fix: avoid overriding existing _headers rules

.changeset/hip-apes-marry.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-cloudflare': patch
+---
+
+fix: avoid overriding user's existing `_headers` rules

packages/adapter-cloudflare/index.js

@@ -7,7 +7,8 @@ import {
 	is_building_for_cloudflare_pages,
 	validate_worker_settings,
 	get_routes_json,
-	parse_redirects
+	parse_redirects,
+	append_headers
 } from './utils.js';
 
 const name = '@sveltejs/adapter-cloudflare';
@@ -133,10 +134,12 @@ export default function (options = {}) {
 			// _headers
 			const headers_src = '_headers';
 			const headers_dest = `${dest}/_headers`;
+			/** @type {string | undefined} */
+			let headers;
 			if (existsSync(headers_src)) {
-				copyFileSync(headers_src, headers_dest);
+				headers = readFileSync(headers_src, 'utf-8');
 			}
-			writeFileSync(headers_dest, generate_headers(builder.getAppPath()), { flag: 'a' });
+			writeFileSync(headers_dest, generate_headers(builder.getAppPath(), headers));
 
 			// _redirects
 			const redirects_src = '_redirects';
@@ -218,19 +221,23 @@ export default function (options = {}) {
 
 /**
  * @param {string} app_dir
+ * @param {string | undefined} content existing `_headers` file content
  * @returns {string}
  */
-function generate_headers(app_dir) {
-	return `
-# === START AUTOGENERATED SVELTE IMMUTABLE HEADERS ===
-/${app_dir}/*
-  X-Robots-Tag: noindex
-	Cache-Control: no-cache
-/${app_dir}/immutable/*
-  ! Cache-Control
-	Cache-Control: public, immutable, max-age=31536000
-# === END AUTOGENERATED SVELTE IMMUTABLE HEADERS ===
-`.trimEnd();
+function generate_headers(app_dir, content = '') {
+	content = append_headers(
+		`/${app_dir}/*`,
+		['X-Robots-Tag: noindex', 'Cache-Control: no-cache'],
+		content
+	);
+
+	content = append_headers(
+		`/${app_dir}/immutable/*`,
+		['! Cache-Control', 'Cache-Control: public, immutable, max-age=31536000'],
+		content
+	);
+
+	return content;
 }
 
 /**

packages/adapter-cloudflare/utils.js

@@ -163,3 +163,29 @@ export function get_routes_json(builder, client_assets, redirects, routes) {
 		exclude
 	};
 }
+
+/**
+ * Adds header rules to the contents of a `_headers` file
+ * @param {string} url
+ * @param {string[]} rules
+ * @param {string} content
+ * @returns {string}
+ */
+export function append_headers(url, rules, content) {
+	const regex = new RegExp(`^(${url.replaceAll('*', '\\*')})$`, 'm');
+	const formatted_headers = rules.map((rule) => `  ${rule}`).join('\n');
+
+	// if the URL already exists, just add header rules to it
+	if (regex.test(content)) {
+		return content.replace(regex, `$1\n${formatted_headers}`);
+	}
+
+	// otherwise, we add the url and header rules
+	return `
+${content}
+# === START AUTOGENERATED SVELTE IMMUTABLE HEADERS ===
+${url}
+${formatted_headers}
+# === END AUTOGENERATED SVELTE IMMUTABLE HEADERS ===
+`.trim();
+}