Deferred Layer

[mookums]

This solves the issue of scripts running while blocking scripts are running by introducing a DeferringLayer. This layer defers the completion of requests while a blocking request is running, clearing them later on staticScriptsDone.

This tracks the currently active request by its ID ensuring that each frame can only have one blocking request at a time and deferring the scripts.

[karlseguin]

I think there's a serious UAF here. The transfer of an deferred request will be deinit'd while the request still lives in the deferred list. I tested this out by writing a log when transfer.deinit is called, and that always happens before the request is "fired".

Another issue is that this defers the done/error/shutdown but the header/data are just as important to block from firing.

[karlseguin]

A while ago, I looked at something that did something similar - delaying new requests until blocking requests were complete. I don't remember exactly what I measured, but it was that it would introduce significant latency. You can imagine:

const xhr = new XMLHttpRequest();
...
xhr.send();
// do something that take 500ms

If the DeferredLayer defers this request, then it adds 500ms before the request starts. You want to start the HTTP request ASAP, just not process the callbacks.

I also think the code current leaks requests, e.g. what happens if deinit is called while there are still deferred requests?

[mookums]

Reworked it a bit around your new changes with Transfer and all.

[karlseguin]

So every callsite that wants to make a blocking request (and there are more), needs to do this?