[Client] Only quit on update when the shutdown actually ran

MainWindow::closeEvent has a static re-entrancy guard that returns early on a
second close event, leaving it in its default accepted state. DlgUpdate reached
from a nested event loop while a shutdown prompt was up could then get close()
== true with no shutdown work done (no settings flush, no tab shutdown) and tear
down the process behind an unanswered prompt.

Add MainWindow::closeForUpdate(), which reports false when a close is already in
progress or was vetoed, and gate the update-exit on it.

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

@@ -246,10 +246,12 @@ void DlgUpdate::downloadSuccessful(const QUrl &filepath)
         // Close the main window synchronously so file locks are released before the NSIS installer
         // (already launched) starts replacing files. This also flushes settings and shuts down the
         // tabs, but only when the close is actually accepted: MainWindow may veto it for a running
-        // card DB update, an open game, or an unsaved deck. In that case keep running so the user
-        // can resolve the blocker, and tell them the installer is already waiting.
+        // card DB update, an open game, or an unsaved deck, and closeForUpdate() also reports a
+        // close already in progress (reached from a nested event loop while a shutdown prompt is
+        // up). Only quit when the shutdown really ran - otherwise keep running so the user can
+        // resolve the blocker, and tell them the installer is already waiting.
         if (auto *window = qobject_cast<MainWindow *>(parent())) {
-            if (window->close()) {
+            if (window->closeForUpdate()) {
                 QTimer::singleShot(0, qApp, &QCoreApplication::quit);
             } else {
                 QMessageBox::warning(this, tr("Update"),

cockatrice/src/interface/window_main.cpp

@@ -851,7 +851,6 @@ void MainWindow::actShow()
 void MainWindow::closeEvent(QCloseEvent *event)
 {
     // workaround Qt bug where closeEvent gets called twice
-    static bool bClosingDown = false;
     if (bClosingDown) {
         return;
     }
@@ -880,6 +879,18 @@ void MainWindow::closeEvent(QCloseEvent *event)
     tabSupervisor->deleteLater();
 }
 
+bool MainWindow::closeForUpdate()
+{
+    // A shutdown is already being handled (e.g. this is reached from a nested event loop while
+    // closeEvent() is blocked on a user prompt). close() would hit the re-entrancy guard and return
+    // true without any shutdown having happened, so report faithfully instead: the caller must keep
+    // the process alive until the user resolves whatever is blocking the close.
+    if (bClosingDown) {
+        return false;
+    }
+    return close();
+}
+
 void MainWindow::changeEvent(QEvent *event)
 {
     if (event->type() == QEvent::LanguageChange) {

cockatrice/src/interface/window_main.h

@@ -167,6 +167,7 @@ private:
     LagMonitor lagMonitor;                        ///< watches the main thread for event loop stalls
     LatencyStatusWidget *latencyStatus = nullptr; ///< status bar widget with live round-trip stats and history graph
     bool bHasActivated, askedForDbUpdater;
+    bool bClosingDown = false; ///< guards closeEvent() against re-entrancy
     bool skipStartupAutoConnect = false;
     bool startupAutoConnectAttempted = false;
     QProcess *cardUpdateProcess;
@@ -204,6 +205,13 @@ public:
         return tabSupervisor;
     }
 
+    /**
+     * @brief Closes the window so an update installer can replace the running binaries.
+     *        Returns true only if the shutdown actually ran (settings flushed, tabs shut down);
+     *        false if the close was vetoed by the user or is already in progress.
+     */
+    bool closeForUpdate();
+
 protected:
     void closeEvent(QCloseEvent *event) override;
     void changeEvent(QEvent *event) override;