feat(federation): implement minimal federation user backend

apps/federation/composer/composer/autoload_classmap.php

@@ -23,4 +23,5 @@
     'OCA\\Federation\\SyncFederationAddressBooks' => $baseDir . '/../lib/SyncFederationAddressBooks.php',
     'OCA\\Federation\\SyncJob' => $baseDir . '/../lib/SyncJob.php',
     'OCA\\Federation\\TrustedServers' => $baseDir . '/../lib/TrustedServers.php',
+    'OCA\\Federation\\User\\Backend' => $baseDir . '/../lib/User/Backend.php',
 );

apps/federation/composer/composer/autoload_static.php

@@ -38,6 +38,7 @@ class ComposerStaticInitFederation
         'OCA\\Federation\\SyncFederationAddressBooks' => __DIR__ . '/..' . '/../lib/SyncFederationAddressBooks.php',
         'OCA\\Federation\\SyncJob' => __DIR__ . '/..' . '/../lib/SyncJob.php',
         'OCA\\Federation\\TrustedServers' => __DIR__ . '/..' . '/../lib/TrustedServers.php',
+        'OCA\\Federation\\User\\Backend' => __DIR__ . '/..' . '/../lib/User/Backend.php',
     );
 
     public static function getInitializer(ClassLoader $loader)

apps/federation/lib/AppInfo/Application.php

@@ -11,11 +11,13 @@
 use OCA\DAV\Events\SabrePluginAuthInitEvent;
 use OCA\Federation\Listener\SabrePluginAuthInitListener;
 use OCA\Federation\Listener\TrustedServerRemovedListener;
+use OCA\Federation\User\Backend;
 use OCP\AppFramework\App;
 use OCP\AppFramework\Bootstrap\IBootContext;
 use OCP\AppFramework\Bootstrap\IBootstrap;
 use OCP\AppFramework\Bootstrap\IRegistrationContext;
 use OCP\Federation\Events\TrustedServerRemovedEvent;
+use OCP\IUserManager;
 
 class Application extends App implements IBootstrap {
 
@@ -36,5 +38,6 @@ public function register(IRegistrationContext $context): void {
 
 	#[\Override]
 	public function boot(IBootContext $context): void {
+		$context->injectFn(fn (IUserManager $userManager, Backend $backend) => $userManager->registerBackend($backend));
 	}
 }

apps/federation/lib/User/Backend.php

@@ -0,0 +1,59 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+declare(strict_types=1);
+
+namespace OCA\Federation\User;
+
+use OCP\Federation\ICloudIdManager;
+use OCP\User\Backend\ABackend;
+use OCP\User\Backend\IGetDisplayNameBackend;
+
+class Backend extends ABackend implements IGetDisplayNameBackend {
+	public function __construct(
+		private ICloudIdManager $cloudIdManager,
+	) {
+	}
+
+	#[\Override]
+	public function getBackendName() {
+		return 'federation';
+	}
+
+	#[\Override]
+	public function deleteUser($uid) {
+		return false;
+	}
+
+	#[\Override]
+	public function getUsers($search = '', $limit = null, $offset = null) {
+		return [];
+	}
+
+	/**
+	 * Check if a user exists. Assume that the user with valid cloud id exists since the account is for a remote user
+	 */
+	#[\Override]
+	public function userExists($uid) {
+		return $this->cloudIdManager->isValidCloudId($uid);
+	}
+
+	#[\Override]
+	public function getDisplayName($uid): string {
+		return $this->cloudIdManager->resolveCloudId($uid)->getDisplayId();
+	}
+
+	#[\Override]
+	public function getDisplayNames($search = '', $limit = null, $offset = null) {
+		return [];
+	}
+
+	#[\Override]
+	public function hasUserListings() {
+		return false;
+	}
+}

apps/federation/tests/User/BackendTest.php

@@ -0,0 +1,82 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+declare(strict_types=1);
+
+namespace OCA\Federation\Tests\User;
+
+use OC\User\Backend as Base;
+use OCA\Federation\User\Backend;
+use OCP\Server;
+use Test\TestCase;
+
+#[\PHPUnit\Framework\Attributes\Group(name: 'DB')]
+class BackendTest extends TestCase {
+	private Backend $backend;
+	private string $userId;
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->backend = Server::get(Backend::class);
+		$this->userId = 'foo@example.com';
+	}
+
+	protected function tearDown(): void {
+		parent::tearDown();
+	}
+
+	public function testGetBackendName(): void {
+		$this->assertEquals($this->backend->getBackendName(), 'federation');
+	}
+
+	#[\PHPUnit\Framework\Attributes\DataProvider('dataTestImplementsActions')]
+	public function testImplementsActions(int $action, bool $implemented): void {
+		$this->assertEquals($this->backend->implementsActions($action), $implemented);
+	}
+
+	public static function dataTestImplementsActions(): array {
+		return [
+			[Base::CREATE_USER, false],
+			[Base::SET_PASSWORD, false],
+			[Base::CHECK_PASSWORD, false],
+			[Base::GET_HOME, false],
+			[Base::GET_DISPLAYNAME, true],
+			[Base::SET_DISPLAYNAME, false],
+			[Base::PROVIDE_AVATAR, false],
+			[Base::COUNT_USERS, false],
+		];
+	}
+
+	public function testDeleteUser(): void {
+		$this->assertFalse($this->backend->deleteUser($this->userId));
+	}
+
+	public function testGetUsers(): void {
+		$this->assertEquals($this->backend->getUsers(), []);
+	}
+
+	public function testUserExists(): void {
+		$this->assertTrue($this->backend->userExists($this->userId));
+		$this->assertFalse($this->backend->userExists('foo'));
+	}
+
+	public function testGetDisplayName(): void {
+		$this->assertEquals($this->backend->getDisplayName($this->userId), $this->userId);
+
+		$this->expectException(\InvalidArgumentException::class);
+		$this->backend->getDisplayName('foo');
+	}
+
+	public function testGetDisplayNames(): void {
+		$this->assertEquals($this->backend->getDisplayNames(), []);
+	}
+
+	public function testHasUserListings(): void {
+		$this->assertFalse($this->backend->hasUserListings());
+	}
+}