Wireshark=>LUA=> Read TCP flags within a dissector before dissecting?

Hi there, I am trying to write a script for reassembing the application segments.

When PSH flag is not set, write the the current buffer to a file, and display proper message to user to scroll to next packet.

When the user reaches the last TCP ssegment, the PSH flag would be set, append this buf to the file and call the regular dissector function to user with proper tree for app specific data.

Please help me to achieve this task.

Thank you in advance.