File - exclusive access

I have two programs that I call from my program (MyProgram) - Program1 and Program2. MyProgram calls Program1 asynchronously and Program1 outputs a file that I know the location of. MyProgram then calls Program2 which opens that file and performs some manipulation on it. I need a way for MyProgram to block until the instant that the file is closed by Program1. Keep in mind I do not have any control over Program1 or Program2, and MyProgram regains control before Program1 closes the file. Any suggestions? Thanks!