Adds support for headless chrome embedder mojo services

headless/lib/renderer/headless_content_renderer_client.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "headless/lib/renderer/headless_content_renderer_client.h"
 
+#include "base/strings/utf_string_conversions.h"
+#include "content/public/renderer/render_frame.h"
+
 namespace headless {
 
 HeadlessContentRendererClient::HeadlessContentRendererClient() {}
 
 HeadlessContentRendererClient::~HeadlessContentRendererClient() {}
 
+void HeadlessContentRendererClient::RunScriptsAtDocumentStart(
+    content::RenderFrame* render_frame) {
+  render_frame->ExecuteJavaScript(base::UTF8ToUTF16(R"(
+      // Shim to let code use define() instead of mojo.define()
+      window.define = (function() {
+        let moduleCache = new Map();
+        return function(name, deps, factory) {
+          let promise = moduleCache.get(name);
+          if (promise === undefined) {
+            // This promise must be cached as mojo.define will only call the
+            // factory function the first time the module is defined.
+            promise = new Promise(resolve => {
+              mojo.define(name, deps, (...modules) => {
+                let result = factory(...modules);
+                resolve(result);
+                return result;
+              });
+            });
+            moduleCache.set(name, promise);
+          }
+          return promise;
+        }
+      })();
+
+      // This code is run before the browser has sent us the mojo bindings so
+      // we need to get fancy and use nested proxy classes to define a promise
+      // to an arbitary window.mojo.services.myModule.myInterface.
+      if (window.hasOwnProperty("mojo")) {
+        let mojoBindings = new Map();
+        let resolvePending = true;
+        window.mojo.services = new Proxy({}, {
+          get: function(target, serviceName) {
+            if (!(serviceName in target)) {
+              var interfaceProxy = new Proxy({}, {
+                get: function(target, interfaceName) {
+                  let name = serviceName + "::" + interfaceName;
+                  let binding = mojoBindings.get(name);
+                  if (binding === undefined) {
+                    binding = {};
+                    binding.promise = new Promise(function(resolve, reject) {
+                      binding.resolve = resolve;
+                      binding.reject = reject;
+                      if (!resolvePending)
+                        reject();
+                    });
+                    mojoBindings.set(name, binding);
+                  }
+                  return binding.promise;
+                },
+                set: function(target, name, value) {
+                  return false;
+                }
+              });
+              target[serviceName] = interfaceProxy;
+              return interfaceProxy;
+            }
+            return target[serviceName];
+          },
+          set: function(target, name, value) {
+            return false;
+          }
+        });
+
+        // Resolve promises for the listed |serviceNames|.
+        window.mojo.resolvePromisesForService = function(serviceNames) {

[Eric Seckler, 2016/06/14 17:50:48]

I'd suggest to rename this function to ..ForServices (plural) to signal that
it's called once for all services rather than multiple times.

[alex clarke (OOO till 29th), 2016/06/14 19:05:08]

Makes sense, done.

+          let numServices = serviceNames.length;
+          for (let i = 0; i < numServices; ++i) {
+            let serviceName = serviceNames[i];
+            // Use mojo to obtain the module binding.
+            define([
+                serviceName,
+                "mojo/public/js/core",
+                "mojo/public/js/router",
+                "content/public/renderer/frame_service_registry",
+                ], function(serviceMojom, mojoCore, routerModule,
+                            serviceProvider) {
+              // A mojom binding may contain bindings for a number of intefaces.

[Eric Seckler, 2016/06/14 17:50:48]

typo (interfaces)

[alex clarke (OOO till 29th), 2016/06/14 19:05:08]

Done.  Note to self install the spell checker on my editor!

+              // Iterate through all of them and resolve any promises.
+              for (var m in serviceMojom) {
+                if (typeof serviceMojom[m] == "object") {
+                  let service = serviceMojom[m];
+                  let binding = mojoBindings.get(service.name);
+                  let interface = new service.proxyClass(
+                      new routerModule.Router(
+                          serviceProvider.connectToService(service.name)));
+                  if (binding === undefined) {
+                    // Store resolved promise in case binding is requested.
+                    mojoBindings.set(
+                        service.name, {"promise": Promise.resolve(interface)});
+                  } else {
+                    binding.resolve(interface);
+                    mojoBindings.delete(service.name);

[Eric Seckler, 2016/06/14 17:50:48]

I might be missing something, but if you delete the binding here, does that mean
that any later use of the service (through e.g.
mojo.services.embedder_test.TestEmbedderService.then()) will fail? It seems to
me that you're supporting resolving-before-use in the if-branch directly above,
how come we don't expect a later use here?

[alex clarke (OOO till 29th), 2016/06/14 19:05:08]

Mmm yes this is a bad idea.  What I really want to to (I think) is remove the
reject attribute because I don't want the code below to try to reject it.

+                  }
+                }
+              }
+            });
+          }
+          // Reject any remaining promises.
+          for (let [key, value] of mojoBindings) {
+            if ("reject" in value) {
+              value.reject();
+            }
+          }
+          // Reject any subsequent unknown properties.
+          resolvePending = false;
+        };
+      } )"));
+}
+
 }  // namespace headless