WellConnections: bound the Cartesian->compressed lookup

WellConnections::init translates each well connection into a compressed cell
index with

    cart_grid_idx = i + nx*(j + ny*k);
    compressed_idx = cartesian_to_compressed[cart_grid_idx];

and does the same for possibleFutureConnections.  Neither lookup checked that
the computed position is inside cartesian_to_compressed before indexing it.

The map is sized by the level-zero Cartesian grid, and a connection is not
obliged to carry a position in that grid -- a completion inside a local grid
refinement, for instance, carries a position local to the refined grid.  When
that position lands past the end of the map the result is an out-of-bounds read
during load balancing.

Route both lookups through a helper that returns -1 for a position outside the
map, which the existing "ignore inactive cells" test already handles.  Nothing
changes for a position that was in range, so grids whose connections all
address level zero are unaffected.

Author: hnil

opm/grid/common/WellConnections.cpp

@@ -115,6 +115,22 @@ void WellConnections::init([[maybe_unused]] const std::vector<OpmWellType>& well
 #if HAVE_OPM_COMMON
     well_indices_.resize(wells.size());
 
+    // Both loops below index cartesian_to_compressed with a position derived
+    // from the connection, and neither checked that the position is actually
+    // inside the map.  Bound the lookup against the container that is being
+    // indexed, so that a position which is not a level-zero Cartesian index
+    // cannot read past the end of it.
+    const auto lookup = [&cartesian_to_compressed](const int cart_grid_idx)
+    {
+        if ((cart_grid_idx < 0) ||
+            (static_cast<std::size_t>(cart_grid_idx) >= cartesian_to_compressed.size()))
+        {
+            return -1;
+        }
+
+        return cartesian_to_compressed[cart_grid_idx];
+    };
+
     // We assume that we know all the wells.
     int index=0;
     for (const auto& well : wells) {
@@ -126,7 +142,7 @@ void WellConnections::init([[maybe_unused]] const std::vector<OpmWellType>& well
             int j = connection.getJ();
             int k = connection.getK();
             int cart_grid_idx = i + cartesianSize[0]*(j + cartesianSize[1]*k);
-            int compressed_idx = cartesian_to_compressed[cart_grid_idx];
+            int compressed_idx = lookup(cart_grid_idx);
             if ( compressed_idx >= 0 ) // Ignore connections in inactive cells.
             {
                 well_indices.insert(compressed_idx);
@@ -135,7 +151,7 @@ void WellConnections::init([[maybe_unused]] const std::vector<OpmWellType>& well
         const auto possibleFutureConnectionSetIt = possibleFutureConnections.find(well.name());
         if (possibleFutureConnectionSetIt != possibleFutureConnections.end()) {
             for (auto& cart_grid_idx : possibleFutureConnectionSetIt->second) {
-                int compressed_idx = cartesian_to_compressed[cart_grid_idx];
+                int compressed_idx = lookup(cart_grid_idx);
                 if ( compressed_idx >= 0 ) // Ignore connections in inactive cells.
                 {
                     well_indices.insert(compressed_idx);