From e51034e33f225cbc0563941cfe3028149b5ee875 Mon Sep 17 00:00:00 2001
From: Mark Abraham <mark.j.abraham@gmail.com>
Date: Mon, 1 Nov 2021 08:58:18 +0100
Subject: [PATCH] Add missing MPI_Barrier to MpiTest

Fixes #4261
---
 src/testutils/tests/mpitest.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/testutils/tests/mpitest.cpp b/src/testutils/tests/mpitest.cpp
index f3d8c0f4ec..8d3b9521fc 100644
--- a/src/testutils/tests/mpitest.cpp
+++ b/src/testutils/tests/mpitest.cpp
@@ -74,6 +74,9 @@ TEST_F(MpiSelfTest, Runs)
     {
         reached_.resize(getNumberOfTestMpiRanks(), 0);
     }
+    // Needed for thread-MPI so that we resize the buffer before we
+    // fill it on non-master ranks.
+    MPI_Barrier(MPI_COMM_WORLD);
     int value = 1;
     MPI_Gather(&value, 1, MPI_INT, reached_.data(), 1, MPI_INT, 0, MPI_COMM_WORLD);
     if (gmx_node_rank() == 0)
-- 
2.22.0