Merge bitcoin-core/gui#336: Do not exit and re-enter main event loop during shutdown

451ca244db qt, refactor: Drop intermediate BitcoinApplication::shutdownResult slot (Hennadii Stepanov)
f3a17bbe5f qt: Do not exit and re-enter main event loop during shutdown (Hennadii Stepanov)
b4e0d2c431 qt, refactor: Allocate SendConfirmationDialog instances on heap (Hennadii Stepanov)
332dea2852 qt, refactor: Keep HelpMessageDialog in the main event loop (Hennadii Stepanov)
c8bae37a7a qt, refactor: Keep PSBTOperationsDialog in the main event loop (Hennadii Stepanov)
7fa91e8312 qt, refactor: Keep AskPassphraseDialog in the main event loop (Hennadii Stepanov)
6f6fde30e7 qt, refactor: Keep EditAddressDialog in the main event loop (Hennadii Stepanov)
59f7ba4fd7 qt, refactor: Keep CoinControlDialog in the main event loop (Hennadii Stepanov)
7830cd0b35 qt, refactor: Keep OptionsDialog in the main event loop (Hennadii Stepanov)
13f618818d qt: Add GUIUtil::ShowModalDialogAndDeleteOnClose (Hennadii Stepanov)

Pull request description:

  On master (1ef34ee25e) during shutdown `QApplication` exits the main event loop, then re-enter again.

  This PR streamlines shutdown process by removing the need to interrupt the main event loop, that is required for #59.

  Also, blocking [`QDialog::exec()`](https://doc.qt.io/qt-5/qdialog.html#exec) calls are replaced with safer [`QDialog::show()`](https://doc.qt.io/qt-5/qwidget.html#show), except for `SendConfirmationDialog` as that change is not trivial (marked as TODO).

  The [`QDialog::open()`](https://doc.qt.io/qt-5/qdialog.html#open) was not used because the actual modality mode (application modal or window modal) of a dialog depends on whether it has a parent.

  This PR does not change behavior, and all touched dialogs are still application modal.
  As a follow up, a design research could suggest to make some dialogs window modal.

  NOTE for reviewers: quitting app while a dialog is open (e.g., via systray icon menu) must work fine.

ACKs for top commit:
  laanwj:
    Code review and lighly tested ACK 451ca244db
  promag:
    ACK 451ca244db, just changed signal to `quitRequested`.

Tree-SHA512: ef01ab6ed803b202e776019a4e1f592e816f7bc786e00574b25a0bf16be2374ddf9db21f0a26da08700df7ef0ab9e879550df46dcfe3b6d940f5ed02ca5f8447

src/qt/bitcoin.cpp

@@ -55,6 +55,7 @@
 #include <QThread>
 #include <QTimer>
 #include <QTranslator>
+#include <QWindow>
 
 #if defined(QT_STATICPLUGIN)
 #include <QtPlugin>
@@ -259,6 +260,7 @@ void BitcoinApplication::createOptionsModel(bool resetSettings)
 void BitcoinApplication::createWindow(const NetworkStyle *networkStyle)
 {
     window = new BitcoinGUI(node(), platformStyle, networkStyle, nullptr);
+    connect(window, &BitcoinGUI::quitRequested, this, &BitcoinApplication::requestShutdown);
 
     pollShutdownTimer = new QTimer(window);
     connect(pollShutdownTimer, &QTimer::timeout, window, &BitcoinGUI::detectShutdown);
@@ -296,7 +298,7 @@ void BitcoinApplication::startThread()
 
     /*  communication to and from thread */
     connect(&m_executor.value(), &InitExecutor::initializeResult, this, &BitcoinApplication::initializeResult);
-    connect(&m_executor.value(), &InitExecutor::shutdownResult, this, &BitcoinApplication::shutdownResult);
+    connect(&m_executor.value(), &InitExecutor::shutdownResult, this, &QCoreApplication::quit);
     connect(&m_executor.value(), &InitExecutor::runawayException, this, &BitcoinApplication::handleRunawayException);
     connect(this, &BitcoinApplication::requestedInitialize, &m_executor.value(), &InitExecutor::initialize);
     connect(this, &BitcoinApplication::requestedShutdown, &m_executor.value(), &InitExecutor::shutdown);
@@ -326,13 +328,17 @@ void BitcoinApplication::requestInitialize()
 
 void BitcoinApplication::requestShutdown()
 {
+    for (const auto w : QGuiApplication::topLevelWindows()) {
+        w->hide();
+    }
+
     // Show a simple window indicating shutdown status
     // Do this first as some of the steps may take some time below,
     // for example the RPC console may still be executing a command.
     shutdownWindow.reset(ShutdownWindow::showShutdownWindow(window));
 
     qDebug() << __func__ << ": Requesting shutdown";
-    window->hide();
+
     // Must disconnect node signals otherwise current thread can deadlock since
     // no event loop is running.
     window->unsubscribeFromCoreSignals();
@@ -409,15 +415,10 @@ void BitcoinApplication::initializeResult(bool success, interfaces::BlockAndHead
         pollShutdownTimer->start(200);
     } else {
         Q_EMIT splashFinished(); // Make sure splash screen doesn't stick around during shutdown
-        quit(); // Exit first main loop invocation
+        requestShutdown();
     }
 }
 
-void BitcoinApplication::shutdownResult()
-{
-    quit(); // Exit second main loop invocation after shutdown finished
-}
-
 void BitcoinApplication::handleRunawayException(const QString &message)
 {
     QMessageBox::critical(
@@ -639,8 +640,6 @@ int GuiMain(int argc, char* argv[])
 #if defined(Q_OS_WIN)
             WinShutdownMonitor::registerShutdownBlockReason(QObject::tr("%1 didn't yet exit safely…").arg(PACKAGE_NAME), (HWND)app.getMainWinId());
 #endif
-            app.exec();
-            app.requestShutdown();
             app.exec();
             rv = app.getReturnValue();
         } else {