backend: add and use for external URL discovery

plugins/proxy-backend/src/service/router.test.ts

@@ -17,16 +17,20 @@
 import { createRouter } from './router';
 import * as winston from 'winston';
 import { ConfigReader } from '@backstage/config';
-import { loadBackendConfig } from '@backstage/backend-common';
+import {
+  loadBackendConfig,
+  SingleHostDiscovery,
+} from '@backstage/backend-common';
 
 describe('createRouter', () => {
   it('works', async () => {
     const logger = winston.createLogger();
     const config = ConfigReader.fromConfigs(await loadBackendConfig());
+    const discovery = SingleHostDiscovery.fromConfig(config);
     const router = await createRouter({
       config,
       logger,
-      pathPrefix: '/proxy',
+      discovery,
     });
     expect(router).toBeDefined();
   });

plugins/proxy-backend/src/service/router.ts

@@ -23,12 +23,12 @@ import createProxyMiddleware, {
 } from 'http-proxy-middleware';
 import { Logger } from 'winston';
 import http from 'http';
+import { PluginEndpointDiscovery } from '@backstage/backend-common';
 
 export interface RouterOptions {
   logger: Logger;
   config: Config;
-  // The URL path prefix that the router itself is mounted as, commonly "/proxy"
-  pathPrefix: string;
+  discovery: PluginEndpointDiscovery;
 }
 
 export interface ProxyConfig extends ProxyMiddlewareConfig {
@@ -76,16 +76,14 @@ export async function createRouter(
 ): Promise<express.Router> {
   const router = Router();
 
+  const externalUrl = await options.discovery.getExternalBaseUrl('proxy');
+  const { pathname: pathPrefix } = new URL(externalUrl);
+
   const proxyConfig = options.config.getOptional('proxy') ?? {};
   Object.entries(proxyConfig).forEach(([route, proxyRouteConfig]) => {
     router.use(
       route,
-      buildMiddleware(
-        options.pathPrefix,
-        options.logger,
-        route,
-        proxyRouteConfig,
-      ),
+      buildMiddleware(pathPrefix, options.logger, route, proxyRouteConfig),
     );
   });
 

plugins/proxy-backend/src/service/standaloneServer.ts

@@ -17,6 +17,7 @@
 import {
   createServiceBuilder,
   loadBackendConfig,
+  SingleHostDiscovery,
 } from '@backstage/backend-common';
 import { Server } from 'http';
 import { Logger } from 'winston';
@@ -37,10 +38,11 @@ export async function startStandaloneServer(
   logger.debug('Creating application...');
 
   const config = ConfigReader.fromConfigs(await loadBackendConfig());
+  const discovery = SingleHostDiscovery.fromConfig(config);
   const router = await createRouter({
     config,
     logger,
-    pathPrefix: '/proxy',
+    discovery,
   });
   const service = createServiceBuilder(module)
     .enableCors({ origin: 'http://localhost:3000' })