GSoC Status Update – Week 3

This is a status update for my Google Summer of Code 2013 project - implementing advanced statistics importers for Amarok. Please read the first post if you would like to know more about the project.

For the previous week I had scheduled "Reimplementing FastForward importer, cleaning up data-reading module. At the end of the week I expect to have working implementation of FastForwardProvider." That's done now, and I can't say it took a lot of coding.

It took a fair amount of thinking, though. Classes that inherit StatSyncing::Provider and StatSyncing::Track have to fulfill certain conditions for working in multi-threaded environment. In short, the former must be reentrant, and the latter thread-safe.

Surprisingly, there are various definitions of reentrancy and thread-safety. The Qt's one basically boils down to this: a class is reentrant when different threads can safely work on different objects, and thread-safe when they can work on the same object. [Wikipedia disagrees]Wikipedia reentrancy. In it's definition, a method is reentrant when it can be interrupted in the middle and started again. In that sense, the function in the following example (courtesy of Wikipedia) is thread- safe, but not reentrant; an invocation interrupted in the function body would leave the mutex locked, and any subsequent invocation would hang on mutex_lock().

int function()
    // ...
    function body
    // ...

But leaving this small tangent aside, that wasn't the only thing I had to think through. I had a small case of premature optimization going on: I didn't want to open a new SQL connection for every request (Amarok stores it's main collection's data in SQL database). It sounds simple enough to do, but coupled with the fact that "a connection can only be used from within the thread that created it", and multithreaded nature of providers, this was an interesting problem to get right. So I rediscovered what every other Qt developer already knew: if you need something to run in a specific thread, use Signals & Slots mechanism. With Qt::BlockingQueuedConnection and using emit foo() instead of slotFoo(), we basically get an effect of executing a function - synchronously - in a different thread, and with an added benefit of not having to bother with synchronization ourselves.

It gets a little bit more awkward when both caller and callee can happen to be in the same thread, as Qt::BlockingQueuedConnection blocks caller's thread until called slot is done:

    connect( this, SIGNAL(retrieveAllData()), SLOT(slotRetrieveAllData()), Qt::BlockingQueuedConnection );
    // ...
    if( thread() == QCoreApplication::instance()->thread() )
        emit retrieveAllData();

But hey, the pros outweigh the cons, and it's what matters. And after I was done with FastForwardProvider and FastForwardTrack, I decided to add some lazy initialization to the mix...

So, to wrap this up. Aside from multi-threaded fun and figuring out the right file system hierarchy, I also spent a lot of time thinking about the bigger picture, that is how and when importers would actually load up the configuration and register themselves with right controllers. It's something I initially overlooked in my proposal, and something I'll try to address this week, possibly skewing my schedule a little bit. More on this topic coming next week.

Oh, and I'm not done with FastForwardProvider, not really. It's more of a prototype. As more providers come to life, they will see a lot of changes aiming at deduplication and easing the creation of new ones.

Thanks for reading!