Undefined behavior in LB halo communication

[jngrad]

The LB halo communication framework relies on undefined behavior to calculate the length of strides. The use of int to calculate a pointer offset leads to an integer overflow for LB grids of size 9x9x9 or larger. UBSAN report:

/work/jgrad/espresso/src/core/grid_based_algorithms/halo.cpp:112:19: runtime error: signed integer overflow: 10648 * 202232 cannot be represented in type 'int'
    #0 in halo_dtset(char*, int, _Fieldtype*) /work/jgrad/espresso/src/core/grid_based_algorithms/halo.cpp:112:19
    #1 in halo_communication(HaloCommunicator const*, char*) /work/jgrad/espresso/src/core/grid_based_algorithms/halo.cpp:307:7
    #2 in lb_lbfluid_on_integration_start() /work/jgrad/espresso/src/core/grid_based_algorithms/lb_interface.cpp:128:5
    #3 in on_integration_start() /work/jgrad/espresso/src/core/event.cpp:109:3
    #4 in integrate(int, int) /work/jgrad/espresso/src/core/integrate.cpp:188:3
    #5 in mpi_integrate_local(int, int) /work/jgrad/espresso/src/core/integrate.cpp:405:3

MWE: compile ESPResSo with UBSAN and run the integration test lb_poiseuille_cylinder.py with the following change:

diff --git a/testsuite/python/lb_poiseuille_cylinder.py b/testsuite/python/lb_poiseuille_cylinder.py
index f0dee7984..981c0c23e 100644
--- a/testsuite/python/lb_poiseuille_cylinder.py
+++ b/testsuite/python/lb_poiseuille_cylinder.py
@@ -90,4 +90,6 @@ class LBPoiseuilleCommon:
 
         """
+        # disable periodicity except in the flow direction
+        self.system.periodicity = np.logical_not(self.params['axis'])
         local_lb_params = LB_PARAMS.copy()
         local_lb_params['ext_force_density'] = np.array(

This was never caught in CI because the part of the code in question is not covered by tests.