From 44f30589c5f5bdfcac8e94837c8fc428b7f3ffbd Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Fri, 6 Jan 2023 17:28:00 -0500
Subject: [PATCH] only prerender a given dependency once - closes #3784

---
 .changeset/breezy-walls-provide.md               |  5 +++++
 packages/kit/src/core/prerender/prerender.js     | 16 +++++++++++++++-
 .../fetch-endpoint/also-not-buffered/+page.js    |  9 +++++++++
 .../also-not-buffered/+page.svelte               |  6 ++++++
 .../fetch-endpoint/not-buffered.json/+server.js  |  5 +++++
 5 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/breezy-walls-provide.md
 create mode 100644 packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/also-not-buffered/+page.js
 create mode 100644 packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/also-not-buffered/+page.svelte

diff --git a/.changeset/breezy-walls-provide.md b/.changeset/breezy-walls-provide.md
new file mode 100644
index 000000000000..28b778bfaef7
--- /dev/null
+++ b/.changeset/breezy-walls-provide.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+only prerender a given dependency once
diff --git a/packages/kit/src/core/prerender/prerender.js b/packages/kit/src/core/prerender/prerender.js
index aa3e732c6835..0e6f86306fda 100644
--- a/packages/kit/src/core/prerender/prerender.js
+++ b/packages/kit/src/core/prerender/prerender.js
@@ -102,10 +102,20 @@ export async function prerender() {
 	/** @type {import('types').SSRManifest} */
 	const manifest = (await import(pathToFileURL(manifest_path).href)).manifest;
 
+	/** @type {Map<string, string>} */
+	const saved = new Map();
+
 	override({
 		building: true,
 		paths: config.paths,
-		read: (file) => readFileSync(join(config.files.assets, file))
+		read: (file) => {
+			// stuff we just wrote
+			const filepath = saved.get(file);
+			if (filepath) return readFileSync(filepath);
+
+			// stuff in `static`
+			return readFileSync(join(config.files.assets, file));
+		}
 	});
 
 	const server = new Server(manifest);
@@ -222,6 +232,7 @@ export async function prerender() {
 			}
 
 			const body = result.body ?? new Uint8Array(await result.response.arrayBuffer());
+
 			save(
 				'dependencies',
 				result.response,
@@ -351,6 +362,9 @@ export async function prerender() {
 		} else if (response_type !== OK) {
 			handle_http_error({ status: response.status, path: decoded, referrer, referenceType });
 		}
+
+		manifest.assets.add(file);
+		saved.set(file, dest);
 	}
 
 	for (const route of manifest._.routes) {
diff --git a/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/also-not-buffered/+page.js b/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/also-not-buffered/+page.js
new file mode 100644
index 000000000000..de4123c2aafe
--- /dev/null
+++ b/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/also-not-buffered/+page.js
@@ -0,0 +1,9 @@
+/** @type {import('@sveltejs/kit').Load} */
+export async function load({ fetch }) {
+	const url = '/fetch-endpoint/not-buffered.json';
+	const res = await fetch(url);
+
+	return {
+		headers: res.headers
+	};
+}
diff --git a/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/also-not-buffered/+page.svelte b/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/also-not-buffered/+page.svelte
new file mode 100644
index 000000000000..bf73bfc65b89
--- /dev/null
+++ b/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/also-not-buffered/+page.svelte
@@ -0,0 +1,6 @@
+<script>
+	/** @type {import('./$types').PageData} */
+	export let data;
+</script>
+
+<h1>content-type: {data.headers.get('content-type')}</h1>
diff --git a/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/not-buffered.json/+server.js b/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/not-buffered.json/+server.js
index 82d3cb7b8345..692b07ced2c6 100644
--- a/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/not-buffered.json/+server.js
+++ b/packages/kit/test/prerendering/basics/src/routes/fetch-endpoint/not-buffered.json/+server.js
@@ -1,5 +1,10 @@
 import { json } from '@sveltejs/kit';
 
+let called = false;
+
 export async function GET() {
+	if (called) throw new Error('should only be called once');
+	called = true;
+
 	return json({ answer: 42 });
 }