Fixes path_prefix flag

PR tensorflow#1512 introduced a bug where it formed a complete path without using
existing pathname. Previously without forming the complete path, it was
treated as a relative path from current window.location.pathname.

Author: stephanwlee

tensorboard/components/tf_backend/router.ts

@@ -28,10 +28,10 @@ export interface Router {
  * Create a router for communicating with the TensorBoard backend. You
  * can pass this to `setRouter` to make it the global router.
  *
- * @param pathPrefix {string=} The base prefix for data endpoints.
+ * @param pathPrefix {string} The base prefix for data endpoints.
  * @param demoMode {boolean=} Whether to modify urls for filesystem demo usage.
  */
-export function createRouter(pathPrefix = 'data', demoMode = false): Router {
+export function createRouter(pathPrefix, demoMode = false): Router {
   if (pathPrefix[pathPrefix.length - 1] === '/') {
     pathPrefix = pathPrefix.slice(0, pathPrefix.length - 1);
   }
@@ -62,7 +62,13 @@ export function createRouter(pathPrefix = 'data', demoMode = false): Router {
   };
 };
 
-let _router: Router = createRouter();
+export function getDefaultRouter(): Router {
+  const sep = window.location.pathname.endsWith('/') ? '' : '/';
+  const pathPrefix = window.location.pathname + sep + 'data';
+  return createRouter(pathPrefix);
+}
+
+let _router: Router = getDefaultRouter();
 
 /**
  * @return {Router} the global router

tensorboard/components/tf_backend/test/backendTests.ts

@@ -81,6 +81,23 @@ describe('backend tests', () => {
       assert.equal(r.runs(), '/foo/runs');
     });
 
+    describe('default router', () => {
+      beforeEach(function() {
+        this.router = getDefaultRouter();
+      });
+
+      it('makes sure test is not degenerate', function() {
+        // Test becomes degenerate if location.pathname is empty.
+        assert.notEqual('/', window.location.pathname);
+      });
+
+      it('creates a URL including the pathname', function() {
+        assert.equal(
+            this.router.runs(),
+            `${window.location.pathname}/data/runs`);
+      });
+    });
+
     describe('prod mode', () => {
       beforeEach(function() {
         this.router = createRouter(base, /*demoMode=*/false);