ServoMedia::get can hang forever when setting document activity

[gterzian]

While debugging, I'm running into a hang of the script-thread at

servo/components/script/dom/document.rs

Line 666 in 072b892

let media = ServoMedia::get().unwrap();

Note this call is using OnceCell.html#method.wait, which will block until another thread sets the value.

In servo, it seems setting the value would be done at

servo/components/servo/lib.rs

Line 156 in 072b892

ServoMedia::init::<servo_media_dummy::DummyBackend>();

I am running into this while running a WPT test: /css/cssom-view/MediaQueryList-addListener-handleEvent.html

[gterzian]

Seems to only happen at 86bcc32

Happens also on latest main.

[gterzian]

So it seems that this particular call ends-up completing, but taking a very long time:

servo/components/servo/lib.rs

Line 125 in 072b892

ServoMedia::init_with_backend(|| {

Note that the closure is called from within a thread:

https://github.com/servo/media/blob/c04bb3025320cf8cd75ecf00ab8cacb39ff81843/servo-media/lib.rs#L103

So the initial call doesn't block, but then later when script call get, it blocks until that previous call in the thread completes.

[mrobinson]

This sounds like GStreamer building the plugin registry. We have this issue on MacOS in particular. The first time that Servo runs it spends over a minute building a new GStreamer plugin registry. I took a little time to investigate how we can eliminate this pause -- since we really only load a single set of plugins, but I'm unsure how we can deal with this.

[mrobinson]

For more information see: https://gstreamer.freedesktop.org/documentation/gstreamer/gstregistry.html