Changes to ServeMux Specification in Go 1.22rc

This article was originally published on bmf-tech.com.

This article is the 19th entry in the Makuake Advent Calendar 2023.

The reason for this significant delay is that I decided to fill in the only unoccupied slot for the 19th at the last minute. Since this was not originally my assigned slot, it's not really a delay.

Overview

Since last year, there have been proposals to extend the functionality of ServeMux included in Go's net/http, and I have been watching these developments, but it seems they have recently been closed.

cf. net/http: enhanced ServeMux routing #61410
cf. Proposal to extend ServeMux functionality in Go has been Accepted

This extension is expected to be included in Go 1.22, and since Go 1.22rc2 has been released, I decided to give it a try. (It seems to have been included since Go 1.22rc1, but I had overlooked it.)

Changes in net/http for Go 1.22

According to the information listed at tip.golang.org/doc/go1.22#net/http, the following changes appear to have been made:

• New additions: ServeFileFSs, FileServerFS, NewFileTransportFS
• The HTTP server and client have been modified to reject requests and responses containing invalid Content-Length headers.
• New additions: Request.PathValue and Request.SetPathValue

From what I can see here, I don't quite understand how the routing specifications have changed, so I referred to the documentation where various details are written.

cf. pkg.go.dev/net/http@go1.22rc2#ServeMux

I haven't been able to follow all the details of the Proposal, but it seems that the routing pattern matching specifications proposed in the Proposal have become as rich as those of third-party routers.

To put it succinctly, it seems that pattern matching using HTTP method names and path parameters has become possible.

Trying Out the New Features of ServeMux in Go 1.22rc2

I downloaded go1.22rc2 from go.dev/dl - All releases and started experimenting.

package main

import (
    "net/http"
)

func main() {
    mux := http.NewServeMux()

    // Patterns without HTTP methods match all methods.
    // {$} is a special wildcard that only matches the end of the URL.
    // /{$} matches only /, but / matches all paths.
    mux.HandleFunc("/{$}", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("/{$}"))
    })

    // GET method matches GET and HEAD.
    // {bar} is a wildcard.
    mux.HandleFunc("GET /foo/{bar}", func(w http.ResponseWriter, r *http.Request) {
        // Retrieve path parameter
        v := r.PathValue("bar")
        w.Write([]byte("GET /foo/" + v))
    })

    // {bar} is a wildcard.
    mux.HandleFunc("POST /foo/{bar}", func(w http.ResponseWriter, r *http.Request) {
        // Retrieve path parameter
        v := r.PathValue("bar")
        w.Write([]byte("POST /foo/" + v))
    })
    http.ListenAndServe(":8080", mux)
}

The combination of {$} and defining methods with routes might not be commonly seen in third-party routers. (Maybe I'm just not aware of it...)

Regarding backward compatibility, it is mentioned in pkg.go.dev/net/http@go1.22rc2#hdr-Compatibility.

If you want to maintain the specifications of Go 1.21, you need to set httpmuuxgo121=1 in the GODEBUG environment variable.

Thoughts

I feel that I no longer need to use my custom router goblin, so I plan to switch my personal projects to use the new ServeMux.