Why not write the application around a queue?

You write out a page with a stop button on submit, if it is "pressed", it pushes a stop command at the queue, and after certain process points, your script checks to see if it should stop, or you have a child that looks for the stop. This way, you aren't relying on catching the "Stop" button.

The constant writing to STDOUT isn't always a good method. Example:

Our intranet has a throttle, and will also time out connections that last longer than X minutes if the connection is going between buildings. Meanwhile, your CGI script is running, but the STOP button on the browser has been rendered useless.

The Web is stateless, don't try to circumvent that. Writing the queue also allows you to update a percent finished or some other kind of process flag to provide feedback to your user. Put a meta refresh on the page, and the user now has an idea of WHEN the process will finish, based on this feed back without even having to hit an 'update' button (which you SHOULD provide so that an impatient user can always clickity-click). Voila, a stateful looking connection to a stateful application in a stateless medium.

C-.

Update: After reading the above, I realized that my writing looks like I haven't slept in days. It's catching up to me.