Add "Socket backlog size" option

The default value in libtorrent is 5 which is too small nowadays.
The new default value 30 is chosen to be in line with
QTcpServer::maxPendingConnections().

src/base/bittorrent/session.cpp

@@ -282,6 +282,7 @@ Session::Session(QObject *parent)
     , m_sendBufferWatermark(BITTORRENT_SESSION_KEY("SendBufferWatermark"), 500)
     , m_sendBufferLowWatermark(BITTORRENT_SESSION_KEY("SendBufferLowWatermark"), 10)
     , m_sendBufferWatermarkFactor(BITTORRENT_SESSION_KEY("SendBufferWatermarkFactor"), 50)
+    , m_socketBacklogSize(BITTORRENT_SESSION_KEY("SocketBacklogSize"), 30)
     , m_isAnonymousModeEnabled(BITTORRENT_SESSION_KEY("AnonymousModeEnabled"), false)
     , m_isQueueingEnabled(BITTORRENT_SESSION_KEY("QueueingSystemEnabled"), true)
     , m_maxActiveDownloads(BITTORRENT_SESSION_KEY("MaxActiveDownloads"), 3, lowerLimited(-1))
@@ -1127,10 +1128,13 @@ void Session::initMetrics()
 
 void Session::configure(lt::settings_pack &settingsPack)
 {
+    // from libtorrent doc:
+    // It will not take affect until the listen_interfaces settings is updated
+    settingsPack.set_int(lt::settings_pack::listen_queue_size, socketBacklogSize());
+
 #ifdef Q_OS_WIN
     QString chosenIP;
 #endif
-
     if (m_listenInterfaceChanged) {
         const ushort port = this->port();
         const std::pair<int, int> ports(port, port);
@@ -2955,6 +2959,19 @@ void Session::setSendBufferWatermarkFactor(const int value)
     configureDeferred();
 }
 
+int Session::socketBacklogSize() const
+{
+    return m_socketBacklogSize;
+}
+
+void Session::setSocketBacklogSize(const int value)
+{
+    if (value == m_socketBacklogSize) return;
+
+    m_socketBacklogSize = value;
+    configureDeferred();
+}
+
 bool Session::isAnonymousModeEnabled() const
 {
     return m_isAnonymousModeEnabled;

src/base/bittorrent/session.h

@@ -340,6 +340,8 @@ namespace BitTorrent
         void setSendBufferLowWatermark(int value);
         int sendBufferWatermarkFactor() const;
         void setSendBufferWatermarkFactor(int value);
+        int socketBacklogSize() const;
+        void setSocketBacklogSize(int value);
         bool isAnonymousModeEnabled() const;
         void setAnonymousModeEnabled(bool enabled);
         bool isQueueingSystemEnabled() const;
@@ -595,6 +597,7 @@ namespace BitTorrent
         CachedSettingValue<int> m_sendBufferWatermark;
         CachedSettingValue<int> m_sendBufferLowWatermark;
         CachedSettingValue<int> m_sendBufferWatermarkFactor;
+        CachedSettingValue<int> m_socketBacklogSize;
         CachedSettingValue<bool> m_isAnonymousModeEnabled;
         CachedSettingValue<bool> m_isQueueingEnabled;
         CachedSettingValue<int> m_maxActiveDownloads;

src/gui/advancedsettings.cpp

@@ -100,7 +100,8 @@ enum AdvSettingsRows
     SEND_BUF_WATERMARK,
     SEND_BUF_LOW_WATERMARK,
     SEND_BUF_WATERMARK_FACTOR,
-    // ports
+    // networking & ports
+    SOCKET_BACKLOG_SIZE,
     OUTGOING_PORT_MIN,
     OUTGOING_PORT_MAX,
     UTP_MIX_MODE,
@@ -171,6 +172,8 @@ void AdvancedSettings::saveAdvancedSettings()
     session->setSendBufferWatermark(spinBoxSendBufferWatermark.value());
     session->setSendBufferLowWatermark(spinBoxSendBufferLowWatermark.value());
     session->setSendBufferWatermarkFactor(spinBoxSendBufferWatermarkFactor.value());
+    // Socket listen backlog size
+    session->setSocketBacklogSize(spinBoxSocketBacklogSize.value());
     // Save resume data interval
     session->setSaveResumeDataInterval(spinBoxSaveResumeDataInterval.value());
     // Outgoing ports
@@ -394,6 +397,12 @@ void AdvancedSettings::loadAdvancedSettings()
     spinBoxSendBufferWatermarkFactor.setValue(session->sendBufferWatermarkFactor());
     addRow(SEND_BUF_WATERMARK_FACTOR, (tr("Send buffer watermark factor") + ' ' + makeLink("https://www.libtorrent.org/reference-Settings.html#send_buffer_watermark_factor", "(?)"))
             , &spinBoxSendBufferWatermarkFactor);
+    // Socket listen backlog size
+    spinBoxSocketBacklogSize.setMinimum(1);
+    spinBoxSocketBacklogSize.setMaximum(std::numeric_limits<int>::max());
+    spinBoxSocketBacklogSize.setValue(session->socketBacklogSize());
+    addRow(SOCKET_BACKLOG_SIZE, (tr("Socket backlog size") + ' ' + makeLink("https://www.libtorrent.org/reference-Settings.html#listen_queue_size", "(?)"))
+            , &spinBoxSocketBacklogSize);
     // Save resume data interval
     spinBoxSaveResumeDataInterval.setMinimum(0);
     spinBoxSaveResumeDataInterval.setMaximum(std::numeric_limits<int>::max());

src/gui/advancedsettings.h

@@ -61,7 +61,7 @@ private:
     QLabel labelQbtLink, labelLibtorrentLink;
     QSpinBox spinBoxAsyncIOThreads, spinBoxCheckingMemUsage, spinBoxCache, spinBoxSaveResumeDataInterval, spinBoxOutgoingPortsMin, spinBoxOutgoingPortsMax, spinBoxListRefresh,
              spinBoxTrackerPort, spinBoxCacheTTL, spinBoxSendBufferWatermark, spinBoxSendBufferLowWatermark,
-             spinBoxSendBufferWatermarkFactor, spinBoxSavePathHistoryLength;
+             spinBoxSendBufferWatermarkFactor, spinBoxSocketBacklogSize, spinBoxSavePathHistoryLength;
     QCheckBox checkBoxOsCache, checkBoxRecheckCompleted, checkBoxResolveCountries, checkBoxResolveHosts, checkBoxSuperSeeding,
               checkBoxProgramNotifications, checkBoxTorrentAddedNotifications, checkBoxTrackerFavicon, checkBoxTrackerStatus,
               checkBoxConfirmTorrentRecheck, checkBoxConfirmRemoveAllTags, checkBoxListenIPv6, checkBoxAnnounceAllTrackers, checkBoxAnnounceAllTiers,