Ensure MPI is initialized before getting rank, which may be called early by
Logger::write() via dolfin::info().

Fixes "MPI_Comm_rank called before MPI_INIT" error from
`demo_stokes-iterative_serial` and `demo_waveguide_serial` tests.

Submitted upstream at https://bitbucket.org/fenics-project/dolfin/issues/1127

--- a/dolfin/common/MPI.cpp
+++ b/dolfin/common/MPI.cpp
@@ -143,6 +143,7 @@ MPI_Info& dolfin::MPIInfo::operator*()
 unsigned int dolfin::MPI::rank(const MPI_Comm comm)
 {
 #ifdef HAS_MPI
+  SubSystemsManager::init_mpi();
   int rank;
   MPI_Comm_rank(comm, &rank);
   return rank;
diff --git a/dolfin/common/MPI.h b/dolfin/common/MPI.h
index b93f6df30..854114a3f 100644
--- a/dolfin/common/MPI.h
+++ b/dolfin/common/MPI.h
@@ -102,12 +102,13 @@ namespace dolfin
       /// communicator
       void reset(MPI_Comm comm);
 
-      /// Return process rank for the communicator
+      /// Return process rank for the communicator. This function will
+      /// also initialize MPI if it hasn't already been initialised.
       unsigned int rank() const;
 
       /// Return size of the group (number of processes) associated
-      /// with the communicator. This function will also intialise MPI
-      /// if it hasn't already been intialised.
+      /// with the communicator. This function will also initialise MPI
+      /// if it hasn't already been initialised.
       unsigned int size() const;
 
       /// Set a barrier (synchronization point)
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<div class='path'>path: <a href='/guix/log/?id=b82a18c8a33482249c634de1e91be18ac10a3c76'>root</a>/<a href='/guix/log/README?id=b82a18c8a33482249c634de1e91be18ac10a3c76'>README</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/README?id=b82a18c8a33482249c634de1e91be18ac10a3c76&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-05-20 01:58:04 +0200'>2021-05-20</span></td><td><a href='/guix/commit/README?id=c0087d33705f53cc3619a1a43a33602aaecc3a8b'>The #guix channel is hosted by Libera Chat.</a><span class='msg-avail'>...<span class='msg-tooltip'>* README (Contact): Update network name.
* ROADMAP: Likewise.
* doc/contributing.texi (Contributing): Likewise.
* doc/guix.texi (After System Installation): Likewise.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-10-23 12:57:20 -0400'>2020-10-23</span></td><td><a href='/guix/commit/README?id=b5aaa3a4fb985d21e21e2a67f48b52f13ebd4728'>doc: Update README.</a><span class='msg-avail'>...<span class='msg-tooltip'>* README (Installation): Do not mention installing Guix from Git anymore.
(Building from Git): Add section.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-10-22 22:47:54 -0400'>2020-10-22</span></td><td><a href='/guix/commit/README?id=503d2bfde08f9bf97bbf430516d2f19faedd4319'>README: Refer to the manual for building from Git.</a><span class='msg-avail'>...<span class='msg-tooltip'>* README (Installing Guix from Guix): Remove section.
(Installation): For installing from Git, refer to the "Building from Git"
section of the manual and suggest using './pre-inst-env guix pull' rather than
'make install' for installing Guix.
</span></span></td><td>Maxim Cournoyer</td></tr>