Fix ensureLoaded-vs-build race condition in HEAD

[dvoytenko]

Closes #32978

This is the #32980 for the current HEAD

[samouri]

is whenBuilt different from resource.build()?

[dvoytenko]

Yes. Unfortunately it is. This is in a nutshell exactly why the bug happened. This is just a quick fix. Our big-fix options:

1. Reimplement Resource.whenBuild and state.
2. Transition some elements to V1 faster. V1 do not have that problem by definition.

[samouri]

what is downstream? would it be cleanest for custom-element to force built signal on Resource?

[dvoytenko]

Unfortunately, no. The problem is not only a built signal, but also all this code:

amphtml/src/service/resource.js

Lines 346 to 362 in c3e45ef

	this.isBuilding_ = false;
	// With IntersectionObserver, measure can happen before build
	// so check if we're "ready for layout" (measured and built) here.
	if (this.intersect_ && this.hasBeenMeasured()) {
	this.state_ = ResourceState.READY_FOR_LAYOUT;
	// The InOb premeasurement couldn't account for fixed position since
	// implementation wasn't loaded yet. Perform a remeasure now that we know it
	// is fixed.
	if (this.element.isAlwaysFixed() && !this.isFixed_) {
	this.requestMeasure();
	}
	this.element.onMeasure(/* sizeChanged */ true);
	} else {
	this.state_ = ResourceState.NOT_LAID_OUT;
	}
	// TODO(dvoytenko): merge with the standard BUILT signal.
	this.element.signals().signal('res-built');

What we could do is to refactor that code to be called by the CustomElement itself in the same microtask as its own build completion code.

[dvoytenko]

This would actually be quite easy to achieve. But I still wonder if doing this would take away time from maybe porting a few more elements to V1 to address most of the issues. Let me know what you think.

[samouri]

Is it that both Resource and CustomElement both have their own isBuilt state and we need to dedupe them or are there other issues? I'm in favor of deduping as much as possible between Resource and CustomElement (sizes, states, etc). It has caused multiple P0s so far

[dvoytenko]

Yes. Big area of concern. V1 elements so far simply do not use Resource interface at all (i.e. they are already deduped). We can definitely dedupe V0 elements as well area-by-area.