Fix race in dark mode detection (#13366)

The current dark mode detection implementation is prone to a race
between an asynchronous D-Bus method call in NixUtils::NixUtils() and
bootstrap logic in Bootstrap::bootstrap().  This race condition causes
dark mode detection to fail intermittently during application startup.

The sequence of events in the failing case is as follows:

 1. In the Application constructor, osUtils is instantiated, causing
    the NixUtils constructor to run.

 2. An asynchronous D-Bus method call is issued to query the value of
    the org.freedesktop.appearance.color-scheme key.

 3. Bootstrap code runs.  In particular, a prctl() system call is
    issued to set the keepassxc process "dumpable" attribute to 0.

 4. xdg-desktop-portal begins handling the D-Bus method call initiated
    in step 2.  Flatpak detection logic is invoked, triggering an
    attempt to access /proc/<pid>/root, where <pid> is the PID of the
    keepassxc process.

 5. Since the keepassxc process had its "dumpable" attribute set to 0
    in step 3, its /proc/<pid>/root entry becomes owned by root.  This
    causes the access attempt from xdg-desktop-portal to fail, which in
    turn causes the D-Bus method call to fail as well.  As a result, the
    registered callback (NixUtils::handleColorSchemeRead()) is never
    invoked, effectively breaking dark mode detection.

If xdg-desktop-portal completes its permission checks before the
prctl() system call is issued (that is, if the order of steps 3 and 4
is reversed), everything works correctly.

Fix by replacing the asynchronous D-Bus method call with a synchronous
one, preventing bootstrap code from executing while the D-Bus call is
still pending.

* Use ReadOne D-Bus method instead of Read

The org.freedesktop.portal.Settings.Read method is deprecated.  Replace
its use with org.freedesktop.portal.Settings.ReadOne and adjust reply
handling accordingly.

Remove NixUtils::handleColorSchemeRead(), which is no longer needed.

Author: kempniu

src/gui/osutils/nixutils/NixUtils.cpp

@@ -23,6 +23,7 @@
 
 #include <QApplication>
 #include <QDBusInterface>
+#include <QDBusReply>
 #include <QDebug>
 #include <QDir>
 #include <QPointer>
@@ -81,10 +82,12 @@ NixUtils::NixUtils(QObject* parent)
                        this,
                        SLOT(handleColorSchemeChanged(QString, QString, QDBusVariant)));
 
-    QDBusMessage msg = QDBusMessage::createMethodCall(
-        "org.freedesktop.portal.Desktop", "/org/freedesktop/portal/desktop", "org.freedesktop.portal.Settings", "Read");
-    msg << QVariant("org.freedesktop.appearance") << QVariant("color-scheme");
-    sessionBus.callWithCallback(msg, this, SLOT(handleColorSchemeRead(QDBusVariant)));
+    QDBusInterface desktopPortal(
+        "org.freedesktop.portal.Desktop", "/org/freedesktop/portal/desktop", "org.freedesktop.portal.Settings");
+    QDBusReply<QDBusVariant> reply = desktopPortal.call("ReadOne", "org.freedesktop.appearance", "color-scheme");
+    if (reply.isValid()) {
+        setColorScheme(reply.value());
+    }
 }
 
 NixUtils::~NixUtils() = default;
@@ -359,12 +362,6 @@ bool NixUtils::unregisterGlobalShortcut(const QString& name)
     return true;
 }
 
-void NixUtils::handleColorSchemeRead(QDBusVariant value)
-{
-    value = qvariant_cast<QDBusVariant>(value.variant());
-    setColorScheme(value);
-}
-
 void NixUtils::handleColorSchemeChanged(QString ns, QString key, QDBusVariant value)
 {
     if (ns == "org.freedesktop.appearance" && key == "color-scheme") {

src/gui/osutils/nixutils/NixUtils.h

@@ -53,7 +53,6 @@ class NixUtils : public OSUtilsBase, QAbstractNativeEventFilter
     quint64 getProcessStartTime() const;
 
 private slots:
-    void handleColorSchemeRead(QDBusVariant value);
     void handleColorSchemeChanged(QString ns, QString key, QDBusVariant value);
     void launchAtStartupRequested(uint response, const QVariantMap& results);