Send a one byte message in send_fd because otherwise it sometimes hangs on Linux. (#41)

src/plasma_client.c

@@ -164,8 +164,10 @@ int plasma_subscribe(plasma_store_conn *conn) {
   plasma_request req = {};
   plasma_send_request(conn->conn, PLASMA_SUBSCRIBE, &req);
   /* Send the file descriptor that the Plasma store should use to push
-   * notifications about sealed objects to this client. */
-  send_fd(conn->conn, fd[1], NULL, 0);
+   * notifications about sealed objects to this client. We include a one byte
+   * message because otherwise it seems to hang on Linux. */
+  char dummy = '\0';
+  send_fd(conn->conn, fd[1], &dummy, 1);
   /* Return the file descriptor that the client should use to read notifications
    * about sealed objects. */
   return fd[0];

src/plasma_store.c

@@ -282,7 +282,8 @@ void send_notifications(event_loop *loop,
 /* Subscribe to notifications about sealed objects. */
 void subscribe_to_updates(plasma_store_state *s, int conn) {
   LOG_DEBUG("subscribing to updates");
-  int fd = recv_fd(conn, NULL, 0);
+  char dummy;
+  int fd = recv_fd(conn, &dummy, 1);
   CHECKM(HASH_CNT(handle, s->open_objects) == 0,
          "plasma_subscribe should be called before any objects are created.");
   CHECKM(HASH_CNT(handle, s->sealed_objects) == 0,