[7.3] [basePathProxy] add __UNSAFE_bypassBasePath route (#4120… (#42451)

docs/development/core/development-basepath.asciidoc

@@ -77,6 +77,15 @@ To accomplish this the `serve` task does a few things:
   - redirects from `/{any}/app/{appName}` to `/{randomBasePath}/app/{appName}` so that refreshes should work
   - proxies all requests starting with `/{randomBasePath}/` to the Kibana server
 
+If you're writing scripts that interact with the Kibana API, the base path proxy will likely
+make this difficult. To bypass the base path proxy for a single request, prefix urls with
+`__UNSAFE_bypassBasePath` and the request will be routed to the development Kibana server.
+
+["source","shell"]
+-----------
+curl "http://elastic:changeme@localhost:5601/__UNSAFE_bypassBasePath/api/status"
+-----------
+
 This proxy can sometimes have unintended side effects in development, so when
 needed you can opt out by passing the `--no-base-path` flag to the `serve` task
 or `yarn start`.

src/core/server/http/base_path_proxy_server.ts

@@ -18,7 +18,8 @@
  */
 
 import { ByteSizeValue } from '@kbn/config-schema';
-import { Server } from 'hapi';
+import { Server, Request } from 'hapi';
+import Url from 'url';
 import { Agent as HttpsAgent, ServerOptions as TlsOptions } from 'https';
 import { sample } from 'lodash';
 import { DevConfig } from '../dev';
@@ -145,6 +146,38 @@ export class BasePathProxyServer {
       path: `${this.httpConfig.basePath}/{kbnPath*}`,
     });
 
+    this.server.route({
+      handler: {
+        proxy: {
+          agent: this.httpsAgent,
+          passThrough: true,
+          xforward: true,
+          mapUri: (request: Request) => ({
+            uri: Url.format({
+              hostname: request.server.info.host,
+              port: this.devConfig.basePathProxyTargetPort,
+              protocol: request.server.info.protocol,
+              pathname: `${this.httpConfig.basePath}/${request.params.kbnPath}`,
+              query: request.query,
+            }),
+            headers: request.headers,
+          }),
+        },
+      },
+      method: '*',
+      options: {
+        pre: [
+          // Before we proxy request to a target port we may want to wait until some
+          // condition is met (e.g. until target listener is ready).
+          async (request, responseToolkit) => {
+            await blockUntil();
+            return responseToolkit.continue;
+          },
+        ],
+      },
+      path: `/__UNSAFE_bypassBasePath/{kbnPath*}`,
+    });
+
     // It may happen that basepath has changed, but user still uses the old one,
     // so we can try to check if that's the case and just redirect user to the
     // same URL, but with valid basepath.