Ensure tests don't depend on fake devices being added synchronously

third_party/WebKit/LayoutTests/usb/resources/usb-helpers.js

 'use strict';
 
 function usb_test(func, name, properties) {
   promise_test(() => {
     return navigator.usb.test.initialize().then(() => {
       let testResult = func();
       let cleanup = () => {
         navigator.usb.test.reset();
       };
       testResult.then(cleanup, cleanup);
       return testResult;
     });
   }, name, properties);
 }
 
+// Returns a promise that is resolved when the next USBConnectionEvent of the
+// given type is received.
+function connectionEventPromise(eventType) {
+  return new Promise(resolve => {
+    let eventHandler = e => {
+      assert_true(e instanceof USBConnectionEvent);
+      navigator.usb.removeEventListener(eventType, eventHandler);
+      resolve(e.device);
+    };
+    navigator.usb.addEventListener(eventType, eventHandler);
+  });
+}
+
+// Returns a promise that is resolved with an object containing a fake
+// USBDevice and its guid (for calling navigator.usb.test.removeFakeDevice().

[ortuno, 2017/04/18 00:44:22]

optional:

// Creates a fake device and returns a promise that resolves once the
// 'connect' event is fired for the fake device. The promise is resolved with
// an object containing a USBDevice that corresponds to the fake USB Device
// and the device's guid.

[Reilly Grant (use Gerrit), 2017/04/18 01:27:52]

Done.

+function getFakeDeviceAndGuid() {
+  let promise = connectionEventPromise('connect');
+  let guid = navigator.usb.test.addFakeDevice(fakeDeviceInit);
+  return promise.then(device => {
+    return { device: device, guid: guid };

[ortuno, 2017/04/18 00:44:22]

optional: Would it make sense/be worth it to return a FakeUSBDevice rather than
the guid. Then tests could call disconnect() directly on that object instead of
passing a guid around.

[Reilly Grant (use Gerrit), 2017/04/18 01:27:52]

Done.

+  });
+}
+
+// Returns a promise that is resolved with just a fake USBDevice.
+function getFakeDevice() {
+  return getFakeDeviceAndGuid().then(({ device, guid }) => {
+    return device;
+  });
+}
+
+// Disconnects the given device and returns a promise that is resolved when it
+// is done.
+function waitForDisconnect(guid) {
+  let promise = connectionEventPromise('disconnect');
+  navigator.usb.test.removeFakeDevice(guid);
+  return promise;
+}
+
 function assertRejectsWithError(promise, name, message) {
   return promise.then(() => {
     assert_unreached('expected promise to reject with ' + name);
   }, error => {
     assert_equals(error.name, name);
     if (message !== undefined)
       assert_equals(error.message, message);
   });
 }
 
@@ skipping 30 matching lines @@
   });
 }
 
 function runGarbageCollection() {
   // Run gc() as a promise.
   return new Promise((resolve, reject) => {
     GCController.collect();
     setTimeout(resolve, 0);
   });
 }