feat(csp): server islands

[ematipico]

Changes

This adds support for CSP for server islands. To support this feature, we have to apply a refactor to how server islands are rendered inside our engine. The new version of the compiler adds the propagation metadata to a route when one of its modules contains the server:defer attribute.

With that information available, the server island has been refactored into a class that contains two methods: init and render.

The method init is the important one, because that's the method that gets called when executing the head propagation. The init is called inside the following function:

astro/packages/astro/src/runtime/server/render/astro/render.ts

Lines 186 to 199 in 1609044

	async function bufferHeadContent(result: SSRResult) {
	const iterator = result._metadata.propagators.values();
	while (true) {
	const { value, done } = iterator.next();
	if (done) {
	break;
	}
	// Call component instances that might have head content to be propagated up.
	const returnValue = await value.init(result);
	if (isHeadAndContent(returnValue)) {
	result._metadata.extraHead.push(returnValue.head);
	}
	}
	}

As you might noticed, we also check if the returned value is an actual head. To comply, I created a new ThinHead type that just holds the Symbol that we use to tag head content. I created this type because the server island, as for now, doesn't return any content. I decided not to return any content to keep the rendering as similar as possible the one we have now. Of course, this changed regardless, because now some scripts are rendered in the head, as you might notice in extraHead.push()

The rendering of the server island changed slightly:

• the slots are rendered inside the init method, but not the fallback slot
• the fallback slot is rendered inside the render method
• inside the init, we save hostId and the content of the scripts so we can render them inside the render function

Finally, when we create a ServerIslandComponent, we add it to the list of propagators. These components are called inside the function I showed at the very beginning :)

Testing

The current tests should still pass.
Created new tests: integration and e2e

Docs

N/A

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[florian-lefebvre]

I'm not very familiar with this part of Astro but I haven't seen anything weird, LGTM!