Per request from Nathan, let the user provide a NULL return list for dstore.fetch to indicate they just want to know if the key is present (but don't care about the actual value). Saves dealing with the list and copying data when not needed.

opal/mca/dstore/hash/dstore_hash.c

@@ -179,6 +179,11 @@ static int fetch(struct opal_dstore_base_module_t *imod,
 
     /* if the key is NULL, that we want everything */
     if (NULL == key) {
+        /* must provide an output list or this makes no sense */
+        if (NULL == kvs) {
+            OPAL_ERROR_LOG(OPAL_ERR_BAD_PARAM);
+            return OPAL_ERR_BAD_PARAM;
+        }
         OPAL_LIST_FOREACH(kv, &proc_data->data, opal_value_t) {
             /* copy the value */
             if (OPAL_SUCCESS != (rc = opal_dss.copy((void**)&knew, kv, OPAL_VALUE))) {
@@ -207,6 +212,12 @@ static int fetch(struct opal_dstore_base_module_t *imod,
         return OPAL_ERR_NOT_FOUND;
     }
 
+    /* if the user provided a NULL list object, then they
+     * just wanted to know if the key was present */
+    if (NULL == kvs) {
+        return OPAL_SUCCESS;
+    }
+    
     /* create the copy */
     if (OPAL_SUCCESS != (rc = opal_dss.copy((void**)&knew, kv, OPAL_VALUE))) {
         OPAL_ERROR_LOG(rc);