[postgres] Avoid crash when fetching default value fails

src/core/qgsvectordataprovider.cpp

@@ -606,7 +606,7 @@ QStringList QgsVectorDataProvider::errors() const
   return mErrors;
 }
 
-void QgsVectorDataProvider::pushError( const QString& msg )
+void QgsVectorDataProvider::pushError( const QString& msg ) const
 {
   QgsDebugMsg( msg );
   mErrors << msg;
@@ -722,4 +722,4 @@ QStringList QgsVectorDataProvider::smEncodings;
 QList<QgsRelation> QgsVectorDataProvider::discoverRelations( const QgsVectorLayer*, const QList<QgsVectorLayer*>& ) const
 {
   return QList<QgsRelation>();
-}
+}

src/core/qgsvectordataprovider.h

@@ -445,7 +445,7 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider
 
   signals:
     /** Signals an error in this provider */
-    void raiseError( const QString& msg );
+    void raiseError( const QString& msg ) const;
 
   protected:
     void clearMinMaxCache();
@@ -465,7 +465,7 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider
     /** The names of the providers native types*/
     QList< NativeType > mNativeTypes;
 
-    void pushError( const QString& msg );
+    void pushError( const QString& msg ) const;
 
     /** Old-style mapping of index to name for QgsPalLabeling fix */
     QgsAttrPalIndexNameHash mAttrPalIndexName;
@@ -479,7 +479,7 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider
     QMap<QString, QVariant::Type> mOldTypeList;
 
     /** List of errors */
-    QStringList mErrors;
+    mutable QStringList mErrors;
 
     static QStringList smEncodings;
 

src/providers/postgres/qgspostgresprovider.cpp

@@ -1707,7 +1707,13 @@ QVariant QgsPostgresProvider::defaultValue( int fieldId ) const
 
     QgsPostgresResult res( connectionRO()->PQexec( QString( "SELECT %1" ).arg( defVal.toString() ) ) );
 
-    return convertValue( fld.type(), fld.subType(), res.PQgetvalue( 0, 0 ) );
+    if ( res.result() )
+      return convertValue( fld.type(), fld.subType(), res.PQgetvalue( 0, 0 ) );
+    else
+    {
+      pushError( tr( "Could not execute query" ) );
+      return QVariant();
+    }
   }
 
   return defVal;