fix cache server startup hang when GLOG_v=0

mindspore/ccsrc/minddata/dataset/engine/cache/cache_admin_arg.cc

@@ -588,26 +588,31 @@ Status CacheAdminArgHandler::StartServer(CommandId command_id) {
     close(fd[1]);
     (void)dup2(fd[0], STDIN_FILENO);
     close(fd[0]);
-    int status;
-    if (waitpid(pid, &status, 0) == -1) {
-      RETURN_STATUS_UNEXPECTED("waitpid fails. errno = " + std::to_string(errno));
-    }
     std::string msg;
     const int32_t buf_sz = 1024;
     msg.resize(buf_sz);
     auto n = read(0, msg.data(), buf_sz);
+    // keep reading until we drain the pipe
+    while (n > 0) {
+      msg.resize(n);
+      // Not an error, some info message goes to stdout
+      std::cout << msg;
+      msg.resize(buf_sz);
+      n = read(0, msg.data(), buf_sz);
+    }
     if (n < 0) {
       std::string err_msg = "Failed to read from pipeline " + std::to_string(errno);
       RETURN_STATUS_UNEXPECTED(err_msg);
     }
-    msg.resize(n);
+
+    int status;
+    if (waitpid(pid, &status, 0) == -1) {
+      RETURN_STATUS_UNEXPECTED("waitpid fails. errno = " + std::to_string(errno));
+    }
     if (WIFEXITED(status)) {
       auto exit_status = WEXITSTATUS(status);
       if (exit_status) {
         return Status(StatusCode::kMDUnexpectedError, msg);
-      } else {
-        // Not an error, some info message goes to stdout
-        std::cout << msg << std::endl;
       }
     }
     return Status::OK();