fix(files_sharing): skip unresolvable share recipients

apps/files_sharing/lib/Listener/SharesUpdatedListener.php

@@ -26,6 +26,7 @@
 use OCP\Share\Events\ShareMovedEvent;
 use OCP\Share\Events\ShareTransferredEvent;
 use OCP\Share\IManager;
+use OCP\User\Exceptions\UserNotFoundException;
 use Psr\Clock\ClockInterface;
 use Psr\Log\LoggerInterface;
 
@@ -135,7 +136,17 @@ private function markOrRun(IUser $user, callable $callback): void {
 		$elapsed = $now - $this->firstRun;
 
 		if ($this->cutOffMarkTime === -1.0 || $elapsed < $this->cutOffMarkTime) {
-			$callback();
+			try {
+				$callback();
+			} catch (UserNotFoundException $e) {
+				// A share recipient may reference a user id that no backend can resolve anymore
+				// (e.g. with LazyUser::getUID()) - like remnant / incorrectly removed user.
+				// Skip this recipient instead of aborting the share operation.
+				$this->logger->warning(
+					'Skipping share mount update for unresolvable user ' . $user->getUID(),
+					['app' => Application::APP_ID, 'exception' => $e],
+				);
+			}
 		} else {
 			$this->markUserForRefresh($user);
 		}

apps/files_sharing/tests/SharesUpdatedListenerTest.php

@@ -19,6 +19,7 @@
 use OCP\Share\Events\ShareCreatedEvent;
 use OCP\Share\IManager;
 use OCP\Share\IShare;
+use OCP\User\Exceptions\UserNotFoundException;
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\MockObject\MockObject;
 use Psr\Clock\ClockInterface;
@@ -117,6 +118,33 @@ public function testShareAddedFilterOwner() {
 		$this->sharesUpdatedListener->handle($event);
 	}
 
+	public function testShareAddedSkipsUnresolvableUser(): void {
+		$share = $this->createMock(IShare::class);
+		$user1 = $this->createUser('user1', '');
+		$user2 = $this->createUser('user2', '');
+
+		$this->manager->method('getUsersForShare')
+			->willReturn([$user1, $user2]);
+
+		$event = new ShareCreatedEvent($share);
+
+		// user1 is an orphaned recipient that no backend can resolve
+		$this->shareRecipientUpdater
+			->expects($this->exactly(2))
+			->method('updateForAddedShare')
+			->willReturnCallback(function (IUser $user) use ($user1): void {
+				if ($user === $user1) {
+					throw new UserNotFoundException('Backends provided no user object');
+				}
+			});
+
+		// the failure is logged, not thrown
+		$this->logger->expects($this->once())->method('warning');
+
+		// must not throw: user2 is still processed
+		$this->sharesUpdatedListener->handle($event);
+	}
+
 	public function testShareAccessUpdated() {
 		$user1 = $this->createUser('user1', '');
 		$user2 = $this->createUser('user2', '');