workaround(node ssr): cancellation support for renderToAsyncIterable (#10319)

* workaround(node ssr): cancellation support for renderToAsyncIterable

* add changeset

* Update .changeset/nice-pets-tie.md

2 files changed, 14 insertions, 1 deletions

diff --git a/.changeset/nice-pets-tie.md b/.changeset/nice-pets-tie.md
new file mode 100644
index 000000000..02d90002e
--- /dev/null
+++ b/.changeset/nice-pets-tie.md
@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Fixes an issue where streaming SSR responses sometimes failed with "`iterator.result` is not a function" on node-based adapters.
diff --git a/packages/astro/src/runtime/server/render/astro/render.ts b/packages/astro/src/runtime/server/render/astro/render.ts
index 4d11254b7..9ee7461f5 100644
--- a/packages/astro/src/runtime/server/render/astro/render.ts
+++ b/packages/astro/src/runtime/server/render/astro/render.ts
@@ -201,10 +201,14 @@ export async function renderToAsyncIterable(
 	// 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>();
+	// keep track of whether the client connection is still interested in the response.
+	let cancelled = false;
 	const buffer: Uint8Array[] = []; // []Uint8Array
 
-	const iterator = {
+	const iterator: AsyncIterator<Uint8Array> = {
 		async next() {
+			if (cancelled) return { done: true, value: undefined };
+
 			await next.promise;
 
 			// If an error occurs during rendering, throw the error as we cannot proceed.
@@ -238,6 +242,10 @@ export async function renderToAsyncIterable(
 
 			return returnValue;
 		},
+		async return() {
+			cancelled = true;
+			return { done: true, value: undefined };
+		}
 	};
 
 	const destination: RenderDestination = {