Don't quit automatically via QEventLoopLocker if there are open windows

As part of df359bcb703db5a8adbf14e88ba4ae0d54f0cfcd the semantics and
interaction between QEventLoopLocker and QGuiApplication was changed,
based on the assumption that these two mechanisms were independent
and should not affect each other.

This had a surprising regression where the use of QEventLoopLocker in
combination with the QCoreApplication::isQuitLockEnabled() automatic
quit would end up quitting the app, even if it had open windows, for
example when the last job of some internal job queue finished.

It could be argued that if the app has open windows that should not
be closed, they should ignore the Close event, and that an application
with running QEventLoopLocker jobs should maintain an active window
showing the progress of those jobs, but still, this is regression
that we want to fix.

We now bail out if !lastWindowClosed() in QGuiApplication's
canQuitAutomatically, which is triggered from QEventLoopLocker's
isQuitLockEnabled() behavior. And we do so regardless of whether
quitOnLastWindowClosed is set or not, as the latter property
determines the behavior when closing a window, not the behavior
when a QEventLoopLocker goes out of scope.

Similarly, we now block quitting of the application when triggered
by quitOnLastWindowClosed() if a QEventLoop is active, regardless of
the isQuitLockEnabled(), as the latter property is determining
whether we should trigger a quit, not whether we should block them.

[ChangeLog][Important behavior changes] Fixed a regression where
the last QEventLoopLocker going out of scope would quit the app,
even if there were open windows, if quitOnLastWindowClosed was
false.

[ChangeLog][Important behavior changes] Fixed a regression where
closing the last window would quit the app, even if there were
active QEventLoopLockers, if isQuitLockEnabled was false.

Fixes: QTBUG-124386
Pick-to: 6.7 6.5
Change-Id: I84fd0ddea78a2f417f3a17b326113c880079cf85
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

2 files changed, 20 insertions, 2 deletions

diff --git a/src/corelib/kernel/qcoreapplication.cpp b/src/corelib/kernel/qcoreapplication.cpp
index 13108cecea..bd00f69c1c 100644
--- a/src/corelib/kernel/qcoreapplication.cpp
+++ b/src/corelib/kernel/qcoreapplication.cpp
@@ -1091,6 +1091,14 @@ bool QCoreApplication::testAttribute(Qt::ApplicationAttribute attribute)
     \brief Whether the use of the QEventLoopLocker feature can cause the
     application to quit.
 
+    When this property is \c true the release of the last remaining
+    QEventLoopLocker operating on the application will attempt to
+    quit the application.
+
+    Note that attempting a quit may not necessarily result in the
+    application quitting, for example if there still are open windows,
+    or the QEvent::Quit event is ignored.
+
     The default is \c true.
 
     \sa QEventLoopLocker
@@ -2094,7 +2102,13 @@ bool QCoreApplicationPrivate::canQuitAutomatically()
     if (!in_exec)
         return false;
 
-    if (quitLockEnabled && quitLockRef.loadRelaxed())
+    // The automatic quit functionality is triggered by
+    // both QEventLoopLocker and maybeLastWindowClosed.
+    // In either case, we don't want to quit if there
+    // are active QEventLoopLockers, even if quitLockEnabled
+    // is not enabled, as the property signals whether to
+    // trigger the automatic quit, not whether to block it.
+    if (quitLockRef.loadRelaxed())
         return false;
 
     return true;
diff --git a/src/corelib/kernel/qeventloop.cpp b/src/corelib/kernel/qeventloop.cpp
index d318069ca0..e314a17ff8 100644
--- a/src/corelib/kernel/qeventloop.cpp
+++ b/src/corelib/kernel/qeventloop.cpp
@@ -346,7 +346,11 @@ static_assert(alignof(QCoreApplication) >= 4);
 /*!
     Creates an event locker operating on the QCoreApplication.
 
-    The application will quit when there are no more QEventLoopLockers operating on it.
+    The application will attempt to quit when there are no more QEventLoopLockers
+    operating on it, as long as QCoreApplication::isQuitLockEnabled() is \c true.
+
+    Note that attempting a quit may not necessarily result in the application quitting,
+    if there for example are open windows, or the QEvent::Quit event is ignored.
 
     \sa QCoreApplication::quit(), QCoreApplication::isQuitLockEnabled()
  */