[CrOS MultiDevice] Reserve 'existing user' notifications for verified hosts

This CL adapts the MultiDevice notification functionality to the new
spec that only considers a new phone added or switched to once it is
verified rather than just set.

I added tests for this behavior for both of the 'existing host' events
and an extra test for the edge case of a set (but unverified) host
Phone A is replace by a different host Phone B that is verified.

Bug: 891822
Change-Id: Iebd9d0d209b020602a284de200fae168f34759c2
Reviewed-on: https://chromium-review.googlesource.com/c/1263425
Commit-Queue: Jordy Greenblatt <jordynass@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597628}

chromeos/services/multidevice_setup/account_status_change_delegate_notifier_impl.cc

@@ -13,9 +13,6 @@
 #include "chromeos/components/proximity_auth/logging/logging.h"
 #include "chromeos/services/multidevice_setup/host_status_provider_impl.h"
 #include "chromeos/services/multidevice_setup/setup_flow_completion_recorder.h"
-#include "components/cryptauth/proto/cryptauth_api.pb.h"
-#include "components/cryptauth/remote_device_ref.h"
-#include "components/cryptauth/software_feature_state.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/pref_service.h"
 
@@ -76,7 +73,7 @@ void AccountStatusChangeDelegateNotifierImpl::RegisterPrefs(
   registry->RegisterInt64Pref(kExistingUserChromebookAddedPrefName,
                               kTimestampNotSet);
   registry->RegisterStringPref(
-      kHostDeviceIdFromMostRecentHostStatusUpdatePrefName, kNoHost);
+      kVerifiedHostDeviceIdFromMostRecentHostStatusUpdatePrefName, kNoHost);
 }
 
 AccountStatusChangeDelegateNotifierImpl::
@@ -103,9 +100,12 @@ const char AccountStatusChangeDelegateNotifierImpl::
     kExistingUserChromebookAddedPrefName[] =
         "multidevice_setup.existing_user_chromebook_added";
 
+// Note that, despite the pref string name, this pref only records the IDs of
+// verified hosts. In particular, if a host has been set but is waiting for
+// verification, it will not recorded.
 // static
 const char AccountStatusChangeDelegateNotifierImpl::
-    kHostDeviceIdFromMostRecentHostStatusUpdatePrefName[] =
+    kVerifiedHostDeviceIdFromMostRecentHostStatusUpdatePrefName[] =
         "multidevice_setup.host_device_id_from_most_recent_sync";
 
 AccountStatusChangeDelegateNotifierImpl::
@@ -118,7 +118,7 @@ AccountStatusChangeDelegateNotifierImpl::
       pref_service_(pref_service),
       setup_flow_completion_recorder_(setup_flow_completion_recorder),
       clock_(clock) {
-  host_device_id_from_most_recent_update_ =
+  verified_host_device_id_from_most_recent_update_ =
       LoadHostDeviceIdFromEndOfPreviousSession();
   host_status_provider_->AddObserver(this);
 }
@@ -137,27 +137,30 @@ void AccountStatusChangeDelegateNotifierImpl::CheckForMultiDeviceEvents(
     return;
   }
 
-  // Track and update host info.
-  base::Optional<std::string> host_device_id_before_update =
-      host_device_id_from_most_recent_update_;
+  // Track and update verified host info.
+  base::Optional<std::string> verified_host_device_id_before_update =
+      verified_host_device_id_from_most_recent_update_;
 
-  // Check if a host has been set.
-  if (host_status_with_device.host_device()) {
-    host_device_id_from_most_recent_update_ =
+  // Check if a host has been verified.
+  if (host_status_with_device.host_status() ==
+      mojom::HostStatus::kHostVerified) {
+    verified_host_device_id_from_most_recent_update_ =
         host_status_with_device.host_device()->GetDeviceId();
     pref_service_->SetString(
-        kHostDeviceIdFromMostRecentHostStatusUpdatePrefName,
-        *host_device_id_from_most_recent_update_);
+        kVerifiedHostDeviceIdFromMostRecentHostStatusUpdatePrefName,
+        *verified_host_device_id_from_most_recent_update_);
   } else {
     // No host set.
-    host_device_id_from_most_recent_update_.reset();
+    verified_host_device_id_from_most_recent_update_.reset();
+    pref_service_->SetString(
+        kVerifiedHostDeviceIdFromMostRecentHostStatusUpdatePrefName, kNoHost);
   }
 
