Subrouting using net/http in golang 1.22, issue with trailing "/" redirection

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27-08-2024

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You are correct; the standard net/http package in Golang 1.22 does not automatically redirect requests without a trailing slash to requests with a trailing slash. The issue you're experiencing is not a bug, but a consequence of the way http.StripPrefix and http.ServeMux function.

Let's break down what's happening and how to address it.

The Problem: Understanding http.StripPrefix and http.ServeMux

1. http.StripPrefix: The primary function of http.StripPrefix is to remove a specified prefix from the incoming request's URL path. It does not handle automatic redirects for missing trailing slashes.

2. http.ServeMux: http.ServeMux is a multiplexer that maps incoming requests to specific handlers based on their URL path. It matches request paths literally and does not perform any automatic redirects.

3. http.StripPrefix Interaction with http.ServeMux: When you combine http.StripPrefix with http.ServeMux, you are effectively delegating routing decisions to the http.ServeMux after stripping the prefix. This means that if the stripped path doesn't match exactly, including the trailing slash, the request will result in a 404 error.

The Solution: Implementing Custom Redirection

To address the issue of missing trailing slashes, you need to implement a custom redirect mechanism. Here are two common approaches:

1. Using a Middleware Function

• Define a Middleware Function: Create a middleware function that checks if a request's URL path lacks a trailing slash and redirects it if necessary.

package routes

import (
    "net/http"
    "net/url"
)

func trailingSlashMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Check if the path needs a trailing slash
        if r.URL.Path != "/" && !strings.HasSuffix(r.URL.Path, "/") {
            // Construct a new URL with a trailing slash
            u, err := url.Parse(r.URL.String())
            if err != nil {
                http.Error(w, err.Error(), http.StatusInternalServerError)
                return
            }
            u.Path += "/"
            // Redirect to the new URL with a 301 (Moved Permanently) status
            http.Redirect(w, r, u.String(), http.StatusMovedPermanently)
            return
        }
        next.ServeHTTP(w, r)
    })
}

func RegisterUserRoutes() http.Handler {
    router := http.NewServeMux()

    router.HandleFunc("/users", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("<p>POST /users</p>"))
    })
    // ... Other routes ...

    return trailingSlashMiddleware(router)
}

• Apply Middleware: Wrap your existing handlers with the middleware function in your RegisterUserRoutes or RegisterV1Routes functions.

2. Using a Custom http.Handler

• Create a Custom http.Handler: Implement a custom http.Handler that checks for trailing slashes and handles redirects accordingly.

package routes

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

type CustomHandler struct {
    inner http.Handler
}

func (h *CustomHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    if r.URL.Path != "/" && !strings.HasSuffix(r.URL.Path, "/") {
        // Construct a new URL with a trailing slash
        u, err := url.Parse(r.URL.String())
        if err != nil {
            http.Error(w, err.Error(), http.StatusInternalServerError)
            return
        }
        u.Path += "/"
        // Redirect to the new URL with a 301 (Moved Permanently) status
        http.Redirect(w, r, u.String(), http.StatusMovedPermanently)
        return
    }
    h.inner.ServeHTTP(w, r)
}

func RegisterUserRoutes() http.Handler {
    router := http.NewServeMux()

    router.HandleFunc("/users", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("<p>POST /users</p>"))
    })
    // ... Other routes ...

    return &CustomHandler{inner: router}
}

• Wrap the Router: Replace your existing router with the CustomHandler instance in your RegisterUserRoutes or RegisterV1Routes functions.

Conclusion

By implementing a custom redirection solution, you can ensure that your API behaves as expected, accepting requests both with and without trailing slashes. Remember to choose the approach that best fits your code structure and maintainability.