Memory leaks with mux.Router in Go

Today we found that our web server written in Go has memory leaks and consume around 300M of memory, which is really a lot for our app. After restart it’s back to ~10M but each hour increased by few more. Golang has nice built-in tools to debug and find leaks.

import _ "net/http/pprof"
// ...
go func() {
	log.Println(http.ListenAndServe("localhost:6060", nil))
}()

go tool pprof http://localhost:6060/debug/pprof/heap
(pprof) top
...

I have fixed one issue with not closed response.Body, but in-use memory is still growing up. But I never thought that it can be related with gorilla packages. We are using mux, sessions and context packages.

There is our current version with memory leaking:

http.ListenAndServe(adrr, r)

The problem is that variables must be cleared at the end of a request, to remove all values that were stored. This can be done in an http.Handler, after a request was served. Just call context.Clear() passing the request.

It’s a documented behavior, but easy to miss, also context package has a special wrapper function, ClearHandler(), which conveniently wraps an http.Handler to clear variables at the end of a request lifetime.

So here is an updated version of server without memory leaks:

http.ListenAndServe(adrr, context.ClearHandler(r))