fixing nested Suspense boundaries

src/index.js

@@ -84,7 +84,7 @@ export function renderToString(vnode, context) {
  * @param {Object} [context={}] Initial root context object
  * @returns {string} serialized HTML
  */
-export function renderToStringAsync(vnode, context) {
+export async function renderToStringAsync(vnode, context) {
 	// Performance optimization: `renderToString` is synchronous and we
 	// therefore don't execute any effects. To do that we pass an empty
 	// array to `options._commit` (`__c`). But we can go one step further
@@ -113,7 +113,18 @@ export function renderToStringAsync(vnode, context) {
 		);
 
 		if (Array.isArray(rendered)) {
-			return Promise.all(rendered).then((rendered) => rendered.join(''));
+			let count = 0;
+			let resolved = rendered;
+
+			// Resolving nested Promises with a maximum depth of 25
+			while (
+				resolved.some((element) => typeof element.then === 'function') &&
+				count++ < 25
+			) {
+				resolved = (await Promise.all(resolved)).flat();
+			}
+
+			return resolved.join('');
 		}
 
 		return rendered;

test/compat/async.test.js

@@ -25,7 +25,7 @@ describe('Async renderToString', () => {
 		expect(rendered).to.equal(expected);
 	});
 
-	it('should render JSX with nested suspense boundary', async () => {
+	it('should render JSX with nested suspended components', async () => {
 		const {
 			Suspender: SuspenderOne,
 			suspended: suspendedOne
@@ -58,4 +58,40 @@ describe('Async renderToString', () => {
 
 		expect(rendered).to.equal(expected);
 	});
+
+	it('should render JSX with nested suspense boundaries', async () => {
+		const {
+			Suspender: SuspenderOne,
+			suspended: suspendedOne
+		} = createSuspender();
+		const {
+			Suspender: SuspenderTwo,
+			suspended: suspendedTwo
+		} = createSuspender();
+
+		const promise = renderToStringAsync(
+			<ul>
+				<Suspense fallback={null}>
+					<SuspenderOne>
+						<li>one</li>
+						<Suspense fallback={null}>
+							<SuspenderTwo>
+								<li>two</li>
+							</SuspenderTwo>
+						</Suspense>
+						<li>three</li>
+					</SuspenderOne>
+				</Suspense>
+			</ul>
+		);
+
+		const expected = `<ul><li>one</li><li>two</li><li>three</li></ul>`;
+
+		suspendedOne.resolve();
+		suspendedTwo.resolve();
+
+		const rendered = await promise;
+
+		expect(rendered).to.equal(expected);
+	});
 });