Parallelize rendering of sibling components to avoid async waterfalls (#7071)

* Parallelize rendering of sibling components to avoid async waterfalls

* Catch and rethrow errors when eagerly rendering children

* Catch `Response` in rendering stage and throw error

* Add changeset

* Fix test error message

* Improve unit tests

* Start async generators in non-buffered mode, and only start buffering once a component doesn't resolve immediatly

* Add more documentation

10 files changed, 210 insertions, 3 deletions

diff --git a/.changeset/clever-garlics-relate.md b/.changeset/clever-garlics-relate.md
new file mode 100644
index 000000000..2e213df09
--- /dev/null
+++ b/.changeset/clever-garlics-relate.md
@@ -0,0 +1,5 @@
+---
+'astro': minor
+---
+
+Render sibling components in parallel
diff --git a/packages/astro/src/runtime/server/render/astro/render-template.ts b/packages/astro/src/runtime/server/render/astro/render-template.ts
index fc1442422..2433596bd 100644
--- a/packages/astro/src/runtime/server/render/astro/render-template.ts
+++ b/packages/astro/src/runtime/server/render/astro/render-template.ts
@@ -3,6 +3,7 @@ import type { RenderInstruction } from '../types';
 import { HTMLBytes, markHTMLString } from '../../escape.js';
 import { isPromise } from '../../util.js';
 import { renderChild } from '../any.js';
+import { EagerAsyncIterableIterator } from '../util.js';
 
 const renderTemplateResultSym = Symbol.for('astro.renderTemplateResult');
 
@@ -35,12 +36,23 @@ export class RenderTemplateResult {
 	async *[Symbol.asyncIterator]() {
 		const { htmlParts, expressions } = this;
 
+		let iterables: Array<EagerAsyncIterableIterator> = [];
+		// all async iterators start running in non-buffered mode to avoid useless caching
+		for (let i = 0; i < htmlParts.length; i++) {
+			iterables.push(new EagerAsyncIterableIterator(renderChild(expressions[i])));
+		}
+		// once the execution of the next for loop is suspended due to an async component,
+		// this timeout triggers and we start buffering the other iterators
+		setTimeout(() => {
+			// buffer all iterators that haven't started yet
+			iterables.forEach((it) => !it.isStarted() && it.buffer());
+		}, 0);
 		for (let i = 0; i < htmlParts.length; i++) {
 			const html = htmlParts[i];
-			const expression = expressions[i];
+			const iterable = iterables[i];
 
 			yield markHTMLString(html);
-			yield* renderChild(expression);
+			yield* iterable;
 		}
 	}
 }
diff --git a/packages/astro/src/runtime/server/render/page.ts b/packages/astro/src/runtime/server/render/page.ts
index f4a02a75f..45ab4236e 100644
--- a/packages/astro/src/runtime/server/render/page.ts
+++ b/packages/astro/src/runtime/server/render/page.ts
@@ -157,6 +157,15 @@ export async function renderPage(
 									}
 								}
 
