Strange redirect behavior with http.StripPrefix

[Christian Stewart]

Hi all,

I was very confused by the behavior I was seeing while testing a simple program with http.StripPrefix:

package main

import (
"fmt"
"net/http"
)

func main() {
mux := http.NewServeMux()
mux.HandleFunc("/other/", func(w http.ResponseWriter, r *http.Request) {
fmt.Println(r.URL.Path)
w.WriteHeader(200)
w.Write([]byte("\n\nSeen path: " + r.URL.Path + "\n"))
})

// adding a / after /some/ here causes the strange redirect
prefixHandler := http.StripPrefix("/some/", mux)

// Start the HTTP server with the stripPrefixHandler
err := http.ListenAndServe(":9080", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
fmt.Println(r.URL.Path)
prefixHandler.ServeHTTP(w, r)
}))
if err != nil {
panic(err)
}
}

Try browsing to http://localhost:9080/some/other/path

This works fine if the StripPrefix does not have the / after /some/

That said: the strange behavior is: I notice with the above code (including the slash after /some/ in StripPrefix) that browsing to http://localhost:9080/some/other/path results in a 404 and effectively a redirect to http://localhost:9080/other/path - the browser's URL changes to the version of the path without the prefix!

Why is the http handler returning a redirect to the stripped version?

< HTTP/1.1 301 Moved Permanently
< Content-Type: text/html; charset=utf-8
< Location: /other/path
< Date: Wed, 20 Dec 2023 00:37:00 GMT
< Content-Length: 46
<
<a href="/other/path">Moved Permanently</a>.

I would expect to instead see a 404 with the given path /some/other/path.

This seems like a bug or otherwise undocumented behavior, the docs just say "StripPrefix handles a request for a path that doesn't begin with prefix by replying with an HTTP 404 not found error."

Thanks,

Christian Stewart

https://github.com/paralin

[j2gg0s]

Why this happens:

1. http.StripPrefix chang reuqest's path from /some/other/path to other/path

2. ServerMux.Handler will return 301 when cleanPath(r.URL.Path) != r.URL.Path

Link: https://github.com/golang/go/blob/go1.21.5/src/net/http/server.go#L2467

This seems to be a issue with ServerMux,

Our prefix cannot end with / when we use StripPrefix with ServerMux.