net/http: precedence in dynamic urls doesn't work as expected when using http.ServeMux

[covicale]

Go version

go1.22.4 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/root/.cache/go-build'
GOENV='/root/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/root/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/root/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.22.4'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build744119443=/tmp/go-build -gno-record-gcc-switches'

What did you do?

// main.go
package main

import (
	"fmt"
	"log"
	"net/http"
)

func staticURLHandler(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("This is a static url"))
}

func dynamicURLHandler(w http.ResponseWriter, r *http.Request) {
	pathVariable := r.PathValue("example")
	text := fmt.Sprintf("This is a dynamic url with path: %v", pathVariable)
	w.Write([]byte(text))
}

func ExampleMux() http.Handler {
	exampleMux := http.NewServeMux()
	exampleMux.HandleFunc("/test", staticURLHandler)
	return exampleMux
}

func main() {
	server := http.NewServeMux()
	port := ":8080"

	exampleMux := ExampleMux()
	server.HandleFunc("/static", staticURLHandler)
	server.Handle("/", exampleMux)
	server.HandleFunc("/{example}", dynamicURLHandler)

	log.Printf("App runing on localhost%v\n", port)
	http.ListenAndServe(port, server)
}

go run main.go

What did you see happen?

Accessing /static ✅

Accessing a dynamic url with {example} ✅

Accessing /test ❌

What did you expect to see?

By the precedence rules, if I access /test, it should use the static URL handler, but the dynamic URL handler is used. This behavior likely has to do with the use of multiplexers. It is intentional?

[gabyhelp]

Similar Issues

• net/http: enhanced ServerMux routing #65231
• net/http: ServeMux behaves inconsistently across operating systems #66288
• net/http: muxer no longer redirects host patterns in go1.10 #23183
• net/http: ServeMux forcibly cleans double forward slash in URLs even when behaving as a gateway #42244
• net/http: document and test behavior of ServeMux with ports #23351

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

This is working as intended. Each layer of net/http.ServeMux works independently without knowledge of any nested muxes.