From 742eebec2d533ef113b595f656dcf6ce2073a181 Mon Sep 17 00:00:00 2001
From: Axel Kohlmeyer <akohlmey@gmail.com>
Date: Tue, 16 Feb 2021 20:22:18 -0500
Subject: [PATCH] support checking the size of MPI communicators and fail if
 LAMMPS and mpi4py have a mismatch

---
 python/lammps/core.py | 4 ++++
 src/library.cpp       | 8 +++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/python/lammps/core.py b/python/lammps/core.py
index 8639743a75..e13bf9585b 100644
--- a/python/lammps/core.py
+++ b/python/lammps/core.py
@@ -325,6 +325,10 @@ class lammps(object):
         else:
           MPI_Comm = c_void_p
 
+        # Detect whether LAMMPS and mpi4py definitely use different MPI libs
+        if sizeof(MPI_Comm) != self.lib.lammps_config_has_mpi_support():
+          raise Exception('Inconsistent MPI library in LAMMPS and mpi4py')
+
         narg = 0
         cargs = None
         if cmdargs:
diff --git a/src/library.cpp b/src/library.cpp
index 71bf205d90..2a7bbf07b3 100644
--- a/src/library.cpp
+++ b/src/library.cpp
@@ -4128,16 +4128,18 @@ void lammps_get_os_info(char *buffer, int buf_size)
 /* ---------------------------------------------------------------------- */
 
 /** This function is used to query whether LAMMPS was compiled with
- *  a real MPI library or in serial.
+ *  a real MPI library or in serial. For the real MPI library it
+ *  reports the size of the MPI communicator in bytes (4 or 8),
+ *  which allows to check for compatibility with a hosting code.
  *
- * \return 0 when compiled with MPI STUBS, otherwise 1 */
+ * \return 0 when compiled with MPI STUBS, otherwise the MPI_Comm size in bytes */
 
 int lammps_config_has_mpi_support()
 {
 #ifdef MPI_STUBS
   return 0;
 #else
-  return 1;
+  return sizeof(MPI_Comm);
 #endif
 }