Race between editing and saving

[David Faure]

I've spent some more time debugging the problems when editing the description 
for a task, and losing characters. I analyzed the issue, but I need help 
choosing a solution.

Here's what happens:

1) User types text, ArtifactEditorModel's m_text is updated.
2) Later on (timer-based), the model saves => Artifact::setText() + async job.
3) Later on (when akonadi notifies of the item having changed),
Akonadi::Serializer::updateTaskFromItem calls Domain::Artifact::setText
again.

One issue is that at that point trailing whitespace was removed (due to 
libical), that's easy to fix in setText with 
 if (m_text.trimmed() == text.trimmed())
        return;
but that only fixes the loss of whitespace, not the race described below.

What can happen is that after step 2 and before step 3, the user has time
to type a bit more text. This change gets lost at step 3. Good news: this is
also easy to fix, if the save timer didn't kick in again yet. Like this:
void ArtifactEditorModel::onTextChanged(const QString &text)
{
    if (isSaveNeeded()) // we have pending changes, don't lose them
        return;

But what if the save timer did kick in again already? To recap:
1, 2, 1b, 2b, 3
so we have two async jobs running, and isSaveNeeded() is false again.
When the first "item changed" notification comes in, the above fix doesn't help, 
and the text editor shows again the OLD text, losing the changes between 1 and 
1b.

I'm writing here instead of sending a patch because I don't know how to fix 
that, with the current akonadi and zanshin designs.

* EditorView not saving again until the current save job has finished?
  (sounds very dangerous, if akonadi is stuck and we quit zanshin, or worse, 
the computer crashes, we would lose the whole set of pending changes)

* Ignoring notifications of changes triggered by zanshin itself? (how?)

* Ignoring outdated notifications? (how? I tried printing out revision() and it 
increases, but when saving we don't know the revision).

* Or even blunter: ignoring all notifications of item changes sent by akonadi?
After all, if I *ever* edit a task in korganizer and then switch back to 
zanshin, I can very well understand that I have to switch tasks and back to 
see my change. This would be a *much* smaller problem than a text editor that 
loses characters because it's saving and reloading (via dbus ICP and ical 
serialization/deserialization) regularly.

* Any other idea?

PS: to reproduce the race more easily I changed 
ArtifactEditorModel::autoSaveDelay() from 500 to 50, but it also happens 
sometimes with 500, all it takes is for akonadi to be a bit busy.

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE Frameworks 5