Golang Modify HTTP Request Parameters Such As URL Path Before Routing - http

Question

It's common practice in some cases to pass plain URIs as suffix of the path instead of a query parameter. Here is an example from Internet Archive's Wayback Machine.
https://web.archive.org/web/20150825082012/http://example.com/
In this example, user is requesting a copy of http://example.com/ as captured at 2015-08-25 08:20:12. If we were to implement similar service in Go, we probably would have a router as follows:
http.HandleFunc("/web/", returnArchivedCopy)
Then in the returnArchivedCopy handler function, we will split r.URL.Path (where r is the Request object) to extract the date-time and the target URL. However there is a problem in this style of URL scheme; Go's net/http package calls cleanPath function on the path portion to sanitize it. This sanitization process does various cleanup tasks such as eeliminating . and .. from the path and replace multiple slashes with a single one. This later operation makes sense when because in Unix systems // in the file path are same as /. However this causes an issue in the above described use case as http://example becomes http:/example and the server internally returns a redirect response to the client with the sanitized path.
I am wondering, what are my options in this case? Is there a way to ask HTTP not to sanitize the request path while still utilizing all the default behavior that is shipped with the default (or slightly modified) server, multiplexer, and handler? Or is there a way to modify the request parameters (path in this case) before it hits the multiplexer's routing patterns. If the later is possible, we might try to perform something like URL encoding to avoid the redirect and later decode the URL back in the handler function before extracting desired bits.
I have experimented with some custom handlers and multiplexers, but I am new to Go, hence I was not quite sure how to delegate the routing back to the default handlers after making changes in the request.

Answer 1

You can implement a wrapper mux, that falls back to the default one, here's a very simple example:
func main() {
http.HandleFunc("/blah", func(w http.ResponseWriter, req *http.Request) {
w.Write([]byte("w00t"))
})
http.ListenAndServe(":9090", http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
p := strings.SplitN(req.URL.RequestURI()[1:] /*trim the first slash*/, "/", 3)
if len(p) != 3 || p[0] != "web" {
http.DefaultServeMux.ServeHTTP(w, req)
return
}
t, err := time.Parse("20060102150405", p[1])
if err != nil {
http.Error(w, "invalid time", 400)
return
}
url := p[2]
fmt.Fprintf(w, "requested url %v # %v", url, t)
}))
}