Correctly handle situations where GetNodes does not have local nodes to share

Author: heplesser

nestkernel/mpi_manager.cpp

@@ -326,7 +326,12 @@ nest::MPIManager::communicate( std::vector< long >& local_nodes, std::vector< lo
   num_nodes_per_rank[ get_rank() ] = local_nodes.size();
   communicate( num_nodes_per_rank );
 
-  size_t num_globals = std::accumulate( num_nodes_per_rank.begin(), num_nodes_per_rank.end(), 0 );
+  const size_t num_globals = std::accumulate( num_nodes_per_rank.begin(), num_nodes_per_rank.end(), 0 );
+  if ( num_globals == 0 )
+  {
+    return; // must return here to avoid passing address to empty global_nodes below
+  }
+
   global_nodes.resize( num_globals, 0L );
 
   // Set up displacements vector. Entry i specifies the displacement (relative
@@ -337,7 +342,9 @@ nest::MPIManager::communicate( std::vector< long >& local_nodes, std::vector< lo
     displacements.at( i ) = displacements.at( i - 1 ) + num_nodes_per_rank.at( i - 1 );
   }
 
-  MPI_Allgatherv( &( *local_nodes.begin() ),
+  // avoid dereferencing empty vector
+  const auto send_ptr = local_nodes.empty() ? nullptr : &local_nodes[ 0 ];
+  MPI_Allgatherv( send_ptr,
     local_nodes.size(),
     MPI_Type< long >::type,
     &global_nodes[ 0 ],

testsuite/pytests/test_getnodes.py

@@ -66,6 +66,16 @@ def test_GetNodes_with_params(self):
 
         self.assertEqual(nodes_exp_ref, nodes_exp)
 
+    def test_GetNodes_no_match(self):
+        """
+        Ensure we get an empty result if nothing matches.
+
+        This would lead to crashes in MPI-parallel code before #3460.
+        """
+
+        nodes = nest.GetNodes({"V_m": 100.0})
+        self.assertEqual(len(nodes), 0)
+
 
 def suite():
     suite = unittest.makeSuite(GetNodesTestCase, "test")