From f1c65fffdda8a2e57d908c8b47002da8ed56486b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Franz=20P=C3=B6schel?= <franz.poeschel@gmail.com>
Date: Wed, 16 Aug 2023 15:13:31 +0200
Subject: [PATCH] explicit collectivism in get_mpi_ranks_meta_info

---
 include/openPMD/Series.hpp    | 12 +++++++++++-
 src/Series.cpp                | 15 +++++++++++++--
 src/binding/python/Series.cpp | 10 +++++++---
 test/SerialIOTest.cpp         |  6 ++++--
 4 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/include/openPMD/Series.hpp b/include/openPMD/Series.hpp
index be73f09e0f..b4861e7cc1 100644
--- a/include/openPMD/Series.hpp
+++ b/include/openPMD/Series.hpp
@@ -362,10 +362,20 @@ class Series : public Attributable
 
     /**
      * @throw   no_such_attribute_error If optional attribute is not present.
+     * @param collective Run this read operation collectively.
+                There might be an enormous IO overhead if running this
+                operation non-collectively.
+                To make this explicit to users, there is no default parameter.
+                Parameter is ignored if compiling without MPI support, (it is
+                present for the sake of a consistent API).
      * @return  Vector with a String per (writing) MPI rank, indicating user-
      *          defined meta information per rank. Example: host name.
      */
-    chunk_assignment::RankMeta mpiRanksMetaInfo();
+#if openPMD_HAVE_MPI
+    chunk_assignment::RankMeta mpiRanksMetaInfo(bool collective);
+#else
+    chunk_assignment::RankMeta mpiRanksMetaInfo(bool collective = false);
+#endif
 
     /**
      * @brief Set the Mpi Ranks Meta Info attribute, i.e. a Vector with
diff --git a/src/Series.cpp b/src/Series.cpp
index 19f132dfa2..2d7054a3c8 100644
--- a/src/Series.cpp
+++ b/src/Series.cpp
@@ -186,7 +186,12 @@ Series &Series::setMeshesPath(std::string const &mp)
     return *this;
 }
 
-chunk_assignment::RankMeta Series::mpiRanksMetaInfo()
+#if openPMD_HAVE_MPI
+chunk_assignment::RankMeta Series::mpiRanksMetaInfo(bool collective)
+#else
+chunk_assignment::RankMeta
+Series::mpiRanksMetaInfo([[maybe_unused]] bool collective)
+#endif
 {
     auto &series = get();
     auto &rankTable = series.m_rankTable;
@@ -263,7 +268,7 @@ chunk_assignment::RankMeta Series::mpiRanksMetaInfo()
     };
 
 #if openPMD_HAVE_MPI
-    if (series.m_communicator.has_value())
+    if (collective && series.m_communicator.has_value())
     {
         auto comm = series.m_communicator.value();
         int rank{0}, size{1};
@@ -275,6 +280,12 @@ chunk_assignment::RankMeta Series::mpiRanksMetaInfo()
         }
         MPI_Bcast(get.get(), writerRanks * lineWidth, MPI_CHAR, 0, comm);
     }
+    else if (collective)
+    {
+        throw error::WrongAPIUsage(
+            "Cannot read rank table collectively: Series was not constructed "
+            "collectively.");
+    }
     else
     {
         doReadDataset();
diff --git a/src/binding/python/Series.cpp b/src/binding/python/Series.cpp
index b24f76ea6e..9fdda90a00 100644
--- a/src/binding/python/Series.cpp
+++ b/src/binding/python/Series.cpp
@@ -222,10 +222,14 @@ this method.
         .def_property("base_path", &Series::basePath, &Series::setBasePath)
         .def_property(
             "meshes_path", &Series::meshesPath, &Series::setMeshesPath)
-        .def_property(
-            "mpi_ranks_meta_info",
+        .def(
+            "get_mpi_ranks_meta_info",
             &Series::mpiRanksMetaInfo,
-            &Series::setMpiRanksMetaInfo)
+            py::arg("collective"))
+        .def(
+            "set_mpi_ranks_meta_info",
+            &Series::setMpiRanksMetaInfo,
+            py::arg("my_rank_info"))
         .def_property(
             "particles_path", &Series::particlesPath, &Series::setParticlesPath)
         .def_property("author", &Series::author, &Series::setAuthor)
diff --git a/test/SerialIOTest.cpp b/test/SerialIOTest.cpp
index 76988217bb..f511d878e6 100644
--- a/test/SerialIOTest.cpp
+++ b/test/SerialIOTest.cpp
@@ -1616,7 +1616,7 @@ inline void write_test(const std::string &backend)
 
     Series read("../samples/serial_write." + backend, Access::READ_ONLY);
     REQUIRE(
-        read.mpiRanksMetaInfo() ==
+        read.mpiRanksMetaInfo(/* collective = */ false) ==
         chunk_assignment::RankMeta{{0, host_info::hostname()}});
 }
 
@@ -2144,7 +2144,9 @@ inline void fileBased_write_test(const std::string &backend)
             std::string fullPath =
                 std::string("../samples/subdir/") + entry->d_name;
             Series single_file(fullPath, Access::READ_ONLY);
-            REQUIRE(single_file.mpiRanksMetaInfo() == compare);
+            REQUIRE(
+                single_file.mpiRanksMetaInfo(/* collective = */ false) ==
+                compare);
         }
         closedir(directory);
         close(dirfd);