Just before Christmas I was working with Eliise and Lawrence and we observed port exhaustion during load testing of a Golang application that makes HTTP requests against an API. I hoped for a quick win and had a quick scan of the code but that confirmed that the was correctly reading the Body and calling Close - time to dig a bit deeper.
After some head-scratching we noticed that the code creates a new http.Client for each request. Not only that, when it creates the http.Client it also assigns a Transport instance. This last piece is very important - the docs for Transport state: