fix(share): Avoid crash when share owner is not found

IShare::getNode is documented to throw only NotFoundException, so catch
 UserNotFoundException from the level below and wrap it. This avoids
 crashes from share api controller when listing shares and one of them
 is broken because its owner has vanished from the backend.

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

Author: come-nc

lib/private/Share20/Share.php

@@ -5,8 +5,10 @@
  * SPDX-FileCopyrightText: 2016 ownCloud, Inc.
  * SPDX-License-Identifier: AGPL-3.0-only
  */
+
 namespace OC\Share20;
 
+use OC\User\NoUserException;
 use OCP\Constants;
 use OCP\Files\Cache\ICacheEntry;
 use OCP\Files\File;
@@ -158,20 +160,24 @@ public function getNode() {
 				throw new NotFoundException();
 			}
 
-			// for federated shares the owner can be a remote user, in this
-			// case we use the initiator
-			if ($this->userManager->userExists($this->shareOwner)) {
-				$userFolder = $this->rootFolder->getUserFolder($this->shareOwner);
-			} else {
-				$userFolder = $this->rootFolder->getUserFolder($this->sharedBy);
+			try {
+				// for federated shares the owner can be a remote user, in this
+				// case we use the initiator
+				if ($this->userManager->userExists($this->shareOwner)) {
+					$userFolder = $this->rootFolder->getUserFolder($this->shareOwner);
+				} else {
+					$userFolder = $this->rootFolder->getUserFolder($this->sharedBy);
+				}
+
+				$node = $userFolder->getFirstNodeById($this->fileId);
+				if (!$node) {
+					throw new NotFoundException('Node for share not found, fileid: ' . $this->fileId);
+				}
+
+				$this->node = $node;
+			} catch (NoUserException $e) {
+				throw new NotFoundException('Owner for share not found, fileid: ' . $this->fileId, previous:$e);
 			}
-
-			$node = $userFolder->getFirstNodeById($this->fileId);
-			if (!$node) {
-				throw new NotFoundException('Node for share not found, fileid: ' . $this->fileId);
-			}
-
-			$this->node = $node;
 		}
 
 		return $this->node;