+								// `chunk` might be a Response that contains a redirect,
+								// that was rendered eagerly and therefore bypassed the early check
+								// whether headers can still be modified. In that case, throw an error
+								if (chunk instanceof Response) {
+									throw new AstroError({
+										...AstroErrorData.ResponseSentError,
+									});
+								}
+
 								const bytes = chunkToByteArray(result, chunk);
 								controller.enqueue(bytes);
 								i++;
diff --git a/packages/astro/src/runtime/server/render/util.ts b/packages/astro/src/runtime/server/render/util.ts
index 37008abbb..5ddd97716 100644
--- a/packages/astro/src/runtime/server/render/util.ts
+++ b/packages/astro/src/runtime/server/render/util.ts
@@ -138,3 +138,111 @@ export function renderElement(
 	}
 	return `<${name}${internalSpreadAttributes(props, shouldEscape)}>${children}</${name}>`;
 }
+
+// This wrapper around an AsyncIterable can eagerly consume its values, so that
+// its values are ready to yield out ASAP. This is used for list-like usage of
+// Astro components, so that we don't have to wait on earlier components to run
+// to even start running those down in the list.
+export class EagerAsyncIterableIterator {
+	#iterable: AsyncIterable<any>;
+	#queue = new Queue<IteratorResult<any, any>>();
+	#error: any = undefined;
+	#next: Promise<IteratorResult<any, any>> | undefined;
+	/**
+	 * Whether the proxy is running in buffering or pass-through mode
+	 */
+	#isBuffering = false;
+	#gen: AsyncIterator<any> | undefined = undefined;
+	#isStarted = false;
+
+	constructor(iterable: AsyncIterable<any>) {
+		this.#iterable = iterable;
+	}
+
+	/**
+	 * Starts to eagerly fetch the inner iterator and cache the results.
+	 * Note: This might not be called after next() has been called once, e.g. the iterator is started
+	 */
+	async buffer() {
+		if (this.#gen) {
+			// If this called as part of rendering, please open a bug report.
+			// Any call to buffer() should verify that the iterator isn't running
+			throw new Error('Cannot not switch from non-buffer to buffer mode');
+		}
+		this.#isBuffering = true;
+		this.#isStarted = true;
+		this.#gen = this.#iterable[Symbol.asyncIterator]();
+		let value: IteratorResult<any, any> | undefined = undefined;
+		do {
+			this.#next = this.#gen.next();
+			try {
+				value = await this.#next;
+				this.#queue.push(value);
+			} catch (e) {
+				this.#error = e;
+			}
+		} while (value && !value.done);
+	}
+
+	async next() {
+		if (this.#error) {
+			throw this.#error;
+		}
+		// for non-buffered mode, just pass through the next result
+		if (!this.#isBuffering) {
+			if (!this.#gen) {
+				this.#isStarted = true;
+				this.#gen = this.#iterable[Symbol.asyncIterator]();
+			}
+			return await this.#gen.next();
+		}
+		if (!this.#queue.isEmpty()) {
+			return this.#queue.shift()!;
+		}
+		await this.#next;
+		// the previous statement will either put an element in the queue or throw,
+		// so we can safely assume we have something now
+		return this.#queue.shift()!;
+	}
+
+	isStarted() {
+		return this.#isStarted;
+	}
+
+	[Symbol.asyncIterator]() {
+		return this;
+	}
+}
+
+interface QueueItem<T> {
+	item: T;
+	next?: QueueItem<T>;
+}
+
+/**
+ * Basis Queue implementation with a linked list
+ */
+class Queue<T> {
+	head: QueueItem<T> | undefined = undefined;
+	tail: QueueItem<T> | undefined = undefined;
+
+	push(item: T) {
+		if (this.head === undefined) {
+			this.head = { item };
+			this.tail = this.head;
+		} else {
+			this.tail!.next = { item };
+			this.tail = this.tail!.next;
+		}
+	}
+
+	isEmpty() {
+		return this.head === undefined;
+	}
+
+	shift(): T | undefined {
+		const val = this.head?.item;
+		this.head = this.head?.next;
+		return val;
+	}
+}
diff --git a/packages/astro/test/fixtures/parallel/package.json b/packages/astro/test/fixtures/parallel/package.json
new file mode 100644
index 000000000..299d904d1
--- /dev/null
+++ b/packages/astro/test/fixtures/parallel/package.json
@@ -0,0 +1,12 @@
+{
+  "name": "@test/parallel-components",
+  "version": "0.0.0",
+  "private": true,
+  "scripts": {
+    "build": "astro build",
+    "dev": "astro dev"
+  },
+  "dependencies": {
+    "astro": "workspace:*"
+  }
+}
diff --git a/packages/astro/test/fixtures/parallel/src/components/Delayed.astro b/packages/astro/test/fixtures/parallel/src/components/Delayed.astro
new file mode 100644
index 000000000..9f540bd70
--- /dev/null
+++ b/packages/astro/test/fixtures/parallel/src/components/Delayed.astro
@@ -0,0 +1,11 @@
+---
+const { ms } = Astro.props
+const start = new Date().valueOf();
+await new Promise(res => setTimeout(res, ms));
+const finished = new Date().valueOf();
+---
+<section>
+	<h1>{ms}ms Delayed</h1>
+	<span class="start">{ start }</span>
+	<span class="finished">{ finished }</span>
+</section>
diff --git a/packages/astro/test/fixtures/parallel/src/pages/index.astro b/packages/astro/test/fixtures/parallel/src/pages/index.astro
new file mode 100644
index 000000000..952e9efce
--- /dev/null
+++ b/packages/astro/test/fixtures/parallel/src/pages/index.astro
@@ -0,0 +1,8 @@
+---
+import Delayed from '../components/Delayed.astro'
+---
+
+<Delayed ms={30} />
+<Delayed ms={20} />
+<Delayed ms={40} />
+<Delayed ms={10} />
diff --git a/packages/astro/test/fixtures/streaming/src/pages/slot.astro b/packages/astro/test/fixtures/streaming/src/pages/slot.astro
index ea918cc6f..5cf79fb61 100644
--- a/packages/astro/test/fixtures/streaming/src/pages/slot.astro
+++ b/packages/astro/test/fixtures/streaming/src/pages/slot.astro
@@ -15,7 +15,7 @@ import Wait from '../components/Wait.astro';
 				<p>Section content</p>
 			</Wait>
 			<h2>Next section</h2>
-			<Wait ms={50}>
+			<Wait ms={60}>
 				<p>Section content</p>
 			</Wait>
 			<p>Paragraph 3</p>
diff --git a/packages/astro/test/parallel.js b/packages/astro/test/parallel.js
new file mode 100644
index 000000000..2ad50f2d6
--- /dev/null
+++ b/packages/astro/test/parallel.js
@@ -0,0 +1,36 @@
+import { expect } from 'chai';
+import * as cheerio from 'cheerio';
+import { loadFixture } from './test-utils.js';
+
+describe('Component parallelization', () => {
+	let fixture;
+
+	before(async () => {
+		fixture = await loadFixture({
+			root: './fixtures/parallel/',
+		});
+		await fixture.build();
+	});
+
+	it('renders fast', async () => {
+		let html = await fixture.readFile('/index.html');
+		let $ = cheerio.load(html);
+
+		const startTimes = Array.from($('.start')).map((element) => Number(element.children[0].data));
+		const finishTimes = Array.from($('.finished')).map((element) =>
+			Number(element.children[0].data)
+		);
+
+		let renderStartWithin = Math.max(...startTimes) - Math.min(...startTimes);
+		expect(renderStartWithin).to.be.lessThan(
+			10, // in theory, this should be 0, so 10ms tolerance
+			"The components didn't start rendering in parallel"
+		);
+
+		const totalRenderTime = Math.max(...finishTimes) - Math.min(...startTimes);
+		expect(totalRenderTime).to.be.lessThan(
+			60, // max component delay is 40ms
+			'The total render time was significantly longer than the max component delay'
+		);
+	});
+});
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 224a78ee6..30368328c 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -2904,6 +2904,12 @@ importers:
         specifier: workspace:*
         version: link:../../..
 
+  packages/astro/test/fixtures/parallel:
+    dependencies:
+      astro:
+        specifier: workspace:*
+        version: link:../../..
+
   packages/astro/test/fixtures/postcss:
     dependencies:
       '@astrojs/solid-js':