append function redirects after copying existing redirects - fixes #4722 (#4890)

* append function redirects after copying existing redirects - fixes #4722

* Update .changeset/ninety-books-jam.md

* add explanatory comment

Author: Rich-Harris

.changeset/ninety-books-jam.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-netlify': patch
+---
+
+Copy existing `_redirects` file before appending function redirects

packages/adapter-netlify/index.js

@@ -54,6 +54,19 @@ export default function ({ split = false, edge = edge_set_in_env_var } = {}) {
 
 			builder.log.minor(`Publishing to "${publish}"`);
 
+			builder.log.minor('Copying assets...');
+			builder.writeStatic(publish);
+			builder.writeClient(publish);
+			builder.writePrerendered(publish);
+
+			builder.log.minor('Writing custom headers...');
+			const headers_file = join(publish, '_headers');
+			builder.copy('_headers', headers_file);
+			appendFileSync(
+				headers_file,
+				`\n\n/${builder.config.kit.appDir}/*\n  cache-control: public\n  cache-control: immutable\n  cache-control: max-age=31536000\n`
+			);
+
 			// for esbuild, use ESM
 			// for zip-it-and-ship-it, use CJS until https://github.com/netlify/zip-it-and-ship-it/issues/750
 			const esm = netlify_config?.functions?.node_bundler === 'esbuild';
@@ -67,19 +80,6 @@ export default function ({ split = false, edge = edge_set_in_env_var } = {}) {
 			} else {
 				await generate_lambda_functions({ builder, esm, split, publish });
 			}
-
-			builder.log.minor('Copying assets...');
-			builder.writeStatic(publish);
-			builder.writeClient(publish);
-			builder.writePrerendered(publish);
-
-			builder.log.minor('Writing custom headers...');
-			const headers_file = join(publish, '_headers');
-			builder.copy('_headers', headers_file);
-			appendFileSync(
-				headers_file,
-				`\n\n/${builder.config.kit.appDir}/*\n  cache-control: public\n  cache-control: immutable\n  cache-control: max-age=31536000\n`
-			);
 		}
 	};
 }
@@ -230,6 +230,9 @@ function generate_lambda_functions({ builder, publish, split, esm }) {
 		redirects.push('* /.netlify/functions/render 200');
 	}
 
+	// this should happen at the end, after builder.writeStatic(...),
+	// so that generated redirects are appended to custom redirects
+	// rather than replaced by them
 	builder.log.minor('Writing redirects...');
 	const redirect_file = join(publish, '_redirects');
 	if (existsSync('_redirects')) {