print a warning when invoking the scripting API's sleep()

Summary:
the sleep is implemented with a nested event loop which may result in
segfaults when something in the new loop stack accesses objects from the
original loop stack that are presently in a bad state because they were
not meant to be accessed (and only can because of the nesting).

to aid in debugging let's leave a trace of the fact that sleep was used
when in fact, using sleep is super risky.

Test Plan: warning gets printed when sleeping

Reviewers: mart

Reviewed By: mart

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21895

shell/scripting/appinterface.cpp

@@ -159,6 +159,13 @@ bool AppInterface::coronaLocked() const
 
 void AppInterface::sleep(int ms)
 {
+    // The nested eventloop here may cause crashes when for example loading a
+    // LnF because the script runs out of the QML stack. Opening an eventloop
+    // there and processing QML events may result in touching members objects
+    // that were meant to be deleted but haven't yet, because the parent event
+    // loop hasn't returned yet.
+    qWarning() << "Scripted sleeping starts a nested event loop." \
+                  " This can cause process instability. Use with care!";
     QEventLoop loop;
     QTimer::singleShot(ms, &loop, &QEventLoop::quit);
     loop.exec(QEventLoop::ExcludeUserInputEvents);