alps/plugins/base/routes.go

package koushinbase

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"mime"
	"net/http"
	"net/url"
	"strconv"
	"strings"

	"git.sr.ht/~emersion/koushin"
	"github.com/emersion/go-imap"
	imapmove "github.com/emersion/go-imap-move"
	imapclient "github.com/emersion/go-imap/client"
	"github.com/emersion/go-message"
	"github.com/emersion/go-message/mail"
	"github.com/emersion/go-smtp"
	"github.com/labstack/echo/v4"
)

func registerRoutes(p *koushin.GoPlugin) {
	p.GET("/", func(ctx *koushin.Context) error {
		return ctx.Redirect(http.StatusFound, "/mailbox/INBOX")
	})

	p.GET("/mailbox/:mbox", handleGetMailbox)
	p.POST("/mailbox/:mbox", handleGetMailbox)

	p.GET("/message/:mbox/:uid", func(ctx *koushin.Context) error {
		return handleGetPart(ctx, false)
	})
	p.GET("/message/:mbox/:uid/raw", func(ctx *koushin.Context) error {
		return handleGetPart(ctx, true)
	})

	p.GET("/login", handleLogin)
	p.POST("/login", handleLogin)

	p.GET("/logout", handleLogout)

	p.GET("/compose", handleComposeNew)
	p.POST("/compose", handleComposeNew)

	p.GET("/message/:mbox/:uid/reply", handleReply)
	p.POST("/message/:mbox/:uid/reply", handleReply)

	p.GET("/message/:mbox/:uid/forward", handleForward)
	p.POST("/message/:mbox/:uid/forward", handleForward)

	p.GET("/message/:mbox/:uid/edit", handleEdit)
	p.POST("/message/:mbox/:uid/edit", handleEdit)

	p.POST("/message/:mbox/:uid/move", handleMove)

	p.POST("/message/:mbox/:uid/delete", handleDelete)

	p.POST("/message/:mbox/:uid/flag", handleSetFlags)

	p.GET("/settings", handleSettings)
	p.POST("/settings", handleSettings)
}

type MailboxRenderData struct {
	koushin.BaseRenderData
	Mailbox            *imap.MailboxStatus
	Mailboxes          []*imap.MailboxInfo
	Messages           []IMAPMessage
	PrevPage, NextPage int
	Query              string
}

func handleGetMailbox(ctx *koushin.Context) error {
	mboxName, err := url.PathUnescape(ctx.Param("mbox"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}

	page := 0
	if pageStr := ctx.QueryParam("page"); pageStr != "" {
		var err error
		if page, err = strconv.Atoi(pageStr); err != nil || page < 0 {
			return echo.NewHTTPError(http.StatusBadRequest, "invalid page index")
		}
	}

	settings, err := loadSettings(ctx.Session.Store())
	if err != nil {
		return err
	}
	messagesPerPage := settings.MessagesPerPage

	query := ctx.QueryParam("query")

	var mailboxes []*imap.MailboxInfo
	var msgs []IMAPMessage
	var mbox *imap.MailboxStatus
	var total int
	err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
		var err error
		if mailboxes, err = listMailboxes(c); err != nil {
			return err
		}
		if query != "" {
			msgs, total, err = searchMessages(c, mboxName, query, page, messagesPerPage)
		} else {
			msgs, err = listMessages(c, mboxName, page, messagesPerPage)
		}
		if err != nil {
			return err
		}
		mbox = c.Mailbox()
		return nil
	})
	if err != nil {
		return err
	}

	prevPage, nextPage := -1, -1
	if query != "" {
		if page > 0 {
			prevPage = page - 1
		}
		if (page+1)*messagesPerPage <= total {
			nextPage = page + 1
		}
	} else {
		if page > 0 {
			prevPage = page - 1
		}
		if (page+1)*messagesPerPage < int(mbox.Messages) {
			nextPage = page + 1
		}
	}

	return ctx.Render(http.StatusOK, "mailbox.html", &MailboxRenderData{
		BaseRenderData: *koushin.NewBaseRenderData(ctx),
		Mailbox:        mbox,
		Mailboxes:      mailboxes,
		Messages:       msgs,
		PrevPage:       prevPage,
		NextPage:       nextPage,
		Query:          query,
	})
}

