[BUGFIX / FEATURE] [OGR] Allow concurrent edition of Shapefiles and Tabfiles in QGIS & MapInfo

- Closes https://hub.qgis.org/issues/14378
- Adds new virtual methods in QgsDataProvider(): enterUpdateMode() and leaveUpdateMode()
  and implement them in the OGR provider. Limited to shapefiles and tabfiles
- Implements QgsOGRProvider:reloadData()
- Robustify OGR provider methods so they don't crash if dataset re-opening fails.

Cherry picked from dc18b5b

Author: rouault

python/core/qgsdataprovider.sip

@@ -223,6 +223,47 @@ class QgsDataProvider : QObject
      */
     virtual void invalidateConnections( const QString& connection );
 
+    /** Enter update mode.
+     *
+     * This is aimed at providers that can open differently the connection to
+     * the datasource, according it to be in update mode or in read-only mode.
+     * A call to this method shall be balanced with a call to leaveUpdateMode(),
+     * if this method returns true.
+     *
+     * Most providers will have an empty implementation for that method.
+     *
+     * For backward compatibility, providers that implement enterUpdateMode() should
+     * still make sure to allow editing operations to work even if enterUpdateMode()
+     * is not explicitly called.
+     *
+     * Several successive calls to enterUpdateMode() can be done. So there is
+     * a concept of stack of calls that must be handled by the provider. Only the first
+     * call to enterUpdateMode() will really turn update mode on.
+     *
+     * @return true in case of success (or no-op implementation), false in case of failure
+     *
+     * @note added in QGIS 2.16
+     */
+    virtual bool enterUpdateMode();
+
+    /** Leave update mode.
+     *
+     * This is aimed at providers that can open differently the connection to
+     * the datasource, according it to be in update mode or in read-only mode.
+     * This method shall be balanced with a succesful call to enterUpdateMode().
+     *
+     * Most providers will have an empty implementation for that method.
+     *
+     * Several successive calls to enterUpdateMode() can be done. So there is
+     * a concept of stack of calls that must be handled by the provider. Only the last
+     * call to leaveUpdateMode() will really turn update mode off.
+     *
+     * @return true in case of success (or no-op implementation), false in case of failure
+     *
+     * @note added in QGIS 2.16
+     */
+    virtual bool leaveUpdateMode();
+
   signals:
 
     /**

src/core/qgsdataprovider.h

@@ -311,6 +311,47 @@ class CORE_EXPORT QgsDataProvider : public QObject
      */
     virtual void invalidateConnections( const QString& connection ) { Q_UNUSED( connection ); }
 
+    /** Enter update mode.
+     *
+     * This is aimed at providers that can open differently the connection to
+     * the datasource, according it to be in update mode or in read-only mode.
+     * A call to this method shall be balanced with a call to leaveUpdateMode(),
+     * if this method returns true.
+     *
+     * Most providers will have an empty implementation for that method.
+     *
+     * For backward compatibility, providers that implement enterUpdateMode() should
+     * still make sure to allow editing operations to work even if enterUpdateMode()
+     * is not explicitly called.
+     *
+     * Several successive calls to enterUpdateMode() can be done. So there is
+     * a concept of stack of calls that must be handled by the provider. Only the first
+     * call to enterUpdateMode() will really turn update mode on.
+     *
+     * @return true in case of success (or no-op implementation), false in case of failure.
+     *
+     * @note added in QGIS 2.16
+     */
+    virtual bool enterUpdateMode() { return true; }
+
+    /** Leave update mode.
+     *
+     * This is aimed at providers that can open differently the connection to
+     * the datasource, according it to be in update mode or in read-only mode.
+     * This method shall be balanced with a succesful call to enterUpdateMode().
+     *
+     * Most providers will have an empty implementation for that method.
+     *
+     * Several successive calls to enterUpdateMode() can be done. So there is
+     * a concept of stack of calls that must be handled by the provider. Only the last
+     * call to leaveUpdateMode() will really turn update mode off.
+     *
+     * @return true in case of success (or no-op implementation), false in case of failure.
+     *
+     * @note added in QGIS 2.16
+     */
+    virtual bool leaveUpdateMode() { return true; }
+
   signals:
 
     /**

src/core/qgsvectorlayer.cpp

@@ -379,6 +379,7 @@ void QgsVectorLayer::reload()
   if ( mDataProvider )
   {
     mDataProvider->reloadData();
+    updateFields();
   }
 }
 
@@ -1380,6 +1381,8 @@ bool QgsVectorLayer::startEditing()
 
   emit beforeEditingStarted();
 
+  mDataProvider->enterUpdateMode();
+
   if ( mDataProvider->transaction() )
   {
     mEditBuffer = new QgsVectorLayerEditPassthrough( this );
@@ -2334,6 +2337,8 @@ bool QgsVectorLayer::commitChanges()
   updateFields();
   mDataProvider->updateExtents();
 
+  mDataProvider->leaveUpdateMode();
+
   emit repaintRequested();
 
   return success;
@@ -2384,6 +2389,8 @@ bool QgsVectorLayer::rollBack( bool deleteBuffer )
   if ( rollbackExtent )
     updateExtents();
 
+  mDataProvider->leaveUpdateMode();
+
   emit repaintRequested();
   return true;
 }

src/providers/ogr/qgsogrfeatureiterator.cpp

@@ -44,6 +44,10 @@ QgsOgrFeatureIterator::QgsOgrFeatureIterator( QgsOgrFeatureSource* source, bool
   mFeatureFetched = false;
 
   mConn = QgsOgrConnPool::instance()->acquireConnection( mSource->mProvider->dataSourceUri() );
+  if ( mConn->ds == nullptr )
+  {
+    return;
+  }
 
   if ( mSource->mLayerName.isNull() )
   {
@@ -53,10 +57,18 @@ QgsOgrFeatureIterator::QgsOgrFeatureIterator( QgsOgrFeatureSource* source, bool
   {
     ogrLayer = OGR_DS_GetLayerByName( mConn->ds, TO8( mSource->mLayerName ) );
   }
+  if ( ogrLayer == nullptr )
+  {
+    return;
+  }
 
   if ( !mSource->mSubsetString.isEmpty() )
   {
     ogrLayer = QgsOgrUtils::setSubsetString( ogrLayer, mConn->ds, mSource->mEncoding, mSource->mSubsetString );
+    if ( ogrLayer == nullptr )
+    {
+      return;
+    }
     mSubsetStringSet = true;
   }
 
@@ -204,7 +216,7 @@ bool QgsOgrFeatureIterator::fetchFeature( QgsFeature& feature )
 {
   feature.setValid( false );
 
-  if ( mClosed )
+  if ( mClosed || ogrLayer == nullptr )
     return false;
 
   if ( mRequest.filterType() == QgsFeatureRequest::FilterFid )
@@ -249,7 +261,7 @@ bool QgsOgrFeatureIterator::fetchFeature( QgsFeature& feature )
 
 bool QgsOgrFeatureIterator::rewind()
 {
-  if ( mClosed )
+  if ( mClosed || ogrLayer == nullptr )
     return false;
 
   OGR_L_ResetReading( ogrLayer );