de-async emissions

[eoghanmurray]

I've got a problem I've traced back to processing of the emission event.
Basically I think it's possible for a DOMMutation event to alter the DOM node tree while I'm processing the initial FullSnapshot event.

This is resulting in an application specific bug, but (assuming I've got the diagnosis right) I'm wondering whether this could be a more general source of bugs, e.g. a mutation event appearing in the recording before the snapshot that it should act upon.

Should we introduce locking code or some sort of a queue to ensure that emissions happen sequentially?

[Yuyz0112]

I've seen similar problems in the checkout feature.

I've considered adding a general generation number to the events, which is the number of the full snapshot. So in the replay, we can check whether an event was happened before the current generation.

[eoghanmurray]

I'm having a look at implementing a simple lock in the wrapEmit function so that the next emission can't happen until the client code deals with the previous emission.
I'm using a queue, and basing my understanding of the js details on https://stackoverflow.com/a/2734311/6691

[eoghanmurray]

This is turning out a little more complicated.

I think I have to delay execution of e.g. mutation.ts::emit() itself as it is the calling of this function that modifies the mirror

(my current queue/lock system is just delaying the processing of the emissions)

Any guidance appreciated!

[eoghanmurray]

Ok trying to directly pause mutations while taking a full snapshot, building on the code added here:
#224
(that pull request hasn't been accepted yet)

[eoghanmurray]

I have all tests passing in #385 and believe things are correct.

I have this deployed on 11 websites that were exhibiting the problem (a FullSnapshot already containing the results of DOM Mutation before the rrweb mutation event is emitted), and it solves the problem; no need for any further work to queue emissions as far as I'm concerned.

[eoghanmurray]

So to wrap up: #385 (now merged) doesn't do any general queueing of emissions (to ensure that only one event can be processed at a time) but does address the specific interaction of FullSnapshot with DomMutation ... i.e. the latter is delayed until the former is completed.