The godoc comments on net/http/httputil ReverseProxy need to be clarified. The ReverseProxy.Director function can not continue to access the provided http.Request after it returns. The code in ReverseProxy that calls the user supplied Director function will sometimes modify the Header field after Director returns, which can lead to a concurrent read/write on http.Request.Header.

My code hit a sporadic race condition due to this behavior. Director launched a goroutine to do some specialized request logging. This lead to a panic after several thousand requests.