✨ Add backwards compatability for DOM library fetching (#134)

This allows us to use the new CLI with any non-JS SDK right now. This is because
the only breaking changes for those non-js SDKs was:

- Change in server path for where to get the DOM library
- Change in function signature for how the dom library works

This commit adds back the `percy-agent.js` server endpoint (with a deprecation
log), and also adds a wrapper to match the old function signature.

packages/core/src/server.js

@@ -1,5 +1,6 @@
 import fs from 'fs';
 import http from 'http';
+import log from '@percy/logger';
 import pkg from '../package.json';
 
 async function getReply({ version, routes }, request) {
@@ -104,6 +105,16 @@ export default function createPercyServer(percy) {
       .readFile(require.resolve('@percy/dom'), 'utf-8')
       .then(content => [200, 'applicaton/javascript', content]),
 
+    // serves the new DOM library, wrapped for compatability to `@percy/agent`
+    '/percy-agent.js': () => fs.promises
+      .readFile(require.resolve('@percy/dom'), 'utf-8')
+      .then(content => {
+        let wrapper = '(window.PercyAgent = class PercyAgent { snapshot(n, o) { return PercyDOM.serialize(o) } });';
+        log.warn('Warning: `percy-agent.js` is deprecated, please update to the latest SDK version');
+
+        return [200, 'applicaton/javascript', content.concat(wrapper)];
+      }),
+
     // forward snapshot requests
     '/percy/snapshot': ({ body }) => percy.snapshot(body)
       .then(() => [200, 'application/json', { success: true }]),

packages/core/test/server.test.js

@@ -3,6 +3,7 @@ import fetch from 'node-fetch';
 import PercyConfig from '@percy/config';
 import Percy from '../src';
 import pkg from '../package.json';
+import { stdio } from './helpers';
 
 describe('Snapshot Server', () => {
   let percy;
@@ -15,6 +16,7 @@ describe('Snapshot Server', () => {
   });
 
   afterEach(async () => {
+    percy.loglevel('error');
     delete percy.stop; // remove own mocks
     await percy.stop();
   });
@@ -70,6 +72,21 @@ describe('Snapshot Server', () => {
     await expect(response.text()).resolves.toBe(bundle);
   });
 
+  it('serves the legacy percy-agent.js dom bundle', async () => {
+    let bundle = require('fs')
+      .readFileSync(require.resolve('@percy/dom'), { encoding: 'utf-8' })
+      .concat('(window.PercyAgent = class PercyAgent { snapshot(n, o) { return PercyDOM.serialize(o) } });');
+
+    await percy.start();
+    percy.loglevel('warn');
+    let response = await stdio.capture(() => fetch('http://localhost:1337/percy-agent.js'));
+
+    await expect(response.text()).resolves.toBe(bundle);
+    expect(stdio[1]).toEqual([
+      '[percy] Warning: `percy-agent.js` is deprecated, please update to the latest SDK version\n'
+    ]);
+  });
+
   it('has a /stop endpoint that calls #stop()', async () => {
     await percy.start();
     percy.stop = async () => (