Fix race-condition in epoll1 poller's begin_worker() and pollset_kick()

which caused the designated poller to miss a kick in some cases

src/core/lib/iomgr/ev_epoll1_linux.c

@@ -600,6 +600,7 @@ static bool begin_worker(grpc_pollset *pollset, grpc_pollset_worker *worker,
     }
     gpr_mu_unlock(&neighbourhood->mu);
   }
+
   worker_insert(pollset, worker);
   pollset->begin_refs--;
   if (worker->kick_state == UNKICKED) {
@@ -628,7 +629,18 @@ static bool begin_worker(grpc_pollset *pollset, grpc_pollset_worker *worker,
             pollset->shutting_down);
   }
 
-  return worker->kick_state == DESIGNATED_POLLER && !pollset->shutting_down;
+  /* We release pollset lock in this function at a couple of places:
+   *   1. Brielfly when assigning pollset to a neighbourhood
+   *   2. When doing gpr_cv_wait()
+   * It is possible that 'kicked_without_poller' was set to true during (1) and
+   * 'shutting_down' is set to true during (1) or (2). If either of them is
+   * true, this worker cannot do polling */
+
+  /* TODO(sreek): Perhaps there is a better way to handle kicked_without_poller
+   * case; especially when the worker is the DESIGNATED_POLLER */
+
+  return worker->kick_state == DESIGNATED_POLLER && !pollset->shutting_down &&
+         !pollset->kicked_without_poller;
 }
 
 static bool check_neighbourhood_for_available_poller(