Prevent client to rerender page over SSR error - fixes #710

runtime/src/server/middleware/get_page_handler.ts

@@ -261,7 +261,7 @@ export function get_page_handler(
 				session: session && try_serialize(session, err => {
 					throw new Error(`Failed to serialize session data: ${err.message}`);
 				}),
-				error: error && try_serialize(props.error)
+				error: error && serialize_error(props.error)
 			};
 
 			let script = `__SAPPER__={${[
@@ -366,6 +366,20 @@ function try_serialize(data: any, fail?: (err) => void) {
 	}
 }
 
+// Ensure we return something truthy so the client will not re-render the page over the error
+function serialize_error(error: Error | { message: string }) {
+	if (!error) return null;
+	let serialized = try_serialize(error);
+	if (!serialized) {
+		const { name, message, stack } = error as Error;
+		serialized = try_serialize({ name, message, stack });
+	}
+	if (!serialized) {
+		serialized = '{}';
+	}
+	return serialized;
+}
+
 function escape_html(html: string) {
 	const chars: Record<string, string> = {
 		'"' : 'quot',

test/apps/errors/src/routes/index.svelte

@@ -2,4 +2,5 @@
 
 <a href="nope">nope</a>
 <a href="blog/nope">blog/nope</a>
-<a href="throw">throw</a>
+<a href="throw">throw</a>
+<a href="preload-reject">preload-reject</a>

test/apps/errors/src/routes/preload-reject.svelte

@@ -0,0 +1,7 @@
+<script context="module">
+	export function preload() {
+		return Promise.reject(new Error('boom'))
+	}
+</script>
+
+<h1>Test has failed</h1>

test/apps/errors/test.ts

@@ -85,6 +85,16 @@ describe('errors', function() {
 		);
 	});
 
+	it('does not replace server side rendered error', async () => {
+		await r.load('/preload-reject');
+		await r.sapper.start();
+
+		assert.equal(
+			await r.text('h1'),
+			'500'
+		);
+	});
+
 	it('does not serve error page for explicit non-page errors', async () => {
 		await r.load('/nope.json');