Basically the implementation we have with JS onEnterFrame callbacks and friends
is not sufficiently performant, and is a real pain to work with on large
complex websites. This implementation instead serializes all of the necessary
information to a ring buffer hanging off of the JSContext and outputs a JS
object containing that information when prompted.
There may be some subtle oddities in the design here, or more indirection in
some areas than you might expect. It should be kept in mind that this evolved
from a prototype which used a shmem to allow visualizing this trace live in the
parent process, which is a model we eventually want to return to, but this felt
like the safest first patch.
Differential Revision: https://phabricator.services.mozilla.com/D215932
alexical