This is a fix for bug Ticket #27

We were stuck in an infinite loop inside the rmaps round_robin
component when the user specified a host, then over subscribed it.
Instead of retuning an error, we looped forever.

For example:
 $ cat hostfile
  A slots=2 max-slots=2
  B slots=2 max-slots=2
 $ mpirun -np 3 --hostfile hostfile --host B
  <hang>

The loop would not terminate because both host A and B are in the 
'nodes' structure as they are both allocated to the job. However,
after allocating 2 slots to host B, we remove it from the node list
leaving us with a 'nodes' structure with just A in it. Since we can't
use host A, we keep looping here until we find a node that we can use.

This patch checks to make sure that if we get into this situation where
rmaps is looping over the list a second time without finding a node
during the first pass then we know that there are no nodes left to
use, so we have a resource allocation error, and should return to the user.

This patch should be moved to all of the release branches

This commit was SVN r10131.

orte/mca/rmaps/round_robin/rmaps_rr.c

@@ -104,7 +104,7 @@ static opal_list_item_t* get_next_mapped(opal_list_item_t *node_item,
                                          int num_mapped, 
                                          opal_list_t* nodes) 
 {
-    opal_list_item_t *item;
+    opal_list_item_t *item, *initial_item = NULL;
 
     /* Wrap around to beginning if we are at the end of the list */
     if (opal_list_get_end(nodes) == opal_list_get_next(node_item)) {
@@ -129,6 +129,11 @@ static opal_list_item_t* get_next_mapped(opal_list_item_t *node_item,
             return NULL;
         }
         
+        /* save the node we started with */
+        if( NULL == initial_item ) {
+            initial_item = item; 
+        }
+        
         /* Access next item in Round Robin Manner */
         if (opal_list_get_end(nodes) ==  opal_list_get_next(item)) {
             item = opal_list_get_first(nodes);
@@ -137,6 +142,12 @@ static opal_list_item_t* get_next_mapped(opal_list_item_t *node_item,
             item = opal_list_get_next(item);
         }
 
+        /* Check to make sure we didn't loop back around without
+         * finding a node in the mapping */
+        if( initial_item == item) {
+            return NULL;
+        }
+        
     } while( true );
 }