Fix "database is locked" errors caused by concurrent writes

src/database.cpp

@@ -171,7 +171,11 @@ bool Database::execute(QSqlQuery &query)
 
 bool Database::transaction(const QString &connectionName)
 {
-    return QSqlDatabase::database(connectionName).transaction();
+    // use IMMEDIATE transaction here to avoid deadlocks with writes happening
+    // in different threads
+    QSqlQuery query(QSqlDatabase::database(connectionName));
+    query.prepare(QStringLiteral("BEGIN IMMEDIATE TRANSACTION;"));
+    return execute(query);
 }
 
 bool Database::commit(const QString &connectionName)

src/datamanager.cpp

@@ -71,6 +71,11 @@ DataManager::DataManager()
 
         // Check for "new" entries
         if (SettingsManager::self()->autoQueue()) {
+            // start an immediate transaction since this non-blocking read query
+            // can change into a blocking write query if the entry needs to be
+            // queued; this can create a deadlock with other concurrent write
+            // operations
+            Database::instance().transaction();
             query.prepare(QStringLiteral("SELECT id FROM Entries WHERE feed=:feed AND new=:new ORDER BY updated ASC;"));
             query.bindValue(QStringLiteral(":feed"), feedurl);
             query.bindValue(QStringLiteral(":new"), true);
@@ -85,6 +90,7 @@ DataManager::DataManager()
                     }
                 }
             }
+            Database::instance().commit();
         }
 
         Q_EMIT feedEntriesUpdated(feedurl);