Fix response to event=stopped

BEP-3 says all tracker responses to valid announce requests should be bencoded dictionaries with at least the "peers" and "interval" keys; the "peers" key can be empty.

Also clarify support for BEP-21.

src/base/bittorrent/tracker.cpp

@@ -349,9 +349,9 @@ void Tracker::processAnnounceRequest()
         || (announceReq.event == ANNOUNCE_REQUEST_EVENT_COMPLETED)
         || (announceReq.event == ANNOUNCE_REQUEST_EVENT_STARTED)
         || (announceReq.event == ANNOUNCE_REQUEST_EVENT_PAUSED)) {
-        // [BEP-21] Extension for partial seeds (partial support)
+        // [BEP-21] Extension for partial seeds
+        // (partial support - we don't support BEP-48 so the part that concerns that is not supported)
         registerPeer(announceReq);
-        prepareAnnounceResponse(announceReq);
     }
     else if (announceReq.event == ANNOUNCE_REQUEST_EVENT_STOPPED) {
         unregisterPeer(announceReq);
@@ -359,6 +359,8 @@ void Tracker::processAnnounceRequest()
     else {
         throw TrackerError("Invalid \"event\" parameter");
     }
+
+    prepareAnnounceResponse(announceReq);
 }
 
 void Tracker::registerPeer(const TrackerAnnounceRequest &announceReq)
@@ -404,6 +406,7 @@ void Tracker::prepareAnnounceResponse(const TrackerAnnounceRequest &announceReq)
         lt::entry::string_type peers;
         lt::entry::string_type peers6;
 
+        if (announceReq.event != ANNOUNCE_REQUEST_EVENT_STOPPED) {
             int counter = 0;
             for (const Peer &peer : asConst(torrentStats.peers)) {
                 if (counter++ >= announceReq.numwant)
@@ -414,6 +417,7 @@ void Tracker::prepareAnnounceResponse(const TrackerAnnounceRequest &announceReq)
                 else if (peer.endpoint.size() == 18)  // IPv6 + port
                     peers6.append(peer.endpoint);
             }
+        }
 
         replyDict[ANNOUNCE_RESPONSE_PEERS] = peers;  // required, even it's empty
         if (!peers6.empty())
@@ -422,6 +426,7 @@ void Tracker::prepareAnnounceResponse(const TrackerAnnounceRequest &announceReq)
     else {
         lt::entry::list_type peerList;
 
+        if (announceReq.event != ANNOUNCE_REQUEST_EVENT_STOPPED) {
             int counter = 0;
             for (const Peer &peer : torrentStats.peers) {
                 if (counter++ >= announceReq.numwant)
@@ -437,6 +442,7 @@ void Tracker::prepareAnnounceResponse(const TrackerAnnounceRequest &announceReq)
 
                 peerList.emplace_back(peerDict);
             }
+        }
 
         replyDict[ANNOUNCE_RESPONSE_PEERS] = peerList;
     }