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76 lines
3.8 KiB
Text
76 lines
3.8 KiB
Text
-*- Mode: text -*-
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Sometimes it's difficult to use GStreamer in real applications where the GUI and
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the GStreamer pipeline are in different threads. You have to somehow make sure
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that the object is not modifying its properties while you get and set them. This
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document is a brief outline of a potential fix to this issue.
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* When is it safe to get or set properties?
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Well, it is only safe to do so when other threads are not accessing an object.
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So, from the main thread, it is only safe to query or modify properties on an
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object when it is not in the RUNNING state, because when it is RUNNING its
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properties are potentially changing in the thread of execution.
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The only place that it is safe to get or set properties while in the RUNNING
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state is from within the thread of execution. Thus, something within the
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iteration loop of the thread must check to see if there are pending property
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changes or queries. There are two places this could be done, from within
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gstthread itself and from within the scheduler. Doing it from gstthread sounds
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like a good idea because it keeps more code out of the scheduler, but is also
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bad in a way because it requires a gstthread-specific api to proxy prop_set and
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prop_get. Setting it in the scheduler sounds good because of its finer
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granularity, but might be bad because it would clutter the scheduler a bit more.
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I propose to go with the scheduler-based solution based on system response time
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and out of potentials for integration with gst_clock_wait.
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* Implementation
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We do want to preserve some measure of generality, however. Considering that
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more threadsafety issues could pop up in the future, it would be nice to have
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one function to call from the scheduler, in the interests of code
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simplification. This function will be present in some elements and not in
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others, and will be modified at run time, so we will make it a function pointer
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within the GstElement struct.
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struct _GstElement {
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...
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void (*pre_run_func) (GstElement *);
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void (*post_run_func) (GstElement *);
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}
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Only the managing bin of an element is allowed to set that function, because
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presumably that bin would know something about how to schedule the element.
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Then, in the scheduler, before we call chain functions and before we switch into
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loop functions:
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if (element->pre_run_func)
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element->pre_run_func (element);
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Then, to get or set properties, we use the new functions gst_element_get or
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gst_element_set. _set would add the property name and a gvalue onto a queue
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(probably a GAsyncQueue). Then the pre_run_func would go ahead and set the
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properties. _get is a little more tricky; _set doesn't hardly block at all,
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although it's not instantaneous. With _get though, you really don't know what
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the properties are until you query them. The best thing would be to connect to
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the ::notify signal, which executes within the thread of interest. However, say
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you really want to use _get. Hmm. I think that it would have to block. On what?
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Well, probably on an element's mutex. So it seems we might need a
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post_run_func too, to unlock the mutex. We can use the GstObject lock for this.
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But we need a little more. How do we know whether or not just calling
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g_object_get/set is ok? I'm thinking this whole prop set/get proxy thing should
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not be abstracted away, that it should be contained in gstelement.c. There are
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more kinds of bins that need threadsafety than just gstthread (I'm thinking
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about jack here). So, we can add on a GAsyncQueue *prop_set_queue; to the
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GstElement struct, and only initialize it in certain managing bins. Then, we can
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set a flag on gstelement, GST_ELEMENT_USE_THREADSAFE_PROPERTIES. If this flag is
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set (by the managing bin), do all this complicated mess; otherwise use the
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gobject native functionality.
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So, this is the plan. We'll see how the implementation goes. This should make MT
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gst programming much easier.
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wingo.
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25 May 2002.
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