Stop processing Flask route if request aborted

Question

I have a flask REST endpoint that does some cpu-intensive image processing and takes a few seconds to return. Often, this endpoint gets called, then aborted by the client. In these situations I would like to cancel processing. How can I do this in flask?

In node.js, I would do something like:

req.on('close', function(){
  //some handler
});

I was expecting flask to have something similar, or a synchronous method (request.isClosed()) that I could check at certain points during my processing and return if it's closed, but I can't find one.

I thought about sending something to test that the connection is still open, and catching the exception if it fails, but it seems Flask buffers all outputs so the exception isn't thrown until the processing completes and tries to return the result:

An established connection was aborted by the software in your host machine

How can I cancel my processing half way through if the client aborts their request?

Answer 1

There is a potentially... hacky solution to your problem. Flask has the ability to stream content back to the user via a generator. The hacky part would be streaming blank data as a check to see if the connection is still open and then when your content is finished the generator could produce the actual image. Your generator could check to see if processing is done and return None or "" or whatever if it's not finished.

from flask import Response

@app.route('/image')
def generate_large_image():
    def generate():
        while True:
            if not processing_finished():
                yield ""
            else:
                yield get_image()
    return Response(generate(), mimetype='image/jpeg')

I don't know what exception you'll get if the client closes the connection but I'm willing to bet its error: [Errno 32] Broken pipe