DB2 fixes for #20337

src/providers/db2/qgsdb2featureiterator.cpp

@@ -51,21 +51,11 @@ QgsDb2FeatureIterator::QgsDb2FeatureIterator( QgsDb2FeatureSource *source, bool
 
   BuildStatement( request );
 
-  // connect to the database
-  QString errMsg;
-  mDatabase = QgsDb2Provider::getDatabase( mSource->mConnInfo, errMsg );
-
-  if ( !errMsg.isEmpty() )
-  {
-    QgsDebugMsg( "Failed to open database: " + errMsg );
-    return;
-  }
-
-  // create sql query
-  mQuery.reset( new QSqlQuery( mDatabase ) );
-
-  // start selection
-  rewind();
+  // WARNING - we can't obtain the database connection now, as this method should be
+  // run from the main thread, yet iteration can be done in a different thread.
+  // This would result in failure, because QSqlDatabase instances cannot be used
+  // from a different thread where they were created. Instead, we defer creation
+  // of the database until the first feature is fetched.
 }
 
 
@@ -308,12 +298,30 @@ bool QgsDb2FeatureIterator::nextFeatureFilterExpression( QgsFeature &f )
 bool QgsDb2FeatureIterator::fetchFeature( QgsFeature &feature )
 {
   feature.setValid( false );
-  if ( mClosed )
+
+  if ( !mDatabase.isValid() )
   {
-    QgsDebugMsg( QStringLiteral( "iterator closed" ) );
+    // No existing connection, so set it up now. It's safe to do here as we're now in
+    // the thread were iteration is actually occurring.  
+  // connect to the database
+  QString errMsg;
+  QgsDebugMsg( QStringLiteral( "fetchFeature getDatabase" ) );
+  mDatabase = QgsDb2Provider::getDatabase( mSource->mConnInfo, errMsg );
+  QgsDebugMsg( QStringLiteral( "fetchFeature back from getDatabase" ) );
+  if ( !errMsg.isEmpty() )
+  {
+    QgsDebugMsg( "Failed to open database: " + errMsg );
     return false;
   }
 
+  // create sql query
+  mQuery.reset( new QSqlQuery( mDatabase ) );
+
+  // start selection
+  if ( !rewind() )
+      return false;
+  }
+
   if ( !mQuery )
   {
     QgsDebugMsg( QStringLiteral( "Read attempt on no query" ) );
@@ -402,7 +410,10 @@ bool QgsDb2FeatureIterator::fetchFeature( QgsFeature &feature )
 bool QgsDb2FeatureIterator::rewind()
 {
   if ( mClosed )
+  {
+    QgsDebugMsg( QStringLiteral( "iterator closed" ) );
     return false;
+  }
 
   if ( mStatement.isEmpty() )
   {

src/providers/db2/qgsdb2provider.cpp

@@ -186,16 +186,23 @@ QSqlDatabase QgsDb2Provider::getDatabase( const QString &connInfo, QString &errM
     connectionName = service;
   }
   QgsDebugMsg( "connectionName: " + connectionName );
+
+  // Starting with Qt 5.11, sharing the same connection between threads is not allowed.
+  // We use a dedicated connection for each thread requiring access to the database,
+  // using the thread address as connection name.
+  const QString threadSafeConnectionName = dbConnectionName( connectionName );
+  QgsDebugMsg( "threadSafeConnectionName: " + threadSafeConnectionName );
+
   /* if new database connection */
-  if ( !QSqlDatabase::contains( connectionName ) )
+  if ( !QSqlDatabase::contains( threadSafeConnectionName ) )
   {
     QgsDebugMsg( QStringLiteral( "new connection. create new QODBC mapping" ) );
-    db = QSqlDatabase::addDatabase( QStringLiteral( "QODBC3" ), connectionName );
+    db = QSqlDatabase::addDatabase( QStringLiteral( "QODBC3" ), threadSafeConnectionName );
   }
   else  /* if existing database connection */
   {
     QgsDebugMsg( QStringLiteral( "found existing connection, use the existing one" ) );
-    db = QSqlDatabase::database( connectionName );
+    db = QSqlDatabase::database( threadSafeConnectionName );
   }
   db.setHostName( host );
   db.setPort( port.toInt() );
@@ -1747,6 +1754,14 @@ QGISEXTERN QgsVectorLayerExporter::ExportError createEmptyLayer(
          );
 }
 
+QString QgsDb2Provider::dbConnectionName( const QString &name )
+{
+  // Starting with Qt 5.11, sharing the same connection between threads is not allowed.
+  // We use a dedicated connection for each thread requiring access to the database,
+  // using the thread address as connection name.
+  const QString threadAddress = QStringLiteral( ":0x%1" ).arg( QString::number( reinterpret_cast< quintptr >( QThread::currentThread() ), 16 ) );
+  return name + threadAddress;
+}
 
 #ifdef HAVE_GUI
 

src/providers/db2/qgsdb2provider.h

@@ -118,6 +118,10 @@ class QgsDb2Provider : public QgsVectorDataProvider
   private:
     static void db2WkbTypeAndDimension( QgsWkbTypes::Type wkbType, QString &geometryType, int &dim );
     static QString db2TypeName( int typeId );
+	/**
+     * Returns a thread-safe connection name for use with QSqlDatabase
+     */
+    static QString dbConnectionName( const QString &name );
 
     QgsFields mAttributeFields; //fields
     QMap<int, QVariant> mDefaultValues;