Address some potential MPI bugs (#688)

* Fix two potential MPI bugs.

Two conditions where a function could be returned from before the end,
and the code after the return makes MPI calls or modifies the MPI state.
This could lead to MPI hanging if it gets left in a invalid or bad
state.

armi/mpiActions.py

@@ -420,6 +420,7 @@ def invokeHook(self):
                         )
                     )
 
+        actionResult = None
         try:
             action = mpiComm.scatter(self._actions, root=0)
             # create a new communicator that only has these specific dudes running
@@ -430,7 +431,7 @@ def invokeHook(self):
             context.MPI_SIZE = context.MPI_COMM.Get_size()
             context.MPI_NODENAMES = context.MPI_COMM.allgather(context.MPI_NODENAME)
             if hasAction:
-                return action.invoke(self.o, self.r, self.cs)
+                actionResult = action.invoke(self.o, self.r, self.cs)
         finally:
             # restore the global variables
             context.MPI_DISTRIBUTABLE = canDistribute
@@ -439,6 +440,8 @@ def invokeHook(self):
             context.MPI_SIZE = mpiSize
             context.MPI_NODENAMES = mpiNodeNames
 
+        return actionResult
+
 
 class MpiActionError(Exception):
     """Exception class raised when error conditions occur during an MpiAction."""

armi/utils/pathTools.py

@@ -240,6 +240,8 @@ def cleanPath(path, mpiRank=0):
     """
     valid = False
     if not os.path.exists(path):
+        if context.MPI_SIZE > 1:
+            context.MPI_COMM.barrier()
         return True
 
     for validPath in [