[Torrent] Add trackers cache

The current way the trackers are being update is either by adding a new
torrent to the session (on startup or new one), or by changing the
trackers.
This leads to data not being updated in the `torrent.trackers` dict.
To solve this, a cache mechanism was added which will update the dict
on changes or every 5 seconds.
This will help us also get an update regarding `lt.announce_endpoint` in
 each of the trackers.

deluge/core/torrent.py

@@ -17,7 +17,7 @@
 import os
 import socket
 import time
-from typing import Optional
+from typing import Optional, List
 from urllib.parse import urlparse
 
 from twisted.internet.defer import Deferred, DeferredList
@@ -238,6 +238,8 @@ def __init__(self, handle, options, state=None, filename=None, magnet=None):
         self.magnet = magnet
         self._status: Optional['lt.torrent_status'] = None
         self._status_last_update: float = 0.0
+        self._trackers: Optional[List['lt.announce_entry']] = None
+        self._trackers_last_update: float = 0.0
 
         self.torrent_info = self.handle.torrent_file()
         self.has_metadata = self.status.has_metadata
@@ -578,7 +580,7 @@ def set_trackers(self, trackers=None):
             trackers (list of dicts): A list of trackers.
         """
         if trackers is None:
-            self.trackers = list(self.handle.trackers())
+            self.trackers = self.handle.trackers()
             self.tracker_host = None
             return
 
@@ -599,12 +601,53 @@ def set_trackers(self, trackers=None):
             for tracker in self.handle.trackers():
                 log.debug(' [tier %s]: %s', tracker['tier'], tracker['url'])
         # Set the tracker list in the torrent object
-        self.trackers = trackers
+        self.trackers = self.handle.trackers()
         if len(trackers) > 0:
             # Force a re-announce if there is at least 1 tracker
             self.force_reannounce()
         self.tracker_host = None
 
+    def get_lt_trackers(self):
+        """Get the torrent trackers fresh, not from cache.
+
+        This should be used when a guaranteed fresh trackers is needed rather than
+        `torrent.handle.tracker()` because it will update the cache as well.
+        """
+        trackers = self.handle.trackers()
+        self.trackers = trackers
+        return trackers
+
+    @property
+    def trackers(self) -> List[dict]:
+        """Cached copy of the libtorrent Trackers for this torrent.
+
+        If it has not been updated within the last five seconds, it will be
+        automatically refreshed.
+        """
+        if self._trackers_last_update < (time.time() - 5):
+            self.trackers = self.handle.trackers()
+        trackers_list = []
+        for tracker in self._trackers:
+            torrent_tracker = {
+                'url': '',
+                'message': '',
+                'tier': 0,
+            }
+            for data_key in torrent_tracker:
+                torrent_tracker[data_key] = tracker.get(data_key)
+            trackers_list.append(torrent_tracker)
+        return trackers_list
+
+    @trackers.setter
+    def trackers(self, trackers: List['lt.announce_entry']) -> None:
+        """Updates the cached status.
+
+        Args:
+            trackers: a libtorrent torrent trackers
+        """
+        self._trackers = trackers
+        self._trackers_last_update = time.time()
+
     def set_tracker_status(self, status):
         """Sets the tracker status.
 

deluge/core/torrentmanager.py

@@ -1389,7 +1389,7 @@ def on_alert_tracker_error(self, alert):
         )
         # libtorrent 1.2 added endpoint struct to each tracker. to prevent false updates
         # we will need to verify that at least one endpoint to the errored tracker is working
-        for tracker in torrent.handle.trackers():
+        for tracker in torrent.get_lt_trackers():
             if tracker['url'] == alert.url:
                 if any(
                     endpoint['last_error']['value'] == 0

deluge/tests/test_torrent.py

@@ -386,3 +386,31 @@ def test_status_cache(self):
             # Advance time and verify cache expires and updates
             mock_time.return_value += 10
             assert torrent.status == 2
+
+    def test_trackers_cache(self):
+        atp = self.get_torrent_atp('test_torrent.file.torrent')
+        handle = self.session.add_torrent(atp)
+        mock_time = mock.Mock(return_value=time.time())
+        with mock.patch('time.time', mock_time):
+            torrent = Torrent(handle, {})
+            counter = itertools.count()
+            handle.trackers = mock.Mock(
+                side_effect=lambda: [
+                    {'tier': counter.__next__(), 'url': '', 'message': ''}
+                ]
+            )
+            first_trackers = torrent.get_lt_trackers()
+            assert first_trackers == [
+                {'tier': 0, 'url': '', 'message': ''}
+            ], 'sanity check'
+            assert first_trackers == torrent.trackers, 'cached trackers should be used'
+            torrent.set_trackers()
+            assert torrent.trackers == [
+                {'tier': 1, 'url': '', 'message': ''}
+            ], 'trackers should update'
+            assert torrent.get_lt_trackers() == [
+                {'tier': 2, 'url': '', 'message': ''}
+            ], 'trackers should update a second time'
+            # Advance time and verify cache expires and updates
+            mock_time.return_value += 10
+            assert torrent.trackers == [{'tier': 3, 'url': '', 'message': ''}]