Fix a leaked lock, and make a debug-only detection for this class of bug

src/core/iomgr/pollset_multipoller_with_poll_posix.c

@@ -72,7 +72,7 @@ static void multipoll_with_poll_pollset_add_fd(grpc_pollset *pollset,
   pollset_hdr *h = pollset->data.ptr;
   /* TODO(ctiller): this is O(num_fds^2); maybe switch to a hash set here */
   for (i = 0; i < h->fd_count; i++) {
-    if (h->fds[i] == fd) return;
+    if (h->fds[i] == fd) goto exit;
   }
   if (h->fd_count == h->fd_capacity) {
     h->fd_capacity = GPR_MAX(h->fd_capacity + 8, h->fd_count * 3 / 2);
@@ -80,6 +80,7 @@ static void multipoll_with_poll_pollset_add_fd(grpc_pollset *pollset,
   }
   h->fds[h->fd_count++] = fd;
   GRPC_FD_REF(fd, "multipoller");
+exit:  
   if (and_unlock_pollset) {
     gpr_mu_unlock(&pollset->mu);
   }

src/core/iomgr/pollset_posix.c

@@ -106,11 +106,21 @@ void grpc_pollset_init(grpc_pollset *pollset) {
 void grpc_pollset_add_fd(grpc_pollset *pollset, grpc_fd *fd) {
   gpr_mu_lock(&pollset->mu);
   pollset->vtable->add_fd(pollset, fd, 1);
+#ifndef NDEBUG
+  /* this will deadlock if the unlocking rules aren't correctly implemented */
+  gpr_mu_lock(&pollset->mu);
+  gpr_mu_unlock(&pollset->mu);
+#endif
 }
 
 void grpc_pollset_del_fd(grpc_pollset *pollset, grpc_fd *fd) {
   gpr_mu_lock(&pollset->mu);
   pollset->vtable->del_fd(pollset, fd, 1);
+#ifndef NDEBUG  
+  /* this will deadlock if the unlocking rules aren't correctly implemented */
+  gpr_mu_lock(&pollset->mu);
+  gpr_mu_unlock(&pollset->mu);
+#endif
 }
 
 static void finish_shutdown(grpc_pollset *pollset) {