func handleLogin(ctx *koushin.Context) error {
	username := ctx.FormValue("username")
	password := ctx.FormValue("password")
	if username != "" && password != "" {
		s, err := ctx.Server.Sessions.Put(username, password)
		if err != nil {
			if _, ok := err.(koushin.AuthError); ok {
				return ctx.Render(http.StatusOK, "login.html", nil)
			}
			return fmt.Errorf("failed to put connection in pool: %v", err)
		}
		ctx.SetSession(s)

		if path := ctx.QueryParam("next"); path != "" && path[0] == '/' && path != "/login" {
			return ctx.Redirect(http.StatusFound, path)
		}
		return ctx.Redirect(http.StatusFound, "/mailbox/INBOX")
	}

	return ctx.Render(http.StatusOK, "login.html", koushin.NewBaseRenderData(ctx))
}

func handleLogout(ctx *koushin.Context) error {
	ctx.Session.Close()
	ctx.SetSession(nil)
	return ctx.Redirect(http.StatusFound, "/login")
}

type MessageRenderData struct {
	koushin.BaseRenderData
	Mailboxes   []*imap.MailboxInfo
	Mailbox     *imap.MailboxStatus
	Message     *IMAPMessage
	Part        *IMAPPartNode
	View        interface{}
	MailboxPage int
	Flags       map[string]bool
}

func handleGetPart(ctx *koushin.Context, raw bool) error {
	mboxName, uid, err := parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}
	partPath, err := parsePartPath(ctx.QueryParam("part"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}

	settings, err := loadSettings(ctx.Session.Store())
	if err != nil {
		return err
	}
	messagesPerPage := settings.MessagesPerPage

	var mailboxes []*imap.MailboxInfo
	var msg *IMAPMessage
	var part *message.Entity
	var mbox *imap.MailboxStatus
	err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
		var err error
		if mailboxes, err = listMailboxes(c); err != nil {
			return err
		}
		if msg, part, err = getMessagePart(c, mboxName, uid, partPath); err != nil {
			return err
		}
		mbox = c.Mailbox()
		return nil
	})
	if err != nil {
		return err
	}

	mimeType, _, err := part.Header.ContentType()
	if err != nil {
		return fmt.Errorf("failed to parse part Content-Type: %v", err)
	}
	if len(partPath) == 0 {
		mimeType = "message/rfc822"
	}

	if raw {
		ctx.Response().Header().Set("Content-Type", mimeType)

		disp, dispParams, _ := part.Header.ContentDisposition()
		filename := dispParams["filename"]
		if len(partPath) == 0 {
			filename = msg.Envelope.Subject + ".eml"
		}

		// TODO: set Content-Length if possible

		// Be careful not to serve types like text/html as inline
		if !strings.EqualFold(mimeType, "text/plain") || strings.EqualFold(disp, "attachment") {
			dispParams := make(map[string]string)
			if filename != "" {
				dispParams["filename"] = filename
			}
			disp := mime.FormatMediaType("attachment", dispParams)
			ctx.Response().Header().Set("Content-Disposition", disp)
		}

		if len(partPath) == 0 {
			return part.WriteTo(ctx.Response())
		} else {
			return ctx.Stream(http.StatusOK, mimeType, part.Body)
		}
	}

	view, err := viewMessagePart(ctx, msg, part)
	if err == ErrViewUnsupported {
		view = nil
	}

	flags := make(map[string]bool)
	for _, f := range mbox.PermanentFlags {
		f = imap.CanonicalFlag(f)
		if f == imap.TryCreateFlag {
			continue
		}
		flags[f] = msg.HasFlag(f)
	}

	return ctx.Render(http.StatusOK, "message.html", &MessageRenderData{
		BaseRenderData: *koushin.NewBaseRenderData(ctx),
		Mailboxes:      mailboxes,
		Mailbox:        mbox,
		Message:        msg,
		Part:           msg.PartByPath(partPath),
		View:           view,
		MailboxPage:    int(mbox.Messages-msg.SeqNum) / messagesPerPage,
		Flags:          flags,
	})
}

type ComposeRenderData struct {
	koushin.BaseRenderData
	Message *OutgoingMessage
}

type messagePath struct {
	Mailbox string
	Uid     uint32
}

type composeOptions struct {
	Draft     *messagePath
	Forward   *messagePath
	InReplyTo *messagePath
}

