Provide a way to look at the head of the ring

This commit was SVN r24832.

opal/class/opal_ring_buffer.c

@@ -144,6 +144,13 @@ void* opal_ring_buffer_pop(opal_ring_buffer_t *ring)
     OPAL_ACQUIRE_THREAD(&(ring->lock), &(ring->cond), &(ring->in_use));
     if (ring->size <= i || -1 == ring->tail) {
         p = NULL;
+    } else if (i < 0) {
+        /* return the value at the head of the ring */
+        if (ring->head == 0) {
+            p = ring->addr[ring->size - 1];
+        } else {
+            p = ring->addr[ring->head - 1];
+        }
     } else {
         /* calculate the offset of the tail in the ring */
         offset = ring->tail + i;

opal/class/opal_ring_buffer.h

@@ -95,7 +95,8 @@ OPAL_DECLSPEC void* opal_ring_buffer_pop(opal_ring_buffer_t *ring);
 
 /*
  * Access an element of the ring, without removing it, indexed
- * starting at the tail
+ * starting at the tail - a value of -1 will return the element
+ * at the head of the ring
  */
 OPAL_DECLSPEC void* opal_ring_buffer_poke(opal_ring_buffer_t *ring, int i);