Make PipelineId a pair of PipelineId and Option<SubpageId>

[jdm]

This would conceivably allow us to move all subpage-related modification out of script and into the constellation, where it arguably belongs.

[jdm]

Blocks #5236.

[frewsxcv]

I'll take a stab at this right now

[glennw]

I was thinking something like:

struct PipelineId {
    window_id: WindowId,
    subpage_id: Option<SubpageId>,
};

And then have the HTMLIFrameElement code be the one that generates the unique pipeline id, since that would simplify a number of other things in the future.

@jdm does that sound reasonable to you? It might hit an unforeseen roadblock, but it sounds like that could work.

[jdm]

@glennw So the iframe would get the window id from the window, and create the new subpage id? That sounds reasonable to me.

[glennw]

Yes, the window_id would be set once on each window creation, and the window would contain the subpage counter (as it does now) which the iframe would query/increment.

[frewsxcv]

FYI, I didn't feel I have the knowledge to complete this right now, so I'm no longer working on it

[KiChjang]

I can take a look at this one.

[KiChjang]

I just took a look at the previous PR, and I came across with the usage of the new WindowId struct. How and where is it going to be used and created?

[benschulz]

In its current form this issue seems to contradict #5241.

As far as @KiChjang's question is concerned, my interpretation is as follows. The constellation manages a (bijective) mapping from PipelineId to WindowId by incrementing a counter for each new PipelineId it receives from ScriptLoadedURLInIFrame messages. Then it sends the generated WindowId as part of the AttachLayout message. However, I'm not 100% certain that's right.

Assuming you get an answer to your question, will you continue working on this, @KiChjang?

[KiChjang]

You can go ahead and work on this issue, I have plenty of other stuff on my plate right now.

[glennw]

This is handled a different way now - iframes are able to generate their own PipelineId.