[BUG] LoggingHandler does not count response bytes written through ReadFrom #284

Responses served by http.FileServer, http.ServeFile and http.ServeContent may be logged with size 0 when ReadFrom() is involved. These copy the body with io.CopyN, which calls ReadFrom() when the ResponseWriter implements it (as net/http's HTTP/1.x writer does).

The writer from makeLogger comes from httpsnoop.Wrap, which exposes the underlying ReadFrom, so io.Copy calls ReadFrom and bypasses responseLogger.Write.

A repro is below.

ReadFrom is (well, should be :-)) involved e.g. when a server wants to serve the response or parts of the response straight from a file descriptor (On Linux this makes golang use sendfile() which can have significant performance impact; of positive nature).

Possible patch (if we still land patches in this project): add a ReadFrom hook in makeLogger that calls the original ReadFrom with the original reader (and does the right book-keeping otherwise).

#207 proposed this hook in 2020; the stale bot closed it :-). There are still projects out there that use gorilla, and I wonder if we can maybe still patch this. I'd be happy to submit a fresh PR.

Repro

http.ServeContent with a strings.Reader goes down the io.CopyN -> ReadFrom path.

package main

import (
        "fmt"
        "io"
        "net/http"
        "net/http/httptest"
        "os"
        "strings"
        "time"

        "github.com/gorilla/handlers"
)

func main() {
        h := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                http.ServeContent(w, r, "", time.Time{}, strings.NewReader("hello, world"))
        })
        srv := httptest.NewServer(handlers.LoggingHandler(os.Stdout, h))
        resp, err := http.Get(srv.URL)
        if err != nil {
                panic(err)
        }
        body, _ := io.ReadAll(resp.Body)
        resp.Body.Close()
        srv.Close()
        fmt.Println("body bytes:", len(body))
}

Output, with handlers v1.5.2 and Go 1.26.3:

127.0.0.1 - - [23/Oct/2026:12:51:19 +0000] "GET / HTTP/1.1" 200 0
body bytes: 12