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107 lines
3.5 KiB
107 lines
3.5 KiB
Threads in anaconda? No!
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David Cantrell <dcantrell@redhat.com>
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INTRODUCTION
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Threads make a lot of people run screaming. That's entirely
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understandable because thread concurrency can be a pain. In this short
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document, I want to explain why threads are in anaconda and how to work
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with them in the code.
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MISCONCEPTIONS
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Just to make sure everyone is on the same page, threads are similar to
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processes. The big advantage we get is easier shared data structures.
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Threads can communicate over more methods than just signals. But,
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multithreaded does not mean that we are taking every operation out to
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a separate thread.
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ANACONDA THREADS
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The idea for anaconda threads is to simplify life for things that can
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or need to run parallel to other operations. So we will reserve the
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use of threads for tasks that fit in to this category well (the logging
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system, for instance) and keep the bulk of the installer in the main
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thread.
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THREADS AND PYTHON
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Python has a nice model for threading. Threads in Python extend the
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threading.Thread class. So an easy way to identify something that will
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run or can be run as a thread is seeing a class definition like this:
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class SomeClass(threading.Thread):
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You still have your __init__ method for the constructor, but threads
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also have a run() method and a join() method (there are others, but
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I will just discuss these).
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The run() method is called when you start the thread. This is where
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you want to do the work. Normally this happens in the class
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constructor, but in threads we need that separated out to a different
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method.
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The join() method is to block execution of other threads. Whatever you
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put in the join() method will run and other threads will be blocked
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while it runs. Now, this method is only run when you call it explicitly
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from another thread, so think of it as similar to waitpid().
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Python has the thread and threading modules. Use threading as it's
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built on top of thread and provides a threading system similar to the
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POSIX thread model.
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More information:
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http://docs.python.org/lib/module-threading.html
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THREAD NAMES
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Threads have names in Python. They are automatically assigned or you
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can specify the name. For anaconda it probably makes more sense to
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name our threads since we won't be launching more than one for the
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same task.
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The convention I'm using is: NameThr
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For example: RelNotesThr
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The name is arbitrary, but we should probably have some sort of
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consistency.
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PYGTK AND THREADS
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GTK+ presents the biggest challenge for threads, but it's not
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impossible. We will be allowing GTK+ calls from any thread, so we
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have to call threads_init() in gui.py as the first thing:
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gtk.gdk.threads_init()
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After this, you can use Python threads as you normally would. When
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you call gtk.main(), you need to call it like this:
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gtk.threads_enter()
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gtk.main()
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gtk.threads_leave()
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Calls to PyGTK methods or fiddling with GTK objects...all that has to
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be wrapped in threads_enter/threads_leave calls. The suggested syntax
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is:
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gtk.threads_enter()
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try:
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# do stuff
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finally:
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gtk.threads_leave()
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Suggested reading:
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http://www.async.com.br/faq/pygtk/index.py?req=show&file=faq20.006.htp
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http://developer.gnome.org/doc/API/2.0/gdk/gdk-Threads.html
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I will try to expand this document as I move through more threading code.
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Email me if you have any questions.
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--
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David Cantrell
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<dcantrell@redhat.com>
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