Fix crashes with parallel access to GRASS vectors

src/providers/grass/qgsgrassfeatureiterator.cpp

@@ -62,9 +62,15 @@ void copy_boxlist_and_destroy( struct boxlist *blist, struct ilist * list )
   }
 #endif
 
+
+QMutex QgsGrassFeatureIterator::sMutex;
+
+
 QgsGrassFeatureIterator::QgsGrassFeatureIterator( QgsGrassFeatureSource* source, bool ownSource, const QgsFeatureRequest& request )
     : QgsAbstractFeatureIteratorFromSource( source, ownSource, request )
 {
+  sMutex.lock();
+
   // Init structures
   mPoints = Vect_new_line_struct();
   mCats = Vect_new_cats_struct();
@@ -344,6 +350,8 @@ bool QgsGrassFeatureIterator::close()
 
   free( mSelection );
 
+  sMutex.unlock();
+
   mClosed = true;
   return true;
 }

src/providers/grass/qgsgrassfeatureiterator.h

@@ -17,6 +17,7 @@
 
 #include "qgsfeatureiterator.h"
 
+#include <QMutex>
 
 class QgsGrassProvider;
 
@@ -111,6 +112,12 @@ class QgsGrassFeatureIterator : public QgsAbstractFeatureIteratorFromSource<QgsG
      *  @param map pointer to map structure
      */
     void allocateSelection( struct Map_info *map );
+
+    //! Mutex that protects GRASS library from parallel access from multiple iterators at once.
+    //! The library uses static/global variables in various places when accessing vector data,
+    //! making it non-reentrant and thus impossible to use from multiple threads.
+    //! (e.g. static LocList in Vect_select_lines_by_box, global BranchBuf in RTreeGetBranches)
+    static QMutex sMutex;
 };
 
 #endif // QGSGRASSFEATUREITERATOR_H