gui: Make polling in ClientModel asynchronous

Summary:
> After #14193 ClientModel::updateTimer can take some time, as such the GUI hangs, like #17112.
>
> Fixes this by polling in a background thread and updating the GUI asynchronously.

This is a backport of Core [[bitcoin/bitcoin#17135 | PR17135]] and Core [[bitcoin/bitcoin#17427 | PR17427]]

The second PR fixes a bug introduced by the first (`ClientModel::mempoolSizeChanged() signal has unregistered parameter type size_t`)

Test Plan: `ninja && ninja check && src/qt/bitcoin-qt`

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8193

src/qt/bitcoin.cpp

@@ -541,15 +541,19 @@ int GuiMain(int argc, char *argv[]) {
 
     BitcoinApplication app(*node);
 
-    // Register meta types used for QMetaObject::invokeMethod
+    // Register meta types used for QMetaObject::invokeMethod and
+    // Qt::QueuedConnection
     qRegisterMetaType<bool *>();
 #ifdef ENABLE_WALLET
     qRegisterMetaType<WalletModel *>();
 #endif
-    //   Need to pass name here as Amount is a typedef (see
-    //   http://qt-project.org/doc/qt-5/qmetatype.html#qRegisterMetaType)
-    //   IMPORTANT if it is no longer a typedef use the normal variant above
+    // Register typedefs (see
+    // http://qt-project.org/doc/qt-5/qmetatype.html#qRegisterMetaType)
+    // IMPORTANT: if Amount is no longer a typedef use the normal variant above
+    // (see https://doc.qt.io/qt-5/qmetatype.html#qRegisterMetaType-1)
     qRegisterMetaType<Amount>("Amount");
+    qRegisterMetaType<size_t>("size_t");
+
     qRegisterMetaType<std::function<void()>>("std::function<void()>");
     qRegisterMetaType<QMessageBox::Icon>("QMessageBox::Icon");
 

src/qt/clientmodel.cpp

@@ -18,6 +18,7 @@
 #include <util/system.h>
 
 #include <QDebug>
+#include <QThread>
 #include <QTimer>
 
 #include <cstdint>
@@ -28,20 +29,37 @@ static int64_t nLastBlockTipUpdateNotification = 0;
 ClientModel::ClientModel(interfaces::Node &node, OptionsModel *_optionsModel,
                          QObject *parent)
     : QObject(parent), m_node(node), optionsModel(_optionsModel),
-      peerTableModel(nullptr), banTableModel(nullptr), pollTimer(nullptr) {
+      peerTableModel(nullptr), banTableModel(nullptr),
+      m_thread(new QThread(this)) {
     cachedBestHeaderHeight = -1;
     cachedBestHeaderTime = -1;
     peerTableModel = new PeerTableModel(m_node, this);
     banTableModel = new BanTableModel(m_node, this);
-    pollTimer = new QTimer(this);
-    connect(pollTimer, &QTimer::timeout, this, &ClientModel::updateTimer);
-    pollTimer->start(MODEL_UPDATE_DELAY);
+
+    QTimer *timer = new QTimer;
+    timer->setInterval(MODEL_UPDATE_DELAY);
+    connect(timer, &QTimer::timeout, [this] {
+        // no locking required at this point
+        // the following calls will acquire the required lock
+        Q_EMIT mempoolSizeChanged(m_node.getMempoolSize(),
+                                  m_node.getMempoolDynamicUsage());
+        Q_EMIT bytesChanged(m_node.getTotalBytesRecv(),
+                            m_node.getTotalBytesSent());
+    });
+    connect(m_thread, &QThread::finished, timer, &QObject::deleteLater);
+    connect(m_thread, &QThread::started, [timer] { timer->start(); });
+    // move timer to thread so that polling doesn't disturb main event loop
+    timer->moveToThread(m_thread);
+    m_thread->start();
 
     subscribeToCoreSignals();
 }
 
 ClientModel::~ClientModel() {
     unsubscribeFromCoreSignals();
+
+    m_thread->quit();
+    m_thread->wait();
 }
 
 int ClientModel::getNumConnections(NumConnections flags) const {
@@ -84,14 +102,6 @@ int64_t ClientModel::getHeaderTipTime() const {
     return cachedBestHeaderTime;
 }
 
-void ClientModel::updateTimer() {
-    // no locking required at this point
-    // the following calls will acquire the required lock
-    Q_EMIT mempoolSizeChanged(m_node.getMempoolSize(),
-                              m_node.getMempoolDynamicUsage());
-    Q_EMIT bytesChanged(m_node.getTotalBytesRecv(), m_node.getTotalBytesSent());
-}
-
 void ClientModel::updateNumConnections(int numConnections) {
     Q_EMIT numConnectionsChanged(numConnections);
 }

src/qt/clientmodel.h

@@ -87,7 +87,8 @@ class ClientModel : public QObject {
     PeerTableModel *peerTableModel;
     BanTableModel *banTableModel;
 
-    QTimer *pollTimer;
+    //! A thread to interact with m_node asynchronously
+    QThread *const m_thread;
 
     void subscribeToCoreSignals();
     void unsubscribeFromCoreSignals();
@@ -109,7 +110,6 @@ class ClientModel : public QObject {
     void showProgress(const QString &title, int nProgress);
 
 public Q_SLOTS:
-    void updateTimer();
     void updateNumConnections(int numConnections);
     void updateNetworkActive(bool networkActive);
     void updateAlert();