main.go


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package main

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

const (
	errSrcInvalid     = "source is not a parsable URL"
	errTgtNotAccepted = "can not process webmentions for this target"
	errInvalidScheme  = "URL scheme is not HTTP(S)"
)

// endpoint is a webmention receiver.
type endpoint struct {
	allowPrefix string // host (or host:port) and path prefix for the targets served by this endpoint
}

// ServeHTTP is http.Handler implementation.
func (ep endpoint) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	source, err := url.Parse(r.PostFormValue("source"))
	if err != nil || source.Host == "" {
		w.WriteHeader(http.StatusBadRequest)
		_, _ = w.Write([]byte(errSrcInvalid))
		return
	}

	if source.Scheme != "http" && source.Scheme != "https" {
		w.WriteHeader(http.StatusBadRequest)
		_, _ = w.Write([]byte(errInvalidScheme))
		return
	}
	target, err := url.Parse(r.PostFormValue("target"))
	if err != nil || !ep.targetAllowed(target) {
		w.WriteHeader(http.StatusBadRequest)
		_, _ = w.Write([]byte(errTgtNotAccepted))
		return
	}
	if target.Scheme != "http" && target.Scheme != "https" {
		w.WriteHeader(http.StatusBadRequest)
		_, _ = w.Write([]byte(errInvalidScheme))
		return
	}
}

// targetAllowed shows whether ep can accept a webmention for the target.
func (ep endpoint) targetAllowed(target *url.URL) bool {
	if !strings.HasSuffix(ep.allowPrefix, "/") {
		ep.allowPrefix = ep.allowPrefix + "/"
	}
	tgt := path.Join(target.Host, target.Path)
	if !strings.HasSuffix(tgt, "/") {
		tgt = tgt + "/"
	}
	return strings.HasPrefix(tgt, ep.allowPrefix)
}