Provide a public API for invalidating connections.

QgsVectorDataProvider::forceReload() is the virtual method. It is implemented
in the OGR provider.
See here for reference:
http://gis.stackexchange.com/questions/159950/make-a-qgis-layer-update-from-a-changed-data-source

python/core/qgsvectordataprovider.sip

@@ -320,6 +320,8 @@ class QgsVectorDataProvider : QgsDataProvider
      */
     virtual QgsTransaction* transaction() const;
 
+    virtual void forceReload();
+
   protected:
     void clearMinMaxCache();
     void fillMinMaxCache();

src/core/qgsvectordataprovider.h

@@ -371,6 +371,15 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider
      */
     virtual QgsTransaction* transaction() const { return 0; }
 
+    /**
+     * Forces a reload of the underlying datasource if the provider implements this
+     * method.
+     * In particular on the OGR provider, a pooled connection will be invalidated.
+     * This forces QGIS to reopen a file or connection.
+     * This can be required if the underlying file is replaced.
+     */
+    virtual void forceReload() {}
+
   protected:
     void clearMinMaxCache();
     void fillMinMaxCache();

src/providers/ogr/qgsogrprovider.cpp

@@ -2439,6 +2439,11 @@ QByteArray QgsOgrProvider::quotedIdentifier( QByteArray field )
   return QgsOgrUtils::quotedIdentifier( field, ogrDriverName );
 }
 
+void QgsOgrProvider::forceReload()
+{
+  QgsOgrConnPool::instance()->invalidateConnections( filePath() );
+}
+
 QByteArray QgsOgrUtils::quotedIdentifier( QByteArray field, const QString& ogrDriverName )
 {
   if ( ogrDriverName == "MySQL" )

src/providers/ogr/qgsogrprovider.h

@@ -263,6 +263,13 @@ class QgsOgrProvider : public QgsVectorDataProvider
 
     QByteArray quotedIdentifier( QByteArray field );
 
+    /**
+     * A forced reload invalidates the underlying connection.
+     * E.g. in case a shapefile is replaced, the old file will be closed
+     * and the new file will be opened.
+     */
+    void forceReload();
+
   protected:
     /** Loads fields from input file to member attributeFields */
     void loadFields();