Fix streaming in Node.js fast path (#11058)

* Fix streaming in Node.js fast path

* Create a new next if the iterator is not done

* Use a flag instead

* Update test

* Add new assertion

* Add explanation of the renderingComplete variable

* Remove flaky assertion

3 files changed, 22 insertions, 7 deletions

diff --git a/.changeset/fluffy-pears-press.md b/.changeset/fluffy-pears-press.md
new file mode 100644
index 000000000..03eadc134
--- /dev/null
+++ b/.changeset/fluffy-pears-press.md
@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Fix streaming in Node.js fast path
diff --git a/packages/astro/src/runtime/server/render/astro/render.ts b/packages/astro/src/runtime/server/render/astro/render.ts
index f918f55c1..e945551d8 100644
--- a/packages/astro/src/runtime/server/render/astro/render.ts
+++ b/packages/astro/src/runtime/server/render/astro/render.ts
@@ -209,14 +209,23 @@ export async function renderToAsyncIterable(
 	let error: Error | null = null;
 	// The `next` is an object `{ promise, resolve, reject }` that we use to wait
 	// for chunks to be pushed into the buffer.
-	let next = promiseWithResolvers<void>();
+	let next: ReturnType<typeof promiseWithResolvers<void>> | null = null;
 	const buffer: Uint8Array[] = []; // []Uint8Array
+	let renderingComplete = false;
 
 	const iterator: AsyncIterator<Uint8Array> = {
 		async next() {
 			if (result.cancelled) return { done: true, value: undefined };
 
-			await next.promise;
+			if(next !== null) {
+				await next.promise;
+			}
+
+			// Only create a new promise if rendering is still ongoing. Otherwise
+			// there will be a dangling promises that breaks tests (probably not an actual app)
+			if(!renderingComplete) {
+				next = promiseWithResolvers();
+			}
 
 			// If an error occurs during rendering, throw the error as we cannot proceed.
 			if (error) {
@@ -276,8 +285,7 @@ export async function renderToAsyncIterable(
 				// Push the chunks into the buffer and resolve the promise so that next()
 				// will run.
 				buffer.push(bytes);
-				next.resolve();
-				next = promiseWithResolvers<void>();
+				next?.resolve();
 			}
 		},
 	};
@@ -286,12 +294,14 @@ export async function renderToAsyncIterable(
 	renderPromise
 		.then(() => {
 			// Once rendering is complete, calling resolve() allows the iterator to finish running.
-			next.resolve();
+			renderingComplete = true;
+			next?.resolve();
 		})
 		.catch((err) => {
 			// If an error occurs, save it in the scope so that we throw it when next() is called.
 			error = err;
-			next.resolve();
+			renderingComplete = true;
+			next?.resolve();
 		});
 
 	// This is the Iterator protocol, an object with a `Symbol.asyncIterator`
diff --git a/packages/astro/test/streaming.test.js b/packages/astro/test/streaming.test.js
index 93172aa72..cbd4fa4f4 100644
--- a/packages/astro/test/streaming.test.js
+++ b/packages/astro/test/streaming.test.js
@@ -37,7 +37,7 @@ describe('Streaming', () => {
 				let chunk = decoder.decode(bytes);
 				chunks.push(chunk);
 			}
-			assert.equal(chunks.length > 1, true);
+			assert.equal(chunks.length > 5, true);
 		});
 
 		it('Body of slots is chunked', async () => {