Add missing function for completion queue

src/core/lib/surface/completion_queue.c

@@ -86,6 +86,7 @@ struct grpc_completion_queue {
 };
 
 #define POLLSET_FROM_CQ(cq) ((grpc_pollset *)(cq + 1))
+#define CQ_FROM_POLLSET(ps) (((grpc_completion_queue *)ps) - 1)
 
 static gpr_mu g_freelist_mu;
 static grpc_completion_queue *g_freelist;
@@ -514,6 +515,10 @@ grpc_pollset *grpc_cq_pollset(grpc_completion_queue *cc) {
   return POLLSET_FROM_CQ(cc);
 }
 
+grpc_completion_queue *grpc_cq_from_pollset(grpc_pollset *ps) {
+  return CQ_FROM_POLLSET(ps);
+}
+
 void grpc_cq_mark_non_listening_server_cq(grpc_completion_queue *cc) {
   cc->is_non_listening_server_cq = 1;
 }

test/core/surface/completion_queue_test.c

@@ -63,6 +63,12 @@ static void test_no_op(void) {
   shutdown_and_destroy(grpc_completion_queue_create(NULL));
 }
 
+static void test_pollset_conversion() {
+  grpc_completion_queue *cq = grpc_completion_queue(NULL);
+  GPR_ASSERT(grpc_cq_from_pollset(grpc_cq_pollset(cq)) == cq);
+  shutdown_and_destroy(cq);
+}
+
 static void test_wait_empty(void) {
   grpc_completion_queue *cc;
   grpc_event event;