// Send message, append it to the Sent mailbox, mark the original message as
// answered
func submitCompose(ctx *koushin.Context, msg *OutgoingMessage, options *composeOptions) error {
	err := ctx.Session.DoSMTP(func(c *smtp.Client) error {
		return sendMessage(c, msg)
	})
	if err != nil {
		if _, ok := err.(koushin.AuthError); ok {
			return echo.NewHTTPError(http.StatusForbidden, err)
		}
		return fmt.Errorf("failed to send message: %v", err)
	}

	if inReplyTo := options.InReplyTo; inReplyTo != nil {
		err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
			return markMessageAnswered(c, inReplyTo.Mailbox, inReplyTo.Uid)
		})
		if err != nil {
			return fmt.Errorf("failed to mark original message as answered: %v", err)
		}
	}

	err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
		if _, err := appendMessage(c, msg, mailboxSent); err != nil {
			return err
		}
		if draft := options.Draft; draft != nil {
			if err := deleteMessage(c, draft.Mailbox, draft.Uid); err != nil {
				return err
			}
		}
		return nil
	})
	if err != nil {
		return fmt.Errorf("failed to save message to Sent mailbox: %v", err)
	}

	return ctx.Redirect(http.StatusFound, "/mailbox/INBOX")
}

func handleCompose(ctx *koushin.Context, msg *OutgoingMessage, options *composeOptions) error {
	if msg.From == "" && strings.ContainsRune(ctx.Session.Username(), '@') {
		msg.From = ctx.Session.Username()
	}

	if ctx.Request().Method == http.MethodPost {
		formParams, err := ctx.FormParams()
		if err != nil {
			return fmt.Errorf("failed to parse form: %v", err)
		}
		_, saveAsDraft := formParams["save_as_draft"]

		msg.From = ctx.FormValue("from")
		msg.To = parseAddressList(ctx.FormValue("to"))
		msg.Subject = ctx.FormValue("subject")
		msg.Text = ctx.FormValue("text")
		msg.InReplyTo = ctx.FormValue("in_reply_to")

		form, err := ctx.MultipartForm()
		if err != nil {
			return fmt.Errorf("failed to get multipart form: %v", err)
		}

		// Fetch previous attachments from original message
		var original *messagePath
		if options.Draft != nil {
			original = options.Draft
		} else if options.Forward != nil {
			original = options.Forward
		}
		if original != nil {
			for _, s := range form.Value["prev_attachments"] {
				path, err := parsePartPath(s)
				if err != nil {
					return fmt.Errorf("failed to parse original attachment path: %v", err)
				}

				var part *message.Entity
				err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
					var err error
					_, part, err = getMessagePart(c, original.Mailbox, original.Uid, path)
					return err
				})
				if err != nil {
					return fmt.Errorf("failed to fetch attachment from original message: %v", err)
				}

				var buf bytes.Buffer
				if _, err := io.Copy(&buf, part.Body); err != nil {
					return fmt.Errorf("failed to copy attachment from original message: %v", err)
				}

				h := mail.AttachmentHeader{part.Header}
				mimeType, _, _ := h.ContentType()
				filename, _ := h.Filename()
				msg.Attachments = append(msg.Attachments, &imapAttachment{
					Mailbox: original.Mailbox,
					Uid:     original.Uid,
					Node: &IMAPPartNode{
						Path:     path,
						MIMEType: mimeType,
						Filename: filename,
					},
					Body: buf.Bytes(),
				})
			}
		} else if len(form.Value["prev_attachments"]) > 0 {
			return fmt.Errorf("previous attachments specified but no original message available")
		}

		for _, fh := range form.File["attachments"] {
			msg.Attachments = append(msg.Attachments, &formAttachment{fh})
		}

		if saveAsDraft {
			err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
				copied, err := appendMessage(c, msg, mailboxDrafts)
				if err != nil {
					return err
				}
				if !copied {
					return fmt.Errorf("no Draft mailbox found")
				}
				if draft := options.Draft; draft != nil {
					if err := deleteMessage(c, draft.Mailbox, draft.Uid); err != nil {
						return err
					}
				}
				return nil
			})
			if err != nil {
				return fmt.Errorf("failed to save message to Draft mailbox: %v", err)
			}
			return ctx.Redirect(http.StatusFound, "/mailbox/INBOX")
		} else {
			return submitCompose(ctx, msg, options)
		}
	}

