Warn when trying to access headers in SSG mode (#3021)

* Warn when trying to access headers in SSG mode

* Adds a changeset

* Warn when accessing headers at all + a test

7 files changed, 60 insertions, 3 deletions

diff --git a/.changeset/odd-squids-rest.md b/.changeset/odd-squids-rest.md
new file mode 100644
index 000000000..fa2c11c58
--- /dev/null
+++ b/.changeset/odd-squids-rest.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Warn when attempting to access headers in SSG mode
diff --git a/packages/astro/src/core/build/generate.ts b/packages/astro/src/core/build/generate.ts
index 909c0f65a..974e2acd6 100644
--- a/packages/astro/src/core/build/generate.ts
+++ b/packages/astro/src/core/build/generate.ts
@@ -191,6 +191,7 @@ async function generatePath(
 		}
 	}
 
+	const ssr = isBuildingToSSR(opts.astroConfig);
 	const url = new URL(origin + pathname);
 	const options: RenderOptions = {
 		legacyBuild: false,
@@ -219,13 +220,13 @@ async function generatePath(
 			const fullyRelativePath = relPath[0] === '.' ? relPath : './' + relPath;
 			return fullyRelativePath;
 		},
-		request: createRequest({ url, headers: new Headers(), logging }),
+		request: createRequest({ url, headers: new Headers(), logging, ssr }),
 		route: pageData.route,
 		routeCache,
 		site: astroConfig.site
 			? new URL(astroConfig.base, astroConfig.site).toString()
 			: astroConfig.site,
-		ssr: isBuildingToSSR(opts.astroConfig),
+		ssr,
 	};
 
 	let body: string;
diff --git a/packages/astro/src/core/request.ts b/packages/astro/src/core/request.ts
index abccbf995..c18d4f2ca 100644
--- a/packages/astro/src/core/request.ts
+++ b/packages/astro/src/core/request.ts
@@ -11,6 +11,7 @@ export interface CreateRequestOptions {
 	method?: string;
 	body?: RequestBody | undefined;
 	logging: LogOptions;
+	ssr: boolean;
 }
 
 export function createRequest({
@@ -19,6 +20,7 @@ export function createRequest({
 	method = 'GET',
 	body = undefined,
 	logging,
+	ssr
 }: CreateRequestOptions): Request {
 	let headersObj =
 		headers instanceof Headers
@@ -50,5 +52,21 @@ export function createRequest({
 		},
 	});
 
+	if(!ssr) {
+		// Warn when accessing headers in SSG mode
+		const _headers = request.headers;
+		const headersDesc = Object.getOwnPropertyDescriptor(request, 'headers') || {};
+		Object.defineProperty(request, 'headers', {
+			...headersDesc,
+			get() {
+				warn(logging,
+					'ssg',
+					`Headers are not exposed in static-site generation (SSG) mode. To enable reading headers you need to set an SSR adapter in your config.`
+				);
+				return _headers;
+			}
+		})
+	}
+
 	return request;
 }
diff --git a/packages/astro/src/vite-plugin-astro-server/index.ts b/packages/astro/src/vite-plugin-astro-server/index.ts
index dba3b861b..1838a5ad4 100644
--- a/packages/astro/src/vite-plugin-astro-server/index.ts
+++ b/packages/astro/src/vite-plugin-astro-server/index.ts
@@ -176,6 +176,7 @@ async function handleRequest(
 		method: req.method,
 		body,
 		logging,
+		ssr: buildingToSSR
 	});
 
 	try {
diff --git a/packages/astro/src/vite-plugin-build-html/index.ts b/packages/astro/src/vite-plugin-build-html/index.ts
index 9221028f2..34f7b3730 100644
--- a/packages/astro/src/vite-plugin-build-html/index.ts
+++ b/packages/astro/src/vite-plugin-build-html/index.ts
@@ -114,6 +114,7 @@ export function rollupPluginAstroScanHTML(options: PluginOptions): VitePlugin {
 							url: new URL(origin + pathname),
 							headers: new Headers(),
 							logging,
+							ssr: false,
 						}),
 						mode: 'production',
 						origin,
diff --git a/packages/astro/test/fixtures/static build/src/pages/index.astro b/packages/astro/test/fixtures/static build/src/pages/index.astro
index 763046d0a..5bd2410ac 100644
--- a/packages/astro/test/fixtures/static build/src/pages/index.astro
+++ b/packages/astro/test/fixtures/static build/src/pages/index.astro
@@ -3,6 +3,9 @@ import MainHead from '../components/MainHead.astro';
 import Nav from '../components/Nav/index.jsx';
 import { test as ssrConfigTest } from '@test/static-build-pkg';
 let allPosts = await Astro.glob('./posts/*.md');
+
+// Note that this just tests a warning
+Astro.request.headers;
 ---
 <html>
 <head>
diff --git a/packages/astro/test/static-build.test.js b/packages/astro/test/static-build.test.js
index 55ce6ca07..e56f40077 100644
--- a/packages/astro/test/static-build.test.js
+++ b/packages/astro/test/static-build.test.js
@@ -6,14 +6,32 @@ function addLeadingSlash(path) {
 	return path.startsWith('/') ? path : '/' + path;
 }
 
+/**
+ * @typedef {import('../src/core/logger/core').LogMessage} LogMessage
+ */
+
 describe('Static build', () => {
+	/** @type {import('./test-utils').Fixture} */
 	let fixture;
+	/** @type {LogMessage[]} */
+	let logs = [];
 
 	before(async () => {
+		/** @type {import('../src/core/logger/core').LogOptions} */
+		const logging = {
+			dest: {
+				write(chunk) {
+					logs.push(chunk);
+				}
+			},
+			level: 'warn',
+		};
+
+
 		fixture = await loadFixture({
 			root: './fixtures/static build/',
 		});
-		await fixture.build();
+		await fixture.build({ logging });
 	});
 
 	it('Builds out .astro pages', async () => {
@@ -137,4 +155,14 @@ describe('Static build', () => {
 		const $ = cheerioLoad(html);
 		expect($('#ssr-config').text()).to.equal('testing');
 	});
+
+	it('warns when accessing headers', async () => {
+		let found = false;
+		for(const log of logs) {
+			if(log.type === 'ssg' && /[hH]eaders are not exposed in static-site generation/.test(log.args[0])) {
+				found = true;
+			}
+		}
+		expect(found).to.equal(true, 'Found the log message');
+	});
 });