Add a detectable edge for the first insert (so far untested)

src/core/support/stack_lockfree.c

@@ -95,7 +95,7 @@ void gpr_stack_lockfree_destroy(gpr_stack_lockfree *stack) {
   gpr_free(stack);
 }
 
-void gpr_stack_lockfree_push(gpr_stack_lockfree *stack, int entry) {
+int gpr_stack_lockfree_push(gpr_stack_lockfree *stack, int entry) {
   lockfree_node head;
   lockfree_node newhead;
 
@@ -112,6 +112,7 @@ void gpr_stack_lockfree_push(gpr_stack_lockfree *stack, int entry) {
   } while (!gpr_atm_rel_cas(&(stack->head.atm),
                             head.atm, newhead.atm));
   /* Use rel_cas above to make sure that entry index is set properly */
+  return head.atm == INVALID_ENTRY_INDEX;
 }
 
 int gpr_stack_lockfree_pop(gpr_stack_lockfree *stack) {

src/core/support/stack_lockfree.h

@@ -42,7 +42,8 @@ gpr_stack_lockfree *gpr_stack_lockfree_create(int entries);
 void gpr_stack_lockfree_destroy(gpr_stack_lockfree *);
 
 /* Pass in a valid entry number for the next stack entry */
-void gpr_stack_lockfree_push(gpr_stack_lockfree *, int entry);
+/* Returns 1 if this is the first element on the stack, 0 otherwise */
+int gpr_stack_lockfree_push(gpr_stack_lockfree *, int entry);
 
 /* Returns -1 on empty or the actual entry number */
 int gpr_stack_lockfree_pop(gpr_stack_lockfree *);