Proposal: add enumeration function to IUserManager which simply calls a callbask for each user in all backends

apps/dav/command/syncsystemaddressbook.php

@@ -7,6 +7,7 @@ use OCA\DAV\CardDAV\Converter;
 use OCA\DAV\Connector\Sabre\Principal;
 use OCP\IConfig;
 use OCP\IDBConnection;
+use OCP\IUser;
 use OCP\IUserManager;
 use Sabre\CardDAV\Plugin;
 use Sabre\VObject\Component\VCard;
@@ -69,11 +70,8 @@ class SyncSystemAddressBook extends Command {
 		$output->writeln('Syncing users ...');
 		$progress = new ProgressBar($output);
 		$progress->start();
-		$page = 0;
-		foreach( $this->userManager->getBackends() as $backend) {
-			$users = $backend->getUsers('', 50, $page++);
-			foreach($users as $user) {
-				$user = $this->userManager->get($user);
+		$this->userManager->callForAllUsers(function($user) use ($systemAddressBook, $converter, $progress) {
+			/** @var IUser $user */
 			$name = $user->getBackendClassName();
 			$userId = $user->getUID();
 
@@ -89,8 +87,7 @@ class SyncSystemAddressBook extends Command {
 				}
 			}
 			$progress->advance();
-			}
-		}
+		});
 		$progress->finish();
 		$output->writeln('');
 	}

lib/private/user/manager.php

@@ -294,21 +294,47 @@ class Manager extends PublicEmitter implements IUserManager {
 		$userCountStatistics = array();
 		foreach ($this->backends as $backend) {
 			if ($backend->implementsActions(\OC_User_Backend::COUNT_USERS)) {
-				$backendusers = $backend->countUsers();
-				if($backendusers !== false) {
+				$backendUsers = $backend->countUsers();
+				if($backendUsers !== false) {
 					if($backend instanceof \OCP\IUserBackend) {
 						$name = $backend->getBackendName();
 					} else {
 						$name = get_class($backend);
 					}
 					if(isset($userCountStatistics[$name])) {
-						$userCountStatistics[$name] += $backendusers;
+						$userCountStatistics[$name] += $backendUsers;
 					} else {
-						$userCountStatistics[$name] = $backendusers;
+						$userCountStatistics[$name] = $backendUsers;
 					}
 				}
 			}
 		}
 		return $userCountStatistics;
 	}
+
+	/**
+	 * The callback is executed for each user on each backend.
+	 * If the callback returns false no further users will be retrieved.
+	 *
+	 * @param \Closure $callback
+	 * @return void
+	 * @since 9.0.0
+	 */
+	public function callForAllUsers(\Closure $callback, $search = '') {
+		foreach($this->getBackends() as $backend) {
+			$limit = 50;
+			$offset = 0;
+			do {
+				$users = $backend->getUsers($search, $limit, $offset);
+				foreach ($users as $user) {
+					$user = $this->get($user);
+					$return = $callback($user);
+					if ($return === false) {
+						break;
+					}
+				}
+				$offset += $limit;
+			} while (count($users) >= $limit);
+		}
+	}
 }

lib/public/iusermanager.php

@@ -134,4 +134,11 @@ interface IUserManager {
 	 * @since 8.0.0
 	 */
 	public function countUsers();
+
+	/**
+	 * @param \Closure $callback
+	 * @return void
+	 * @since 9.0.0
+	 */
+	public function callForAllUsers (\Closure $callback, $search = '');
 }