   CheckForNewUserPotentialHostExistsEvent(host_status_with_device);
   CheckForExistingUserHostSwitchedEvent(host_status_with_device,
-                                        host_device_id_before_update);
-  CheckForExistingUserChromebookAddedEvent(host_status_with_device,
-                                           host_device_id_before_update);
+                                        verified_host_device_id_before_update);
+  CheckForExistingUserChromebookAddedEvent(
+      host_status_with_device, verified_host_device_id_before_update);
 }
 
 void AccountStatusChangeDelegateNotifierImpl::
@@ -165,7 +168,7 @@ void AccountStatusChangeDelegateNotifierImpl::
         const HostStatusProvider::HostStatusWithDevice&
             host_status_with_device) {
   // We only check for new user events if there is no enabled host.
-  if (host_device_id_from_most_recent_update_)
+  if (verified_host_device_id_from_most_recent_update_)
     return;
 
   // If the observer has been notified of this event before, the user is not
@@ -190,14 +193,17 @@ void AccountStatusChangeDelegateNotifierImpl::
 void AccountStatusChangeDelegateNotifierImpl::
     CheckForExistingUserHostSwitchedEvent(
         const HostStatusProvider::HostStatusWithDevice& host_status_with_device,
-        const base::Optional<std::string>& host_device_id_before_update) {
-  // The host switched event requires both a pre-update and a post-update host.
-  if (!host_device_id_from_most_recent_update_ ||
-      !host_device_id_before_update) {
+        const base::Optional<std::string>&
+            verified_host_device_id_before_update) {
+  // The host switched event requires both a pre-update and a post-update
+  // verified host.
+  if (!verified_host_device_id_from_most_recent_update_ ||
+      !verified_host_device_id_before_update) {
     return;
   }
   // If the host stayed the same, there was no switch.
-  if (*host_device_id_from_most_recent_update_ == *host_device_id_before_update)
+  if (*verified_host_device_id_from_most_recent_update_ ==
+      *verified_host_device_id_before_update)
     return;
 
   delegate()->OnConnectedHostSwitchedForExistingUser(
@@ -209,11 +215,13 @@ void AccountStatusChangeDelegateNotifierImpl::
 void AccountStatusChangeDelegateNotifierImpl::
     CheckForExistingUserChromebookAddedEvent(
         const HostStatusProvider::HostStatusWithDevice& host_status_with_device,
-        const base::Optional<std::string>& host_device_id_before_update) {
-  // The Chromebook added event requires that a set host was found by the
-  // update, i.e. there was no host before the host status update but afterward
-  // there is a set host.
-  if (!host_device_id_from_most_recent_update_ || host_device_id_before_update)
+        const base::Optional<std::string>&
+            verified_host_device_id_before_update) {
+  // The Chromebook added event requires that a verified host was found by the
+  // update, i.e. there was no verified host before the host status update but
+  // afterward there was a verified host.
+  if (!verified_host_device_id_from_most_recent_update_ ||
+      verified_host_device_id_before_update)
     return;
 
   delegate()->OnNewChromebookAddedForExistingUser(
@@ -224,11 +232,12 @@ void AccountStatusChangeDelegateNotifierImpl::
 
 base::Optional<std::string> AccountStatusChangeDelegateNotifierImpl::
     LoadHostDeviceIdFromEndOfPreviousSession() {
-  std::string host_device_id_from_most_recent_update = pref_service_->GetString(
-      kHostDeviceIdFromMostRecentHostStatusUpdatePrefName);
-  if (host_device_id_from_most_recent_update.empty())
+  std::string verified_host_device_id_from_most_recent_update =
+      pref_service_->GetString(
+          kVerifiedHostDeviceIdFromMostRecentHostStatusUpdatePrefName);
+  if (verified_host_device_id_from_most_recent_update.empty())
     return base::nullopt;
-  return host_device_id_from_most_recent_update;
+  return verified_host_device_id_from_most_recent_update;
 }
 
