Purge communicator before free on mpich4+.

Mpich from 4.0 on seems to bug when there is a message dangling on
freeing the communicator.

Author: vehre

src/runtime-libraries/mpi/CMakeLists.txt

@@ -154,6 +154,10 @@ if (mpi_version_out MATCHES "[Oo]pen[ -][Mm][Pp][Ii]")
   if(NOT DEFINED ENV{TRAVIS})
     message( STATUS "Open-MPI back end detected, passing --allow-run-as-root to allow tests to pass when run with sudo or as root." )
   endif()
+elseif (mpi_version_out MATCHES "HYDRA")
+  message(STATUS "MPICH detected")
+  target_compile_definitions(caf_mpi PRIVATE MPI_CLEAR_COMM_BEFORE_FREE)
+  target_compile_definitions(caf_mpi_static PRIVATE MPI_CLEAR_COMM_BEFORE_FREE)
 endif ()
 
 if("${CMAKE_Fortran_COMPILER_ID}" STREQUAL "GNU")

src/runtime-libraries/mpi/mpi_caf.c

@@ -1104,6 +1104,24 @@ finalize_internal(int status_code)
     ierr = MPI_Finalize(); chk_err(ierr);
   }
 #else
+#ifdef MPI_CLEAR_COMM_BEFORE_FREE
+  {
+    int probe_flag;
+    MPI_Status status;
+    do {
+      ierr = MPI_Iprobe(MPI_ANY_SOURCE, MPI_ANY_TAG, CAF_COMM_WORLD, &probe_flag,
+                        &status); /* error is not of interest. */
+      if (probe_flag) {
+        int cnt;
+        MPI_Get_count(&status, MPI_BYTE, &cnt);
+        void * buf = alloca(cnt);
+        ierr = MPI_Recv(buf, cnt, MPI_BYTE, status.MPI_SOURCE, status.MPI_TAG,
+                        CAF_COMM_WORLD, &status); chk_err(ierr);
+      }
+    } while (probe_flag);
+  }
+#endif
+  dprint("freeing caf's communicator.\n");
   ierr = MPI_Comm_free(&CAF_COMM_WORLD); chk_err(ierr);
 
   CAF_Win_unlock_all(*stat_tok);
@@ -1112,6 +1130,7 @@ finalize_internal(int status_code)
   /* Only call Finalize if CAF runtime Initialized MPI. */
   if (caf_owns_mpi)
   {
+    dprint("Finalizing MPI.\n");
     ierr = MPI_Finalize(); chk_err(ierr);
   }
 #endif