[Interest] QAbstractEventDispatcher - registerSocketNotifier(), unregisterSocketNotifier()

Narolewski Jakub izowiuz at gmail.com
Sun Jan 19 17:57:55 CET 2020

I finally had some time to really look at this and even very basic
implementation works great in my use case.
Adding just functions enabling and disabling socket notifiers allows me to
have finer control over when the stuff on my end gets deleted.
There are still some crashes that I have to hunt down. Mostly around of
line 'QObjectPrivate *d = receiver->d_func()' from:

// Qt code -- >

// qcoreapplication.cpp, from line 1083 as of Qt 5.14

// Qt enforces the rule that events can only be sent to objects in
// the current thread, so receiver->d_func()->threadData is
// equivalent to QThreadData::current(), just without the function
// call overhead.
QObjectPrivate *d = receiver->d_func();
QThreadData *threadData = d->threadData;
QScopedScopeLevelCounter scopeLevelCounter(threadData);
if (!selfRequired)
    return doNotify(receiver, event);
return self->notify(receiver, event);

// < -- Qt code

but it might be just some error on my end.

One more question I have is about potential optimizations concerning
bookkeeping relations between QSocketNotifier and 'foreign' part on the
side of QAbstractEventDispatcher.
>From what I saw, currently subclasses of QAED tend to keep relations
between socket notifiers and some 'system' counterpart in indexes - QHashes
for example.
Do you think it would be possible to implement an interface allowing client
to inject some additional information directly into the QSocketNotifier to
skip 'finding' it in QAED?
Maybe something similar to 'std::any attachedData' directly in
QSocketNotifier being QObject can of course be \'setPropertied\' to hold
some external data but, to be honest, I never actually benchmarked
performance of this.

pon., 30 gru 2019 o 22:00 Thiago Macieira <thiago.macieira at intel.com>

> On Monday, 30 December 2019 15:31:14 -03 Narolewski Jakub wrote:
> > I did not notice if unregisterTimer() and registerTimer() are also called
> > so often but their non-destructive switching could also come in handy.
> > Do you think that changing current QSocketNotifier::setEnabled( bool )
> > behaviour to actually enable / disable notifiers is a good idea?
> I don't see why not. In fact, I'm positive it is a good idea. In all
> implementations, there's a cost associated with registering a notifier. So
> the
> worst case scenario is that in an implementation it's impossible to
> disable,
> forcing us to fall back to deregistering with the low-level
> implementation.
> That's exactly what we do today, so it's no worse. We do need some
> bookkeeing
> so we can re-register on setSocketDescriptorEnabled(), but I guess that
> it's
> the bookkeeping that's already there.
> The drawback is of the increased memory consumption by QSocketNotifiers
> that
> stay mostly disabled. That is the case for the write notifiers for all
> sockets, which are only enabled when they have something to write. I think
> that's an acceptable trade-off.
> > I don't know much about Qt at this level or how much of current code
> > depends on them actually registering and deregistering.
> >
> > Also, does QSocketNotifier 'know' when it is supposed to release
> underlying
> > socket?
> Today, on setEnabled(false) or destruction. If we implement the new
> feature,
> only on destruction.
> > Could registering and deregistering be moved entirely to its constructor
> > and destructor?
> That's what I hope.
> > I don't know how QAbstractEventDispatcher interops with QTimer, I will
> try
> > to research it a bit more.
> It's basically the same.
> The one thing to look after is that QAbstractSocket has a shortcut to skip
> some of the signal-slot mechanisms of QSocketNotifier and be notified
> immediately by way of a virtual. So there's a second place to deal with
> registering/unregistering, aside from QSocketNotifier.
> --
> Thiago Macieira - thiago.macieira (AT) intel.com
>   Software Architect - Intel System Software Products
> _______________________________________________
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> Interest at qt-project.org
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