 }  // namespace multidevice_setup

chromeos/services/multidevice_setup/account_status_change_delegate_notifier_impl.h

@@ -62,7 +62,8 @@ class AccountStatusChangeDelegateNotifierImpl
   static const char kExistingUserHostSwitchedPrefName[];
   static const char kExistingUserChromebookAddedPrefName[];
 
-  static const char kHostDeviceIdFromMostRecentHostStatusUpdatePrefName[];
+  static const char
+      kVerifiedHostDeviceIdFromMostRecentHostStatusUpdatePrefName[];
 
   AccountStatusChangeDelegateNotifierImpl(
       HostStatusProvider* host_status_provider,
@@ -84,17 +85,17 @@ class AccountStatusChangeDelegateNotifierImpl
       const HostStatusProvider::HostStatusWithDevice& host_status_with_device);
   void CheckForExistingUserHostSwitchedEvent(
       const HostStatusProvider::HostStatusWithDevice& host_status_with_device,
-      const base::Optional<std::string>& host_device_id_before_update);
+      const base::Optional<std::string>& verified_host_device_id_before_update);
   void CheckForExistingUserChromebookAddedEvent(
       const HostStatusProvider::HostStatusWithDevice& host_status_with_device,
-      const base::Optional<std::string>& host_device_id_before_update);
+      const base::Optional<std::string>& verified_host_device_id_before_update);
 
   // Loads data from previous session using PrefService.
   base::Optional<std::string> LoadHostDeviceIdFromEndOfPreviousSession();
 
   // Set to base::nullopt if there was no enabled host in the most recent
   // host status update.
-  base::Optional<std::string> host_device_id_from_most_recent_update_;
+  base::Optional<std::string> verified_host_device_id_from_most_recent_update_;
 
   mojom::AccountStatusChangeDelegatePtr delegate_ptr_;
   HostStatusProvider* host_status_provider_;

chromeos/services/multidevice_setup/account_status_change_delegate_notifier_impl_unittest.cc

@@ -15,6 +15,7 @@
 #include "chromeos/services/multidevice_setup/fake_host_status_provider.h"
 #include "chromeos/services/multidevice_setup/fake_setup_flow_completion_recorder.h"
 #include "chromeos/services/multidevice_setup/public/mojom/multidevice_setup.mojom.h"
+#include "components/cryptauth/remote_device_ref.h"
 #include "components/cryptauth/remote_device_test_util.h"
 #include "components/sync_preferences/testing_pref_service_syncable.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -90,7 +91,7 @@ class MultiDeviceSetupAccountStatusChangeDelegateNotifierTest
         cryptauth::RemoteDevice::GenerateDeviceId(old_host_key);
     test_pref_service_->SetString(
         AccountStatusChangeDelegateNotifierImpl::
-            kHostDeviceIdFromMostRecentHostStatusUpdatePrefName,
+            kVerifiedHostDeviceIdFromMostRecentHostStatusUpdatePrefName,
         old_host_device_id);
   }
 
@@ -112,6 +113,12 @@ class MultiDeviceSetupAccountStatusChangeDelegateNotifierTest
             kExistingUserChromebookAddedPrefName);
   }
 
+  std::string GetMostRecentVerifiedHostDeviceIdPref() {
+    return test_pref_service_->GetString(
+        AccountStatusChangeDelegateNotifierImpl::
+            kVerifiedHostDeviceIdFromMostRecentHostStatusUpdatePrefName);
+  }
+
   FakeAccountStatusChangeDelegate* fake_delegate() {
     return fake_delegate_.get();
   }
@@ -216,35 +223,35 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
        NotifiesObserverForHostSwitchEvents) {
   BuildAccountStatusChangeDelegateNotifier();
   SetAccountStatusChangeDelegatePtr();
-  // Verify the delegate initializes to 0.
+  // Check the delegate initializes to 0.
   EXPECT_EQ(0u,
             fake_delegate()->num_existing_user_host_switched_events_handled());
 
