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Handle client disconnects better in workhorse

Craig Miskell requested to merge workhorse-499-on-client-disconnect into master

What does this MR do and why?

When a client disconnects before it has received a response, Workhorse currently detects this as a 502 (logged and put into metrics). However this ends up in metrics which for and GitLab Dedicated is counted as an error against the SLA, when it is entirely a client-side problem. Nginx logs this as a 499 (a non-standard code, but a useful one).

This patch extends that behavior to Workhorse, such that when a client disconnects early, it is counted as a 499 and not a server error, because it is strictly the client's problem (and probably just a browser window being closed or similar).

The original problem was seen in GitLab Dedicated ( - internal access only), and the patch was supplied by @jacobvosmaer-gitlab in that issue, I've just put it into an MR and done some testing.

I tried to write a golang test for this, but got tangled up in goroutines and contexts and couldn't quite make it work, but this seems like something that would be good to have if a smarter Go programmer could help out.

How to set up and validate locally

Run a simple webserver that accepts any requests, but contains a delay. Example:

from http.server import HTTPServer, BaseHTTPRequestHandler
import time

class MyHandler(BaseHTTPRequestHandler):
    def do_GET(self):
      # send 200 response
      # send response headers
      # send the body of the response
      self.wfile.write(bytes("It Works!", "utf-8"))

httpd = HTTPServer(('localhost', 8080), MyHandler)

Run workhorse, and then use curl to make a request but with a smaller timeout than the sleep

curl -m 0.5 http://localhost:8081/api/v4/foo

Observe that without the patch workhorse logs a 502. Optionally also enable the metrics server on workhorse and check metrics like gitlab_workhorse_http_requests_total to confirm how the status code is being recorded there.

Repeat with the patched workhorse and observe that it now logs a 499 and that also shows up in the metrics under the same status code.

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