[okular] [Bug 505649] New: Okular crashes on shutdown inconsistently if closed immediately after opening a PDF

[Michael Pyne]

https://bugs.kde.org/show_bug.cgi?id=505649

            Bug ID: 505649
           Summary: Okular crashes on shutdown inconsistently if closed
                    immediately after opening a PDF
    Classification: Applications
           Product: okular
      Version First unspecified
       Reported In:
          Platform: Compiled Sources
                OS: Linux
            Status: REPORTED
          Severity: crash
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: mpyne at kde.org
  Target Milestone: ---

Created attachment 182298
  --> https://bugs.kde.org/attachment.cgi?id=182298&action=edit
Okular crash backtrace with debug symbols

SUMMARY

Okular crashes on shutdown for me, seemingly due to a timing issue on
destruction of the U/I shell when lambdas are in use. I have a patch that works
for me but may not be the best way to fix.

STEPS TO REPRODUCE
1.  Open Okular
2.  Open a PDF from the welcome screen
3.  Immediately close Okular by clicking the 'X' on the titlebar.

OBSERVED RESULT
Okular shuts down, but crashes.

kde at midna ~ $ okular
QObject::disconnect: wildcard call disconnects from destroyed signal of
KToggleAction::mouse_toggle_annotate
QObject::disconnect: wildcard call disconnects from destroyed signal of
KToggleAction::show_leftpanel
KCrash: Application 'okular' crashing... crashRecursionCounter = 2
Segmentation fault (core dumped)

EXPECTED RESULT

Okular shuts down with no crash.

SOFTWARE/OS VERSIONS
Okular: Compiled from git: v24.20.85-604-g696bb55ae
Linux: 6.16.0-rc1
KDE Plasma Version: 6.4 dev
KDE Frameworks Version: 6.14.0
Qt Version: 6.8.3 (qt-copy 6.8 branch)

ADDITIONAL INFORMATION

This patch fixes the crash for me.

```diff
diff --git a/shell/shell.cpp b/shell/shell.cpp
index ac9916282..c4beab21e 100644
--- a/shell/shell.cpp
+++ b/shell/shell.cpp
@@ -251,7 +251,7 @@ Shell::Shell(const QString &serializedOptions)
         m_sidebar->setObjectName(QStringLiteral("okular_sidebar"));
         m_sidebar->setContextMenuPolicy(Qt::ActionsContextMenu);
         m_sidebar->setWindowTitle(i18n("Sidebar"));
-        connect(m_sidebar, &QDockWidget::visibilityChanged, this, [this](bool
visible) {
+        connect(m_sidebar, &QDockWidget::visibilityChanged,
m_centralStackedWidget, [this](bool visible) {
             // sync sidebar visibility with the m_showSidebarAction only if
welcome screen is hidden
             if (m_showSidebarAction && m_centralStackedWidget->currentWidget()
!= m_welcomeScreen) {
                 m_showSidebarAction->setChecked(visible);
```

This leads me to believe that the issue is due to the Shell's m_sidebar holding
a pointer to a sibling U/I element (the centralStackedWidget). If during
automatic QObject destruction during shutdown, events happen in this order:

1. the centralStackedWidget is deleted
2. the m_sibling receives the QDockWidget::visibilityChanged signal
3. the m_sibling is deleted

Then the Qt event loop processing won't know to ignore the event's lambda
handler, as the signal/slot connection is keyed to the overarching parent Shell
(which won't be finally deleted until all its children are deleted).

There are probably various other ways to address the issue but this at least
makes the crash go away from me. A better solution is probably to manually
delete all the Shell's U/I children in a specific order in Shell's destructor
rather than relying on Qt's auto-destruction, especially if there are other
possible sibling-to-sibling events involving stored pointers.

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