-  // Set initial host.
+  // Set initially verified host.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
   // Host was set but has never been switched.
   EXPECT_EQ(0u,
             fake_delegate()->num_existing_user_host_switched_events_handled());
 
-  // Switch hosts.
+  // Switch to new verified host.
   SetHostWithStatus(mojom::HostStatus::kHostVerified,
                     cryptauth::RemoteDeviceRefBuilder()
-                        .SetPublicKey(kFakePhoneKey + "#1")
-                        .SetName(kFakePhoneName + "#1")
+                        .SetPublicKey(kFakePhoneKey + "A")
+                        .SetName(kFakePhoneName + "A")
                         .Build());
   EXPECT_EQ(1u,
             fake_delegate()->num_existing_user_host_switched_events_handled());
 
-  // Switch to another new host.
+  // Switch to a different new verified host.
   SetHostWithStatus(mojom::HostStatus::kHostVerified,
                     cryptauth::RemoteDeviceRefBuilder()
-                        .SetPublicKey(kFakePhoneKey + "#2")
-                        .SetName(kFakePhoneName + "#2")
+                        .SetPublicKey(kFakePhoneKey + "B")
+                        .SetName(kFakePhoneName + "B")
                         .Build());
   EXPECT_EQ(2u,
             fake_delegate()->num_existing_user_host_switched_events_handled());
 
-  // Switch back to initial host.
+  // Switch back to initial host (verified).
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
   EXPECT_EQ(3u,
             fake_delegate()->num_existing_user_host_switched_events_handled());
@@ -254,7 +261,7 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
        SettingSameHostTriggersNoHostSwitchedEvent) {
   BuildAccountStatusChangeDelegateNotifier();
   SetAccountStatusChangeDelegatePtr();
-  // Set initial host.
+  // Set initially verified host.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
   // Set to host with identical information.
   SetHostWithStatus(mojom::HostStatus::kHostVerified,
@@ -270,9 +277,9 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
        ChangingHostDevicesTriggersHostSwitchEventWhenHostNameIsUnchanged) {
   BuildAccountStatusChangeDelegateNotifier();
   SetAccountStatusChangeDelegatePtr();
-  // Set initial host.
+  // Set initially verified host.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
-  // Set to host with same name but different key.
+  // Set to verified host with same name but different key.
   SetHostWithStatus(mojom::HostStatus::kHostVerified,
                     cryptauth::RemoteDeviceRefBuilder()
                         .SetPublicKey(kFakePhoneKey + "alternate")
@@ -283,13 +290,70 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
 }
 
 TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
-       VerifyingHostTriggersNoHostSwtichEvent) {
+       VerifyingSetHostTriggersNoHostSwtichEvent) {
   BuildAccountStatusChangeDelegateNotifier();
   SetAccountStatusChangeDelegatePtr();
-  // Set initial host but do not verify
+  // Set initial host but do not verify.
   SetHostWithStatus(mojom::HostStatus::kHostSetButNotYetVerified, kFakePhone);
-  // Verify host
+  // Verify host.
+  SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
+  EXPECT_EQ(0u,
+            fake_delegate()->num_existing_user_host_switched_events_handled());
+}
+
+TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
+       OnlyVerifiedHostCausesHostSwitchedEvent) {
+  BuildAccountStatusChangeDelegateNotifier();
+  SetAccountStatusChangeDelegatePtr();
+  // Set initially verified host.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
+
+  EXPECT_EQ(0u,
+            fake_delegate()->num_existing_user_host_switched_events_handled());
+
+  // Set a new host without verifying.
+  SetHostWithStatus(mojom::HostStatus::kHostSetButNotYetVerified,
+                    cryptauth::RemoteDeviceRefBuilder()
+                        .SetPublicKey(kFakePhoneKey + "A")
+                        .SetName(kFakePhoneName + "A")
+                        .Build());
+  EXPECT_EQ(0u,
+            fake_delegate()->num_existing_user_host_switched_events_handled());
+
+  // Set a different new host without confirming with backend so host is
+  // unverified.
+  SetHostWithStatus(
+      mojom::HostStatus::kHostSetLocallyButWaitingForBackendConfirmation,
+      cryptauth::RemoteDeviceRefBuilder()
+          .SetPublicKey(kFakePhoneKey + "B")
+          .SetName(kFakePhoneName + "B")
+          .Build());
+  EXPECT_EQ(0u,
+            fake_delegate()->num_existing_user_host_switched_events_handled());
+}
+
+TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
+       ForgettingAndThenSwitchingHostsDoesNotTriggerHostSwitchedEvent) {
+  BuildAccountStatusChangeDelegateNotifier();
+  SetAccountStatusChangeDelegatePtr();
+  // Set initially verified host.
+  SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
+
+  EXPECT_EQ(0u,
+            fake_delegate()->num_existing_user_host_switched_events_handled());
+
+  // Forget host.
+  SetHostWithStatus(mojom::HostStatus::kEligibleHostExistsButNoHostSet,
+                    base::nullopt /* host_device */);
+  EXPECT_EQ(0u,
+            fake_delegate()->num_existing_user_host_switched_events_handled());
+
+  // Set a new verified host.
+  SetHostWithStatus(mojom::HostStatus::kHostVerified,
+                    cryptauth::RemoteDeviceRefBuilder()
+                        .SetPublicKey(kFakePhoneKey + "alternate")
+                        .SetName(kFakePhoneName + "alternate")
+                        .Build());
   EXPECT_EQ(0u,
             fake_delegate()->num_existing_user_host_switched_events_handled());
 }
@@ -298,7 +362,7 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
        HostSwitchedBetweenSessions) {
   SetHostFromPreviousSession(kFakePhoneKey + "-old");
   BuildAccountStatusChangeDelegateNotifier();
-  // Host switched between sessions.
+  // Host switched and verified between sessions.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
   SetAccountStatusChangeDelegatePtr();
   EXPECT_EQ(1u,
@@ -312,7 +376,7 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
   // No enabled host initially.
   SetHostWithStatus(mojom::HostStatus::kEligibleHostExistsButNoHostSet,
                     base::nullopt /* host_device */);
-  // Set host.
+  // Set and verify host.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
   EXPECT_EQ(0u,
             fake_delegate()->num_existing_user_host_switched_events_handled());
@@ -321,7 +385,7 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
 TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
        NoHostSwitchedEventWithoutObserverSet) {
   BuildAccountStatusChangeDelegateNotifier();
-  // Set initial host.
+  // Set initially verified host.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
   // All conditions for host switched event are satisfied except for setting a
   // delegate.
@@ -338,20 +402,66 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
        NotifiesObserverForChromebookAddedEvents) {
   BuildAccountStatusChangeDelegateNotifier();
   SetAccountStatusChangeDelegatePtr();
-  // Verify the delegate initializes to 0.
+  // Check the delegate initializes to 0.
   EXPECT_EQ(
       0u, fake_delegate()->num_existing_user_chromebook_added_events_handled());
 
-  // Host is set from another Chromebook while this one is logged in.
+  // Host is set and verified from another Chromebook while this one is logged
+  // in.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
   EXPECT_EQ(
       1u, fake_delegate()->num_existing_user_chromebook_added_events_handled());
 }
 
+TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
+       OnlyVerifiedHostCausesChromebookAddedEvent) {
+  BuildAccountStatusChangeDelegateNotifier();
+  SetAccountStatusChangeDelegatePtr();
+  // Start with potential hosts but none set.
+  SetHostWithStatus(mojom::HostStatus::kEligibleHostExistsButNoHostSet,
+                    base::nullopt /* host_device */);
+
+  // Set a host without verifying.
+  SetHostWithStatus(mojom::HostStatus::kHostSetButNotYetVerified, kFakePhone);
+  EXPECT_EQ(
+      0u, fake_delegate()->num_existing_user_chromebook_added_events_handled());
+
+  // Verify the new host.
+  SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
+  EXPECT_EQ(
+      1u, fake_delegate()->num_existing_user_chromebook_added_events_handled());
+}
+
+TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
+       ReplacingUnverifiedHostAWithVerifiedHostBCausesChromebookAddedEvent) {
+  BuildAccountStatusChangeDelegateNotifier();
+  SetAccountStatusChangeDelegatePtr();
+  // Start with potential hosts but none set.
+  // Set initial host but do not verify.
+  SetHostWithStatus(mojom::HostStatus::kHostSetButNotYetVerified,
+                    cryptauth::RemoteDeviceRefBuilder()
+                        .SetPublicKey(kFakePhoneKey + "A")
+                        .SetName(kFakePhoneName + "A")
+                        .Build());
+
+  // Replace unverified Phone A with verified Phone B.
+  SetHostWithStatus(mojom::HostStatus::kHostVerified,
+                    cryptauth::RemoteDeviceRefBuilder()
+                        .SetPublicKey(kFakePhoneKey + "B")
+                        .SetName(kFakePhoneName + "B")
+                        .Build());
+  // This causes a 'Chromebook added' event.
+  EXPECT_EQ(
+      1u, fake_delegate()->num_existing_user_chromebook_added_events_handled());
+  // It does not cause a 'host switched' event.
+  EXPECT_EQ(0u,
+            fake_delegate()->num_existing_user_host_switched_events_handled());
+}
+
 TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
        ChromebookAddedBetweenSessionsTriggersEvents) {
   BuildAccountStatusChangeDelegateNotifier();
-  // Host is set before this Chromebook is logged in.
+  // Host is set and verified before this Chromebook is logged in.
   SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
 
   SetAccountStatusChangeDelegatePtr();
@@ -387,6 +497,40 @@ TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
       0u, fake_delegate()->num_existing_user_chromebook_added_events_handled());
 }
 
+TEST_F(MultiDeviceSetupAccountStatusChangeDelegateNotifierTest,
+       VerifiedHostIdStaysUpToDateInPrefs) {
+  BuildAccountStatusChangeDelegateNotifier();
+  SetAccountStatusChangeDelegatePtr();
+  // Check the delegate initializes to empty.
+  EXPECT_EQ(GetMostRecentVerifiedHostDeviceIdPref(), "");
+
+  // Set initially verified host.
+  SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhone);
+  EXPECT_EQ(GetMostRecentVerifiedHostDeviceIdPref(), kFakePhone.GetDeviceId());
+
+  const cryptauth::RemoteDeviceRef kFakePhoneAlternate =
+      cryptauth::RemoteDeviceRefBuilder()
+          .SetPublicKey(kFakePhoneKey + "alternate")
+          .SetName(kFakePhoneName + "alternate")
+          .Build();
+
+  // Switch to an unverified host.
+  SetHostWithStatus(mojom::HostStatus::kHostSetButNotYetVerified,
+                    kFakePhoneAlternate);
+  // The host is set but not verified so the pref should be set to empty.
+  EXPECT_EQ(GetMostRecentVerifiedHostDeviceIdPref(), "");
+
+  // Verify the new host.
+  SetHostWithStatus(mojom::HostStatus::kHostVerified, kFakePhoneAlternate);
+  EXPECT_EQ(GetMostRecentVerifiedHostDeviceIdPref(),
+            kFakePhoneAlternate.GetDeviceId());
+
+  // Forget host.
+  SetHostWithStatus(mojom::HostStatus::kEligibleHostExistsButNoHostSet,
+                    base::nullopt /* host_device */);
+  EXPECT_EQ(GetMostRecentVerifiedHostDeviceIdPref(), "");
+}
+
 }  // namespace multidevice_setup
 
 }  // namespace chromeos