ui: unbreak make watch

By telling Webpack to proxy the path `/` through to the Cockroach process.
Webpack used to serve `/` itself from a static file, but as of cockroachdb#25195 we
generate `/` dynamically in Go, so we can use it to pass login state to
the UI.

Fixes cockroachdb#25858

Release note: None

pkg/ui/package.json

@@ -106,7 +106,7 @@
     "typescript": "^2.2.1",
     "url-loader": "^0.5.8",
     "webpack": "^3.5.6",
-    "webpack-dev-server": "^2.7.1"
+    "webpack-dev-server": "^2.11.2"
   },
   "engines": {
     "node": ">=6",

pkg/ui/webpack.app.js

@@ -25,6 +25,16 @@ try {
   DashboardPlugin = class { apply() { /* no-op */ } };
 }
 
+const proxyPrefixes = ["/_admin", "/_status", "/ts", "/_auth"];
+function shouldProxy(reqPath) {
+  if (reqPath === "/") {
+    return true;
+  }
+  return proxyPrefixes.some((prefix) => (
+    reqPath.startsWith(prefix)
+  ));
+}
+
 // tslint:disable:object-literal-sort-keys
 module.exports = (distDir, ...additionalRoots) => ({
   entry: ["./src/index.tsx"],
@@ -116,10 +126,21 @@ module.exports = (distDir, ...additionalRoots) => ({
 
   devServer: {
     contentBase: path.join(__dirname, distDir),
-    proxy: [{
-      context: ["/_admin", "/_status", "/ts", "/_auth"],
-      secure: false,
-      target: process.env.TARGET,
-    }],
+    index: "",
+    proxy: {
+      // Note: this shouldn't require a custom bypass function to work;
+      // docs say that setting `index: ''` is sufficient to proxy `/`.
+      // However, that did not work, and may require upgrading to webpack 4.x.
+      "/": {
+        secure: false,
+        target: process.env.TARGET,
+        bypass: (req) => {
+          if (shouldProxy(req.path)) {
+            return false;
+          }
+          return req.path;
+        },
+      },
+    },
   },
 });