Don't mount shares with failed underlying storages

apps/files_sharing/lib/MountProvider.php

@@ -82,7 +82,7 @@ public function getMountsForUser(IUser $user, IStorageFactory $storageFactory) {
 		$mounts = [];
 		foreach ($superShares as $share) {
 			try {
-				$mounts[] = new SharedMount(
+				$shareMount = new SharedMount(
 					'\OCA\Files_Sharing\SharedStorage',
 					$mounts,
 					[
@@ -94,6 +94,13 @@ public function getMountsForUser(IUser $user, IStorageFactory $storageFactory) {
 					],
 					$storageFactory
 				);
+
+				$shareStorage = $shareMount->getStorage();
+				if ($shareStorage !== null && $shareStorage->getPermissions('') !== 0) {
+					// permissions = 0 implies that the underlying storage
+					// being shared isn't valid
+					$mounts[] = $shareMount;
+				}
 			} catch (\Exception $e) {
 				$this->logger->logException($e);
 				$this->logger->error('Error while trying to create shared mount');