[Plugins] Fix missing description with metadata 2.1

Changes to the metadata specs in v2.1 meant that Description field
might appear in the body of the message instead of as a header key.

Replaced custom parser with email parser (as outlined in the document
using compat32 policy) to simplify extracting the message header and
body.

Ref: https://dev.deluge-torrent.org/ticket/3476
Ref: https://packaging.python.org/en/latest/specifications/core-metadata/#description

deluge/pluginmanagerbase.py

@@ -8,6 +8,7 @@
 
 
 """PluginManagerBase"""
+import email
 import logging
 import os.path
 
@@ -266,25 +267,13 @@ def get_plugin_info(self, name):
 
     @staticmethod
     def parse_pkg_info(pkg_info):
-        last_header = ''
-        cont_lines = []
-        info = {}.fromkeys(METADATA_KEYS, '')
+        metadata_msg = email.message_from_string(pkg_info)
+        metadata_ver = metadata_msg.get('Metadata-Version')
 
-        for line in pkg_info.splitlines():
-            if not line:
-                continue
+        info = {key: metadata_msg.get(key, '') for key in METADATA_KEYS}
 
-            if line[0] in ' \t' and (
-                len(line.split(':', 1)) == 1 or line.split(':', 1)[0] not in info
-            ):
-                # This is a continuation
-                cont_lines.append(line.strip())
-                continue
+        # Optional Description field in body (Metadata spec >=2.1)
+        if not info['Description'] and metadata_ver.startswith('2'):
+            info['Description'] = metadata_msg.get_payload().strip()
 
-            if cont_lines:
-                info[last_header] = '\n'.join(cont_lines).strip()
-                cont_lines = []
-            if line.split(':', 1)[0] in info:
-                last_header = line.split(':', 1)[0]
-                info[last_header] = line.split(':', 1)[1].strip()
         return info

deluge/tests/test_plugin_metadata.py

@@ -20,10 +20,30 @@ def test_get_plugin_info(self):
         pm = PluginManagerBase('core.conf', 'deluge.plugin.core')
         for p in pm.get_available_plugins():
             for key, value in pm.get_plugin_info(p).items():
-                self.assertTrue(isinstance(f'{key}: {value}', str))
+                self.assertIsInstance(key, str)
+                self.assertIsInstance(value, str)
 
     def test_get_plugin_info_invalid_name(self):
         pm = PluginManagerBase('core.conf', 'deluge.plugin.core')
         for key, value in pm.get_plugin_info('random').items():
             result = 'not available' if key in ('Name', 'Version') else ''
             self.assertEqual(value, result)
+
+    def test_parse_pkg_info_metadata_2_1(self):
+        pkg_info = """Metadata-Version: 2.1
+Name: AutoAdd
+Version: 1.8
+Summary: Monitors folders for .torrent files.
+Home-page: http://dev.deluge-torrent.org/wiki/Plugins/AutoAdd
+Author: Chase Sterling, Pedro Algarvio
+Author-email: chase.sterling@gmail.com, pedro@algarvio.me
+License: GPLv3
+Platform: UNKNOWN
+
+Monitors folders for .torrent files.
+        """
+        plugin_info = PluginManagerBase.parse_pkg_info(pkg_info)
+        for value in plugin_info.values():
+            self.assertNotEqual(value, '')
+        result = 'Monitors folders for .torrent files.'
+        self.assertEqual(plugin_info['Description'], result)