fix: avoid xdg_activation_v1 destroy on a torn-down display at exit

[yixinshark]

Summary

• DDEIntegration::XdgActivationV1 was a function-local static (Meyers singleton). Its destructor runs at __cxa_finalize — after main() returns and ~QGuiApplication has already torn down the Wayland connection — so xdg_activation_v1::destroy() marshals a request onto an already-freed wl_proxy/wl_display and crashes (SIGSEGV in wl_map_insert_at).
• Triggered on normal process exit, notably when the session exits during a VT/TTY switch.
• Fix: instance() now heap-allocates the object and parents it to qApp (held via a non-owning QPointer) so it is destroyed within Qt's shutdown sequence, not at static-destruction time. The destructor is additionally guarded with qApp (which is already null when qApp deletes its children), so destroy() is never marshalled onto a dead wl_display.

Crash backtrace (before fix)

#0 wl_map_insert_at (data=0x0)
#1 proxy_destroy
#3 wl_proxy_marshal_array_flags
#4 wl_proxy_marshal_flags
#5 QtWayland::xdg_activation_v1::destroy()
#6 DDEIntegration::XdgActivationV1::~XdgActivationV1   (launchpadcommon.so, src/ddeintegration/xdgactivation.cpp)
#7 __cxa_finalize / exit

Test plan

• Run on a treeland/Wayland session; trigger an app-launch activation so the singleton is initialized.
• Switch VT to a text console and back (or otherwise cause the desktop dde-shell process to exit); verify no dde-shell coredump in coredumpctl with the xdg_activation_v1::destroy frame.

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[deepin-ci-robot]

deepin pr auto review

这段代码是对XdgActivationV1类的改进，主要关注了单例模式的生命周期管理和Qt应用程序退出时的安全性。以下是我的审查意见：

语法逻辑

1. 语法正确，逻辑清晰。代码使用了QPointer来管理单例实例，这是一个很好的实践，可以避免在Qt对象被删除后访问无效指针。
2. 单例模式的实现方式更加安全，通过将单例对象设置为QCoreApplication的子对象，确保其与Qt应用程序的生命周期保持一致。

代码质量

1. 代码质量有明显提升。原来的静态变量在程序退出时可能引发问题，因为Qt的事件循环和Wayland连接可能已经销毁。
2. 添加了必要的注释，解释了为什么使用QPointer而不是普通的静态指针，以及为什么在析构函数中检查qApp的存在性。

代码性能

1. 性能方面没有明显问题。QPointer的开销很小，主要是增加了额外的指针检查。
2. 单例的懒加载模式（第一次使用时创建）是合理的，避免了不必要的资源占用。

代码安全

1. 安全性显著提高。原来的实现可能在Qt应用程序退出后访问已销毁的Wayland资源，导致崩溃。
2. 通过检查qApp的存在性，确保在Qt应用程序已经销毁的情况下不会尝试销毁Wayland资源。
3. 使用QPointer而不是原始指针，可以防止在对象被删除后访问无效指针。

改进建议

1. 可以考虑在XdgActivationV1类中添加一个标志位，标记单例是否已经被销毁，以防止在极端情况下重复创建或访问已销毁的实例。
2. 可以考虑使用Qt的Q_DISABLE_COPY宏来禁用拷贝构造和赋值操作，确保单例的唯一性。
3. 在XdgActivationV1的构造函数中，可以添加更多的错误检查，确保在初始化失败时能够正确处理。

总体而言，这次改进很好地解决了原代码中可能存在的生命周期管理问题，提高了代码的健壮性和安全性。

[deepin-ci-robot]

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