From 3646decebcb5208b252f9117c121c794f1d92b86 Mon Sep 17 00:00:00 2001
From: Sam Clarke-Green <sam.clarke@metoffice.gov.uk>
Date: Wed, 26 Nov 2025 13:19:56 +0000
Subject: [PATCH 1/4] Fix return type of MPIContext::get_handle

The get_handle() method should return an MPI_Comm and not an int,
because this causes failures when building with OpenMPI.
---
 src/c++/mpi_context.cpp | 2 +-
 src/c++/mpi_context.h   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/c++/mpi_context.cpp b/src/c++/mpi_context.cpp
index 551273af..72e8b7b3 100644
--- a/src/c++/mpi_context.cpp
+++ b/src/c++/mpi_context.cpp
@@ -123,7 +123,7 @@ int meto::MPIContext::get_size() { return comm_size_; }
  * @returns The MPI communicator handle.
  */
 
-int meto::MPIContext::get_handle() { return comm_handle_; }
+MPI_Comm meto::MPIContext::get_handle() { return comm_handle_; }
 
 /**
  * @brief Gets the identifying tag.
diff --git a/src/c++/mpi_context.h b/src/c++/mpi_context.h
index cb6668a7..ce0baabd 100644
--- a/src/c++/mpi_context.h
+++ b/src/c++/mpi_context.h
@@ -55,7 +55,7 @@ class MPIContext {
   // Getters
   int get_size();
   int get_rank();
-  int get_handle();
+  MPI_Comm get_handle();
 
   std::string get_tag() const;
 };

From c73512fa261fdbe2882c77c88708873edbb044ff Mon Sep 17 00:00:00 2001
From: Sam Clarke-Green <sam.clarke@metoffice.gov.uk>
Date: Thu, 8 Jan 2026 19:53:44 +0000
Subject: [PATCH 2/4] Protect MPI_Abort in error handler

Check whether MPI is initialised before trying to call MPI_Abort.
This improves compliance with the standard and prevents test failures
when building with OpenMPI.
---
 src/c++/error_handler.cpp | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/c++/error_handler.cpp b/src/c++/error_handler.cpp
index acd47992..4818d567 100644
--- a/src/c++/error_handler.cpp
+++ b/src/c++/error_handler.cpp
@@ -21,6 +21,13 @@
 meto::error_handler::error_handler(const std::string &customError,
                                    int errorCode) {
   MPI_Comm comm = MPI_COMM_WORLD;
+  int flag = 0;
+
   std::cerr << customError << "\n";
-  MPI_Abort(comm, errorCode);
+
+  if (MPI_Initialized(&flag)) {
+    MPI_Abort(comm, errorCode);
+  } else {
+    exit(errorCode);
+  }
 }

From fc074894abedd0acb26a6207d6b208d4d9c05711 Mon Sep 17 00:00:00 2001
From: Sam Clarke-Green <sam.clarke@metoffice.gov.uk>
Date: Tue, 20 Jan 2026 15:34:00 +0000
Subject: [PATCH 3/4] Replace exit with std::exit

---
 src/c++/error_handler.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/c++/error_handler.cpp b/src/c++/error_handler.cpp
index 4818d567..e8fbb85b 100644
--- a/src/c++/error_handler.cpp
+++ b/src/c++/error_handler.cpp
@@ -7,6 +7,7 @@
 
 #include <iostream>
 #include <string>
+#include <cstdlib>
 
 #include "error_handler.h"
 #include "vernier_mpi.h"
@@ -28,6 +29,6 @@ meto::error_handler::error_handler(const std::string &customError,
   if (MPI_Initialized(&flag)) {
     MPI_Abort(comm, errorCode);
   } else {
-    exit(errorCode);
+    std::exit(errorCode);
   }
 }

From 2ed46f25ae454d97a123bbb4f23926bd6483a508 Mon Sep 17 00:00:00 2001
From: Sam Clarke-Green <sam.clarke@metoffice.gov.uk>
Date: Tue, 20 Jan 2026 15:35:42 +0000
Subject: [PATCH 4/4] Let clang format reorder the includes

---
 src/c++/error_handler.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/c++/error_handler.cpp b/src/c++/error_handler.cpp
index e8fbb85b..62295144 100644
--- a/src/c++/error_handler.cpp
+++ b/src/c++/error_handler.cpp
@@ -5,9 +5,9 @@
  * -----------------------------------------------------------------------------
  */
 
+#include <cstdlib>
 #include <iostream>
 #include <string>
-#include <cstdlib>
 
 #include "error_handler.h"
 #include "vernier_mpi.h"