MThreadPool

Framework for managing threads. More...

Collaboration diagram for MThreadPool:

Classes
class	MThread
	This is a wrapper around QThread that does several additional things. More...
class	MThreadPool

Detailed Description

Framework for managing threads.

Commit 47d67e6722 added two new classes MThread and MThreadPool to be used instead of QThread and QThreadPool. These mirror the API's of the Qt classes but also add a requirement for a QString naming the thread or naming the runnable which are used for debugging. These wrappers also make sure mysql connections are torn down properly and have a number of safety checks and assurances. The destructor for MThread also makes sure that a thread has stopped running before we allow the QThread to be deleted. And MThreaPool makes sure all it's threads have stopped running. MThread has a Cleanup() method that attempts to shut down any threads that are running, it only works for threads that exit on a call to exit(), which includes any threads using the Qt event loop. It will also print out the names of the threads that are still running for debugging purposes.

MThread has one special constructor. It takes a QRunnable* and runs it instead of the Qt event loop in the default run() implementation. Unlike MThreadPool and QThreadPool, MThread does not delete this runnable when it exits. This allows you to run something with less boilerplate code. Just like with QThread in Qt4 it really is better to use the Qt event loop rather than write your own. As you don't use the Qt event loop QObject deleteLater's are not executed until the thread exits, Qt events and timers do not work making Qt networking classes misbehave and signals/slots are generally unsafe. However, many of the threads with event loops in MythTV were written before Qt allowed an event loop outside the main thread so this syntactic sugar just lets us handle that with less lines of code.

There are a few other differences with respect to Qt's classes other than requiring a name.

• MThread does not inherit from QObject, if you need QObject services in a threading class you will need to inherit from QObject separately. Note when you inherit from QObject, you must inherit from no more than one QObject based class and it must be first in the list of classes your class inherits from (a Qt restriction).

• QThread doesn't implement most of QThread's static methods, notably QThread::currentThread(). It does make the underlying QThread available via the qthread() method. But the usual reason to use QThread::currentThread() is to check which thread you are running in. For this there is a set of helper functions is_current_thread(MThread&), is_current_thread(QThread*), and is_current_thread(MThread*) which you can use instead.

• MThreadPool does not have reserveThread() and releaseThread() calls; instead use startReserved(). The reserveThread() and releaseThread() API is not a good API because it doesn't ensure that your runnable starts right away even though the thread will eventually not be counted amoung those forcing other threads to go to the queue rather than running right away.