[CrOS MultiDevice] Sort eligible devices before returning them.

This CL sorts returned devices from most-recently-updated to
least-recently-updated. This increases the likelihood that users who go
through multi-device setup will set up the correct device, since usually
the most-recently-updated the device is the one that users want to set.

Bug: 870133
Change-Id: I954f3bde8ccb62c1f796ec62e8f12af65012e528
Reviewed-on: https://chromium-review.googlesource.com/c/1285769Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600426}

chromeos/services/multidevice_setup/multidevice_setup_impl.cc

@@ -157,6 +157,17 @@ void MultiDeviceSetupImpl::GetEligibleHostDevices(
     eligible_remote_devices.push_back(remote_device_ref.GetRemoteDevice());
   }
 
+  // Sort from most-recently-updated to least-recently-updated. The timestamp
+  // used is provided by the back-end and indicates the last time at which the
+  // device's metadata was updated on the server. Note that this does not
+  // provide us with the last time that a user actually used this device, but it
+  // is a good estimate.
+  std::sort(eligible_remote_devices.begin(), eligible_remote_devices.end(),
+            [](const auto& first_device, const auto& second_device) {
+              return first_device.last_update_time_millis >
+                     second_device.last_update_time_millis;
+            });
+
   std::move(callback).Run(eligible_remote_devices);
 }
 

chromeos/services/multidevice_setup/multidevice_setup_impl_unittest.cc

@@ -1218,6 +1218,22 @@ TEST_F(MultiDeviceSetupImplTest, TestSetHostDeviceWithoutAuthToken) {
       test_devices()[0], observer.get(), 1u /* expected_observer_index */);
 }
 
+TEST_F(MultiDeviceSetupImplTest, GetEligibleHostDevicesSortedByTimestamp) {
+  cryptauth::RemoteDeviceRefList devices = test_devices();
+
+  // Update the devices in the list such that they are sorted from
+  // least-recently-updated to most-recently-updated. This is the opposite order
+  // that is expected from GetEligibleDevices();
+  for (size_t i = 0; i < devices.size(); ++i)
+    GetMutableRemoteDevice(devices[i])->last_update_time_millis = i;
+  fake_eligible_host_devices_provider()->set_eligible_host_devices(devices);
+
+  // Now, reverse the original list and verify that the returned devices are
+  // sorted in the correct order.
+  std::reverse(devices.begin(), devices.end());
+  EXPECT_EQ(RefListToRawList(devices), CallGetEligibleHostDevices());
+}
+
 }  // namespace multidevice_setup
 
 }  // namespace chromeos