Comment by optymizer1

Interesting. Now, what does SetUser(r, "dave") do?

Since you're passing in 'r', it must keep a map of all requests and their data, i.e. map[*http.Request]string ("dave" being the string in this case).

This map should be protected by a mutex because it could be written from multiple go-routines.

Is this the case? If so, how do we avoid contention on this mutex?