added validation that prohibits last tenant admin deletion

application/src/main/java/org/thingsboard/server/controller/UserController.java

@@ -266,6 +266,9 @@ public class UserController extends BaseController {
         if (user.getAuthority() == Authority.SYS_ADMIN && getCurrentUser().getId().equals(userId)) {
             throw new ThingsboardException("Sysadmin is not allowed to delete himself", ThingsboardErrorCode.PERMISSION_DENIED);
         }
+        if (user.getAuthority() == Authority.TENANT_ADMIN && userService.countTenantAdmins(user.getTenantId()) == 1) {
+            throw new ThingsboardException("At least one tenant administrator must remain!", ThingsboardErrorCode.BAD_REQUEST_PARAMS);
+        }
         tbUserService.delete(getTenantId(), getCurrentUser().getCustomerId(), user, getCurrentUser());
     }
 

application/src/test/java/org/thingsboard/server/controller/UserControllerTest.java

@@ -284,6 +284,26 @@ public class UserControllerTest extends AbstractControllerTest {
                 ActionType.ADDED, new DataValidationException(msgError));
     }
 
+    @Test
+    public void testShouldNotDeleteLastTenantAdmin() throws Exception {
+        loginSysAdmin();
+
+        User tenantAdmin2 = new User();
+        tenantAdmin2.setAuthority(Authority.TENANT_ADMIN);
+        tenantAdmin2.setTenantId(tenantId);
+        tenantAdmin2.setEmail("tenant2@thingsboard.io");
+        tenantAdmin2 = doPost("/api/user", tenantAdmin2, User.class);
+
+        // delete second tenant admin - ok
+        doDelete("/api/user/" + tenantAdmin2.getId().getId().toString())
+                .andExpect(status().isOk());
+
+        // delete last tenant admin - forbidden
+        doDelete("/api/user/" + tenantAdminUser.getId().getId().toString())
+                .andExpect(status().isBadRequest())
+                .andExpect(statusReason(containsString("At least one tenant administrator must remain!")));
+    }
+
     @Test
     public void testSaveUserWithInvalidEmail() throws Exception {
         loginSysAdmin();

common/dao-api/src/main/java/org/thingsboard/server/dao/user/UserService.java

@@ -109,4 +109,5 @@ public interface UserService extends EntityDaoService {
 
     void removeMobileSession(TenantId tenantId, String mobileToken);
 
+    int countTenantAdmins(TenantId tenantId);
 }

dao/src/main/java/org/thingsboard/server/dao/sql/user/JpaUserDao.java

@@ -136,6 +136,11 @@ public class JpaUserDao extends JpaAbstractDao<UserEntity, User> implements User
                 DaoUtil.toPageable(pageLink)));
     }
 
+    @Override
+    public int countTenantAdmins(UUID tenantId) {
+        return userRepository.countByTenantIdAndAuthority(tenantId, Authority.TENANT_ADMIN);
+    }
+
     @Override
     public Long countByTenantId(TenantId tenantId) {
         return userRepository.countByTenantId(tenantId.getId());

dao/src/main/java/org/thingsboard/server/dao/sql/user/UserRepository.java

@@ -78,4 +78,6 @@ public interface UserRepository extends JpaRepository<UserEntity, UUID> {
             "u.customerId, u.version, u.firstName, u.lastName, u.email, u.phone, u.additionalInfo) " +
             "FROM UserEntity u WHERE u.id > :id ORDER BY u.id")
     List<UserFields> findNextBatch(@Param("id") UUID id, Limit limit);
+
+    int countByTenantIdAndAuthority(UUID tenantId, Authority authority);
 }

dao/src/main/java/org/thingsboard/server/dao/user/UserDao.java

@@ -101,4 +101,5 @@ public interface UserDao extends Dao<User>, TenantEntityDao<User> {
 
     PageData<User> findByAuthorityAndTenantProfilesIds(Authority authority, List<TenantProfileId> tenantProfilesIds, PageLink pageLink);
 
+    int countTenantAdmins(UUID tenantId);
 }

dao/src/main/java/org/thingsboard/server/dao/user/UserServiceImpl.java

@@ -485,6 +485,11 @@ public class UserServiceImpl extends AbstractCachedEntityService<UserCacheKey, U
         }
     }
 
+    @Override
+    public int countTenantAdmins(TenantId tenantId) {
+        return userDao.countTenantAdmins(tenantId.getId());
+    }
+
     private Optional<UserMobileSessionInfo> findMobileSessionInfo(TenantId tenantId, UserId userId) {
         return Optional.ofNullable(userSettingsService.findUserSettings(tenantId, userId, UserSettingsType.MOBILE))
                 .map(UserSettings::getSettings).map(settings -> JacksonUtil.treeToValue(settings, UserMobileSessionInfo.class));