perf: Allow only getting user ids from a group

lib/private/Group/Group.php

@@ -104,6 +104,24 @@
 			return $this->users;
 		}
 
+		$this->users = $this->getVerifiedUsers($this->getBackendUserIds());
+		$this->usersLoaded = true;
+		return $this->users;
+	}
+
+	#[\Override]
+	public function getUserIds(): array {
+		if ($this->usersLoaded) {
+			return array_keys($this->users);
+		}
+
+		return $this->getBackendUserIds();
+	}
+
+	/**
+	 * @return list<string>
+	 */
+	private function getBackendUserIds(): array {
 		$userIds = [];
 		foreach ($this->backends as $backend) {
 			$diff = array_diff(
@@ -114,10 +132,7 @@
 				$userIds = array_merge($userIds, $diff);
 			}
 		}
-
-		$this->users = $this->getVerifiedUsers($userIds);
-		$this->usersLoaded = true;
-		return $this->users;
+		return array_values($userIds);
 	}
 
 	/**

lib/private/Share20/DefaultShareProvider.php

@@ -1472,9 +1472,7 @@ public function getAccessList($nodes, $currentAccess) {
 					continue;
 				}
 
-				$userList = $group->getUsers();
-				foreach ($userList as $user) {
-					$uid = $user->getUID();
+				foreach ($group->getUserIds() as $uid) {
 					$users[$uid] = $users[$uid] ?? [];
 					$users[$uid][$row['id']] = $row;
 				}

lib/public/IGroup.php

@@ -46,6 +46,17 @@ public function setDisplayName(string $displayName): bool;
 	 */
 	public function getUsers(): array;
 
+	/**
+	 * Get the ids of all users in the group
+	 *
+	 * Unlike {@see self::getUsers()} the ids are not checked against the user
+	 * backends, so they can contain users that no longer exist.
+	 *
+	 * @return list<string>
+	 * @since 36.0.0
+	 */
+	public function getUserIds(): array;
+
 	/**
 	 * check if a user is in the group
 	 *

tests/lib/Group/GroupTest.php

@@ -8,7 +8,9 @@
 
 namespace Test\Group;
 
+use OC\Group\Database;
 use OC\Group\Group;
+use OC\User\Manager;
 use OC\User\User;
 use OCP\EventDispatcher\IEventDispatcher;
 use OCP\Group\Events\BeforeGroupChangedEvent;
@@ -116,6 +118,40 @@ public function testGetUsersMultipleBackends(): void {
 		$this->assertEquals('user3', $user3->getUID());
 	}
 
+	public function testGetUserIdsMultipleBackends(): void {
+		$backend1 = $this->createMock(Database::class);
+		$backend2 = $this->createMock(Database::class);
+		$userManager = $this->createMock(Manager::class);
+		$userManager->expects($this->never())
+			->method('get');
+		$group = new Group('group1', [$backend1, $backend2], $this->dispatcher, $userManager);
+
+		$backend1->expects($this->once())
+			->method('usersInGroup')
+			->with('group1')
+			->willReturn(['user1', 'user2']);
+		$backend2->expects($this->once())
+			->method('usersInGroup')
+			->with('group1')
+			->willReturn(['user2', '3']);
+
+		$this->assertSame(['user1', 'user2', '3'], $group->getUserIds());
+	}
+
+	public function testGetUserIdsReusesLoadedUsers(): void {
+		$backend = $this->createMock(Database::class);
+		$userManager = $this->getUserManager();
+		$group = new Group('group1', [$backend], $this->dispatcher, $userManager);
+
+		$backend->expects($this->once())
+			->method('usersInGroup')
+			->with('group1')
+			->willReturn(['user1', 'user2']);
+
+		$group->getUsers();
+		$this->assertSame(['user1', 'user2'], $group->getUserIds());
+	}
+
 	public function testInGroupSingleBackend(): void {
 		$backend = $this->getMockBuilder('OC\Group\Database')
 			->disableOriginalConstructor()