Improve add torrent error handling

PR #22468.

src/base/bittorrent/addtorrenterror.h

@@ -0,0 +1,49 @@
+/*
+ * Bittorrent Client using Qt and libtorrent.
+ * Copyright (C) 2025  Vladimir Golovnev <glassez@yandex.ru>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ *
+ * In addition, as a special exception, the copyright holders give permission to
+ * link this program with the OpenSSL project's "OpenSSL" library (or with
+ * modified versions of it that use the same license as the "OpenSSL" library),
+ * and distribute the linked executables. You must obey the GNU General Public
+ * License in all respects for all of the code used other than "OpenSSL".  If you
+ * modify file(s), you may extend this exception to your version of the file(s),
+ * but you are not obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ */
+
+#pragma once
+
+#include <QMetaType>
+#include <QString>
+
+namespace BitTorrent
+{
+    struct AddTorrentError
+    {
+        enum Kind
+        {
+            DuplicateTorrent,
+            Other
+        };
+
+        Kind kind = Other;
+        QString message;
+    };
+}
+
+Q_DECLARE_METATYPE(BitTorrent::AddTorrentError)

src/base/bittorrent/session.h

@@ -34,6 +34,7 @@
 
 #include "base/pathfwd.h"
 #include "base/tagset.h"
+#include "addtorrenterror.h"
 #include "addtorrentparams.h"
 #include "categoryoptions.h"
 #include "sharelimitaction.h"
@@ -485,7 +486,7 @@ namespace BitTorrent
 
     signals:
         void startupProgressUpdated(int progress);
-        void addTorrentFailed(const InfoHash &infoHash, const QString &reason);
+        void addTorrentFailed(const InfoHash &infoHash, const AddTorrentError &reason);
         void allTorrentsFinished();
         void categoryAdded(const QString &categoryName);
         void categoryRemoved(const QString &categoryName);

src/base/bittorrent/sessionimpl.cpp

@@ -2772,7 +2772,10 @@ bool SessionImpl::addTorrent_impl(const TorrentDescriptor &source, const AddTorr
     // We should not add the torrent if it is already
     // processed or is pending to add to session
     if (m_loadingTorrents.contains(id) || (infoHash.isHybrid() && m_loadingTorrents.contains(altID)))
+    {
+        emit addTorrentFailed(infoHash, {AddTorrentError::DuplicateTorrent, tr("Duplicate torrent")});
         return false;
+    }
 
     if (Torrent *torrent = findTorrent(infoHash))
     {
@@ -2786,16 +2789,20 @@ bool SessionImpl::addTorrent_impl(const TorrentDescriptor &source, const AddTorr
 
         if (!isMergeTrackersEnabled())
         {
+            const QString message = tr("Merging of trackers is disabled");
             LogMsg(tr("Detected an attempt to add a duplicate torrent. Existing torrent: %1. Result: %2")
-                    .arg(torrent->name(), tr("Merging of trackers is disabled")));
+                    .arg(torrent->name(), message));
+            emit addTorrentFailed(infoHash, {AddTorrentError::DuplicateTorrent, message});
             return false;
         }
 
         const bool isPrivate = torrent->isPrivate() || (hasMetadata && source.info()->isPrivate());
         if (isPrivate)
         {
+            const QString message = tr("Trackers cannot be merged because it is a private torrent");
             LogMsg(tr("Detected an attempt to add a duplicate torrent. Existing torrent: %1. Result: %2")
-                    .arg(torrent->name(), tr("Trackers cannot be merged because it is a private torrent")));
+                    .arg(torrent->name(), message));
+            emit addTorrentFailed(infoHash, {AddTorrentError::DuplicateTorrent, message});
             return false;
         }
 
@@ -2803,8 +2810,10 @@ bool SessionImpl::addTorrent_impl(const TorrentDescriptor &source, const AddTorr
         torrent->addTrackers(source.trackers());
         torrent->addUrlSeeds(source.urlSeeds());
 
+        const QString message = tr("Trackers are merged from new source");
         LogMsg(tr("Detected an attempt to add a duplicate torrent. Existing torrent: %1. Result: %2")
-                .arg(torrent->name(), tr("Trackers are merged from new source")));
+                .arg(torrent->name(), message));
+        emit addTorrentFailed(infoHash, {AddTorrentError::DuplicateTorrent, message});
         return false;
     }
 
@@ -5747,7 +5756,9 @@ void SessionImpl::handleAddTorrentAlert(const lt::add_torrent_alert *alert)
         if (const auto loadingTorrentsIter = m_loadingTorrents.constFind(TorrentID::fromInfoHash(infoHash))
                 ; loadingTorrentsIter != m_loadingTorrents.cend())
         {
-            emit addTorrentFailed(infoHash, msg);
+            const AddTorrentError::Kind errorKind = (alert->error == lt::errors::duplicate_torrent)
+                    ? AddTorrentError::DuplicateTorrent : AddTorrentError::Other;
+            emit addTorrentFailed(infoHash, {errorKind, msg});
             m_loadingTorrents.erase(loadingTorrentsIter);
         }
         else if (const auto downloadedMetadataIter = m_downloadedMetadata.constFind(TorrentID::fromInfoHash(infoHash))