[Core] Assign magnet arg if magnet URI in filename

A magnet that prefetched the metadata is added as a normal torrent but
with the filename set to the magnet URI. So we need to assign the URI to
magnet arg and set filename to None before creating Torrent object.

Updated the is_magnet function to prevent AttributeError with None type.

deluge/common.py

@@ -672,11 +672,11 @@ def is_magnet(uri):
     True
 
     """
-
-    if uri.startswith(MAGNET_SCHEME) and XT_BTIH_PARAM in uri:
-        return True
+    if not uri:
         return False
 
+    return uri.startswith(MAGNET_SCHEME) and XT_BTIH_PARAM in uri
+
 
 def get_magnet_info(uri):
     """Parse torrent information from magnet link.

deluge/core/torrentmanager.py

@@ -23,7 +23,7 @@ from twisted.internet.task import LoopingCall
 
 import deluge.component as component
 from deluge._libtorrent import lt
-from deluge.common import archive_files, decode_bytes, get_magnet_info
+from deluge.common import archive_files, decode_bytes, get_magnet_info, is_magnet
 from deluge.configmanager import ConfigManager, get_config_dir
 from deluge.core.authmanager import AUTH_LEVEL_ADMIN
 from deluge.core.torrent import Torrent, TorrentOptions, sanitize_filepath
@@ -555,6 +555,11 @@ class TorrentManager(component.Component):
         return d
 
     def _add_torrent_obj(self, handle, options, state, filename, magnet, resume_data, filedump, save_state):
+        # For magnets added with metadata, filename is used so set as magnet.
+        if not magnet and is_magnet(filename):
+            magnet = filename
+            filename = None
+
         # Create a Torrent object and add to the dictionary.
         torrent = Torrent(handle, options, state, filename, magnet)
         self.torrents[torrent.torrent_id] = torrent

deluge/tests/test_common.py

@@ -74,6 +74,7 @@ class CommonTestCase(unittest.TestCase):
 
     def test_is_magnet(self):
         self.assertTrue(is_magnet('magnet:?xt=urn:btih:SU5225URMTUEQLDXQWRB2EQWN6KLTYKN'))
+        self.assertFalse(is_magnet(None))
 
     def test_is_infohash(self):
         self.assertTrue(is_infohash('2dc5d0e71a66fe69649a640d39cb00a259704973'))