On Mon, Mar 24, 2014 at 03:18:19PM -0700, Botond Ballo wrote:
> Hello dev-platform,
>
> I recently fixed an APZ bug [1] that was caused by an IPDL message,
> PBrowser::UpdateFrame, being compressed when it shouldn't have been.
>
> I think the compression was correct back when we didn't have subframe
> scrolling, and so all messages pertained to the same frame, in which
> case it's appropriate to only act on the most recent message. With
> subframe scrolling, subsequent messages could apply to different
> frames, and dropping all but the most recent message could lead to a
> frame missing an important update.
>
> I fixed the bug by getting rid of the compression, but it occurred to
> me that if we had a mechanism for specifying that two messages
> applying to the same frame could be suppressed, but two messages
> applying to different frames could not, we'd probably want to use it.
>
> Is there any interest in adding such a mechanism to IPDL?
seems reasonable enough.
> Here's a straw man proposal for how it could work. Obviously, the
> syntax is not the interesting part and I'm happy to change it to
> whatever other syntax people might like:
>
> In addition to having a 'compress' attribute, have a 'compress_if'
> attribute that takes a function name as argument. Such a function
> would be expected to be provided in C++ code, to have 2*n arguments,
> where 'n' is the argument to the message, and to return 'bool'
> indicating whether or not two messages with those arguments should
> be suppressed.
>
> For example:
>
> RetType myMessage(Foo foo, Bar bar) compress_if(CompressMyMessage);
>
> And then one would provide:
>
> bool CompressMyMessage(const Foo& foo1, const Bar& bar1,
> const Foo& foo2, const Bar& bar2)
> {
> // whatever
> }
There's a part of me that wants to do this with a well known name for a
static method on the actor (though I guess that's tricky since the code
gen doesn't currently know the static type of the actor) but then you
could write
bool
FooChild::CoalesceMyMessage(const Bar& aM1, const bar& aM2)
{
return something or other;
}
Then we could use this same mechanism to implement always compressing,
but maybe that's too magical...
Trev
>
> which would decide whether or not myMessage(foo1, bar1) and
> myMessage(foo2, bar2) should be compressed.
>
> For example, for UpdateFrame we could do:
>
> UpdateFrame(FrameMetrics frame) compress_if(CompressUpdateFrame);
>
> and provide:
>
> bool CompressUpdateFrame(const FrameMetrics& frame1,
> const FrameMetrics& frame2)
> {
> // Two UpdateFrame messages can be compressed if they
> // apply to the same frame.
> return frame1.mScrollId == frame2.mScrollId;
> }
>
> I can think of at least one other use case in code that I'm
> currently working on: for bug 976605 [2], I'd find it
> convenient to conditionally compress PBrowser::RealTouchMoveEvent,
> to make sure that certain touch-move events (those with a
> particular flag set which is important for the child side to
> know about) are not compressed away.
>
> Presumably there will be other use cases in other protocols as well.
>
> Any thoughts about this / interest in this?
>
> Cheers,
> Botond
>
> [1] https://bugzilla.mozilla.org/show_bug.cgi?id=984673
> [2] https://bugzilla.mozilla.org/show_bug.cgi?id=976605
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