go-webring/handlers.go

// SPDX-FileCopyrightText: 2021 Amolith <amolith@secluded.site>
//
// SPDX-License-Identifier: BSD-2-Clause

package main

import (
	"fmt"
	"html/template"
	"log"
	"math/rand"
	"net/http"
	"net/url"
	"strings"
)

// Serves the webpage created by createRoot()
func (m Ring) root(writer http.ResponseWriter, request *http.Request) {
	if m.modify("ring") {
		log.Println("Ring modified, clearing field and re-parsing")
		m.parseList()
	}
	if m.modify("index") {
		log.Println("Index modified, clearing field and re-parsing")
		m.parseIndex()
	}
	var table string
	for _, member := range m.ring {
		table = table + "  <tr>\n"
		table = table + "    <td>" + member.id + "</td>\n"
		table = table + "    <td>" + link(member.url) + "</td>\n"
		table = table + "  </tr>\n"
	}
	m.index.Execute(writer, template.HTML(table))
}

type MemberSelector struct {
	id      string // exact match
	urlpart string
}

func parseQuery(query url.Values) (*MemberSelector, error) {
	urlpart := query.Get("urlpart")
	if urlpart == "" {
		urlpart = query.Get("host")
	}
	n_valid := 0
	if urlpart != "" {
		n_valid += 1
	}
	id := query.Get("id")
	if id != "" {
		n_valid += 1
	}
	if n_valid != 1 {
		return nil, fmt.Errorf("Please specify urlpart=xxx or id=xxx")
	}

	return &MemberSelector{
		id:      id,
		urlpart: urlpart,
	}, nil
}

func (m Ring) match(selector *MemberSelector, item RingMember) bool {
	if selector.id != "" {
		return item.id == selector.id	
	}
	if selector.urlpart != "" {
		return strings.Contains(item.url, selector.urlpart)
	}
	panic("unreachable")
}

// Redirects the visitor to the next member, wrapping around the list if the
// next would be out-of-bounds, and ensuring the destination returns a 200 OK
// status before performing the redirect.
func (m Ring) next(writer http.ResponseWriter, request *http.Request) {
	if m.modify("ring") {
		log.Println("Ring modified, clearing field and re-parsing")
		m.parseList()
	}
	success := false
	query := request.URL.Query()
	selector, err := parseQuery(query)
	if err != nil {
		http.Error(writer, fmt.Errorf("invalid query: %w", err).Error(), http.StatusBadRequest)
	}
	length := len(m.ring)
	for i, item := range m.ring {
		if m.match(selector, item) {
			for j := i + 1; j < length+i; j++ {
				dest := m.ring[j%length].url
				log.Println("Checking '" + dest + "'")
				if isTargetValid(request.URL, dest) {
					log.Println("Redirecting visitor to '" + dest + "'")
					http.Redirect(writer, request, dest, http.StatusFound)
					success = true
					break
				}
				log.Println("Something went wrong accessing '" + dest + "', skipping site")
			}
		}
	}
	if !success {
		http.Error(writer, "Ring member '"+query.Encode()+"' not found.", 404)
	}
}

// Redirects the visitor to the previous member, wrapping around the list if the
// next would be out-of-bounds, and ensuring the destination returns a 200 OK
// status before performing the redirect.
func (m Ring) previous(writer http.ResponseWriter, request *http.Request) {
	if m.modify("ring") {
		log.Println("Ring modified, clearing field and re-parsing")
		m.parseList()
	}
	query := request.URL.Query()
	selector, err := parseQuery(query)
	if err != nil {
		http.Error(writer, fmt.Errorf("invalid query: %w", err).Error(), http.StatusBadRequest)
	}
	length := len(m.ring)
	for index, item := range m.ring {
		if m.match(selector, item) {
			// from here to start of list
			for i := index - 1; i > 0; i-- {
				dest := m.ring[i].url
				if isTargetValid(request.URL, dest) {
					log.Println("Redirecting visitor to '" + dest + "'")
					http.Redirect(writer, request, dest, http.StatusFound)
					return
				}
			}
			// from end of list to here
			for i := length - 1; i > index; i-- {
				dest := m.ring[i].url
				if isTargetValid(request.URL, dest) {
					log.Println("Redirecting visitor to '" + dest + "'")
					http.Redirect(writer, request, dest, http.StatusFound)
					return
				}
			}
			http.Error(writer, `It would appear that either none of the ring members are accessible
(unlikely) or the backend is broken (more likely). In either case,
please email amolith@secluded.site and let him (me) know what's up.`, 500)
			return
		}
	}
	http.Error(writer, "Ring member '"+query.Encode()+"' not )found.", 404)
}

// Redirects the visitor to a random member
func (m Ring) random(writer http.ResponseWriter, request *http.Request) {
	// todo: isTargetValid

	if m.modify("ring") {
		log.Println("Ring modified, clearing field and re-parsing")
		m.parseList()
	}
	dest := m.ring[rand.Intn(len(m.ring)-1)].url
	http.Redirect(writer, request, dest, http.StatusFound)
}