNewer);
+    s.setConflictDefault(DeckSyncConflictKeepMine);
+    ASSERT_EQ(s.getConflictDefault(), DeckSyncConflictKeepMine);
+    s.setConflictDefault(DeckSyncConflictAsk);
+    ASSERT_EQ(s.getConflictDefault(), DeckSyncConflictAsk);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_AutoSync_Default)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    ASSERT_EQ(s.getAutoSync(), true);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_AutoSync_SetAndGet)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    s.setAutoSync(false);
+    ASSERT_EQ(s.getAutoSync(), false);
+    s.setAutoSync(true);
+    ASSERT_EQ(s.getAutoSync(), true);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_SyncIntervalMinutes_DefaultAndBounds)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    ASSERT_EQ(s.getSyncIntervalMinutes(), 5);
+    s.setSyncIntervalMinutes(15);
+    ASSERT_EQ(s.getSyncIntervalMinutes(), 15);
+    s.setSyncIntervalMinutes(0);
+    ASSERT_EQ(s.getSyncIntervalMinutes(), 1);
+    s.setSyncIntervalMinutes(600);
+    ASSERT_EQ(s.getSyncIntervalMinutes(), 60);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_HistoryLimit_DefaultAndBounds)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    ASSERT_EQ(s.getHistoryLimit(), 20);
+    s.setHistoryLimit(50);
+    ASSERT_EQ(s.getHistoryLimit(), 50);
+    s.setHistoryLimit(0);
+    ASSERT_EQ(s.getHistoryLimit(), 1);
+    s.setHistoryLimit(500);
+    ASSERT_EQ(s.getHistoryLimit(), 100);
+}
+
 // --- NetworkSettings ---
 
 TEST_F(SettingsDefaultsTest, Network_ClientID_Default)