From 6f2f9c4a2f192f1587734c7cbe647a5169da4ccb Mon Sep 17 00:00:00 2001
From: Timo Heister <timo.heister@gmail.com>
Date: Tue, 22 Oct 2019 22:01:14 +0200
Subject: [PATCH] add DuplicatedCommunicator

---
 include/deal.II/base/mpi.h                    | 55 +++++++++++
 tests/mpi/communicator_duplicated.cc          | 91 +++++++++++++++++++
 .../communicator_duplicated.mpirun=2.output   |  2 +
 3 files changed, 148 insertions(+)
 create mode 100644 tests/mpi/communicator_duplicated.cc
 create mode 100644 tests/mpi/communicator_duplicated.mpirun=2.output

diff --git a/include/deal.II/base/mpi.h b/include/deal.II/base/mpi.h
index 2c03fabdf3..5f749a40e8 100644
--- a/include/deal.II/base/mpi.h
+++ b/include/deal.II/base/mpi.h
@@ -211,6 +211,61 @@ namespace Utilities
     void
     free_communicator(MPI_Comm &mpi_communicator);
 
+    /**
+     * Helper class to automatically duplicate and free an MPI
+     * @ref GlossMPICommunicator "communicator".
+     *
+     * This class duplicates the communicator given in the constructor
+     * using duplicate_communicator() and frees it automatically when
+     * this object gets destroyed by calling free_communicator(). You
+     * can access the wrapped communicator using operator*.
+     *
+     * This class exists to easily allow duplicating communicators without
+     * having to worry when and how to free it after usage.
+     */
+    class DuplicatedCommunicator
+    {
+    public:
+      /**
+       * Create a duplicate of the given @p communicator.
+       */
+      explicit DuplicatedCommunicator(const MPI_Comm &communicator)
+        : comm (duplicate_communicator(communicator))
+      {}
+
+      /**
+       * The destructor will free the communicator automatically.
+       */
+      ~DuplicatedCommunicator()
+      {
+        free_communicator(comm);
+      }
+
+      /**
+       * Access the stored communicator.
+       */
+      const MPI_Comm &operator*() const
+      {
+        return comm;
+      }
+
+      /**
+       * Do not allow making copies.
+       */
+      DuplicatedCommunicator(const DuplicatedCommunicator &) = delete;
+
+      /**
+       * Do not allow assignment of this class.
+       */
+      DuplicatedCommunicator& operator=(const DuplicatedCommunicator &) = delete;
+
+    private:
+	/**
+	 * The communicator of course.
+	 */
+      MPI_Comm comm;
+    };
+
     /**
      * If @p comm is an intracommunicator, this function returns a new
      * communicator @p newcomm with communication group defined by the
diff --git a/tests/mpi/communicator_duplicated.cc b/tests/mpi/communicator_duplicated.cc
new file mode 100644
index 0000000000..10b408d00f
--- /dev/null
+++ b/tests/mpi/communicator_duplicated.cc
@@ -0,0 +1,91 @@
+// ---------------------------------------------------------------------
+//
+// Copyright (C) 2017 - 2018 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// The deal.II library is free software; you can use it, redistribute
+// it, and/or modify it under the terms of the GNU Lesser General
+// Public License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+// The full text of the license can be found in the file LICENSE.md at
+// the top level directory of deal.II.
+//
+// ---------------------------------------------------------------------
+
+
+// check MPI::duplicate_communicator, free_communicator, and
+// DuplicatedCommunicator
+
+
+#include <deal.II/base/mpi.h>
+
+#include "../tests.h"
+
+
+void test(const MPI_Comm comm)
+{
+  int tag = 12345;
+
+  const auto my_rank = Utilities::MPI::this_mpi_process(comm);
+
+  MPI_Comm comm2 = Utilities::MPI::duplicate_communicator(comm);
+  Utilities::MPI::DuplicatedCommunicator pcomm3(comm);
+
+  if (my_rank==1)
+    {
+      int value[3]={1,2,3};
+      int dest = 0;
+      
+      MPI_Send(&value[0],
+	       1,
+	       MPI_UNSIGNED,
+	       dest,
+	       tag,
+	       comm);
+      MPI_Send(&value[1],
+	       1,
+	       MPI_UNSIGNED,
+	       dest,
+	       tag,
+	       comm2);
+      MPI_Send(&value[2],
+	       1,
+	       MPI_UNSIGNED,
+	       dest,
+	       tag,
+	       *pcomm3);
+    }
+
+  if (my_rank == 0)
+    {
+      int value[3];
+      int src = 1;
+
+      // receive in reverse order, if duplication of communicators worked,
+      // these won't be mixed up!
+      MPI_Recv(&value[2], 1, MPI_UNSIGNED,
+	       src, tag, *pcomm3, MPI_STATUS_IGNORE);
+      MPI_Recv(&value[1], 1, MPI_UNSIGNED,
+	       src, tag, comm2, MPI_STATUS_IGNORE);
+      MPI_Recv(&value[0], 1, MPI_UNSIGNED,
+	       src, tag, comm, MPI_STATUS_IGNORE);
+
+      deallog << value[0] << " " << value[1] << " " << value[2] << std::endl;
+    }
+
+  Utilities::MPI::free_communicator(comm2);
+}
+
+
+
+int
+main(int argc, char **argv)
+{
+  Utilities::MPI::MPI_InitFinalize mpi_initialization(
+    argc, argv, testing_max_num_threads());
+
+  mpi_initlog();
+
+  test(MPI_COMM_WORLD);
+}
diff --git a/tests/mpi/communicator_duplicated.mpirun=2.output b/tests/mpi/communicator_duplicated.mpirun=2.output
new file mode 100644
index 0000000000..ed62a0ef21
--- /dev/null
+++ b/tests/mpi/communicator_duplicated.mpirun=2.output
@@ -0,0 +1,2 @@
+
+DEAL::1 2 3
-- 
2.39.5