	return ctx.Render(http.StatusOK, "compose.html", &ComposeRenderData{
		BaseRenderData: *koushin.NewBaseRenderData(ctx),
		Message:        msg,
	})
}

func handleComposeNew(ctx *koushin.Context) error {
	// These are common mailto URL query parameters
	// TODO: cc, bcc
	return handleCompose(ctx, &OutgoingMessage{
		To:        strings.Split(ctx.QueryParam("to"), ","),
		Subject:   ctx.QueryParam("subject"),
		Text:      ctx.QueryParam("body"),
		InReplyTo: ctx.QueryParam("in-reply-to"),
	}, &composeOptions{})
}

func unwrapIMAPAddressList(addrs []*imap.Address) []string {
	l := make([]string, len(addrs))
	for i, addr := range addrs {
		l[i] = addr.Address()
	}
	return l
}

func handleReply(ctx *koushin.Context) error {
	var inReplyToPath messagePath
	var err error
	inReplyToPath.Mailbox, inReplyToPath.Uid, err = parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}

	var msg OutgoingMessage
	if ctx.Request().Method == http.MethodGet {
		// Populate fields from original message
		partPath, err := parsePartPath(ctx.QueryParam("part"))
		if err != nil {
			return echo.NewHTTPError(http.StatusBadRequest, err)
		}

		var inReplyTo *IMAPMessage
		var part *message.Entity
		err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
			var err error
			inReplyTo, part, err = getMessagePart(c, inReplyToPath.Mailbox, inReplyToPath.Uid, partPath)
			return err
		})
		if err != nil {
			return err
		}

		mimeType, _, err := part.Header.ContentType()
		if err != nil {
			return fmt.Errorf("failed to parse part Content-Type: %v", err)
		}

		if !strings.EqualFold(mimeType, "text/plain") {
			err := fmt.Errorf("cannot reply to %q part", mimeType)
			return echo.NewHTTPError(http.StatusBadRequest, err)
		}

		// TODO: strip HTML tags if text/html
		msg.Text, err = quote(part.Body)
		if err != nil {
			return err
		}

		msg.InReplyTo = inReplyTo.Envelope.MessageId
		// TODO: populate From from known user addresses and inReplyTo.Envelope.To
		replyTo := inReplyTo.Envelope.ReplyTo
		if len(replyTo) == 0 {
			replyTo = inReplyTo.Envelope.From
		}
		msg.To = unwrapIMAPAddressList(replyTo)
		msg.Subject = inReplyTo.Envelope.Subject
		if !strings.HasPrefix(strings.ToLower(msg.Subject), "re:") {
			msg.Subject = "Re: " + msg.Subject
		}
	}

	return handleCompose(ctx, &msg, &composeOptions{InReplyTo: &inReplyToPath})
}

func handleForward(ctx *koushin.Context) error {
	var sourcePath messagePath
	var err error
	sourcePath.Mailbox, sourcePath.Uid, err = parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}

	var msg OutgoingMessage
	if ctx.Request().Method == http.MethodGet {
		// Populate fields from original message
		partPath, err := parsePartPath(ctx.QueryParam("part"))
		if err != nil {
			return echo.NewHTTPError(http.StatusBadRequest, err)
		}

		var source *IMAPMessage
		var part *message.Entity
		err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
			var err error
			source, part, err = getMessagePart(c, sourcePath.Mailbox, sourcePath.Uid, partPath)
			return err
		})
		if err != nil {
			return err
		}

		mimeType, _, err := part.Header.ContentType()
		if err != nil {
			return fmt.Errorf("failed to parse part Content-Type: %v", err)
		}

		if !strings.EqualFold(mimeType, "text/plain") {
			err := fmt.Errorf("cannot forward %q part", mimeType)
			return echo.NewHTTPError(http.StatusBadRequest, err)
		}

		msg.Text, err = quote(part.Body)
		if err != nil {
			return err
		}

		msg.Subject = source.Envelope.Subject
		if !strings.HasPrefix(strings.ToLower(msg.Subject), "fwd:") &&
			!strings.HasPrefix(strings.ToLower(msg.Subject), "fw:") {
			msg.Subject = "Fwd: " + msg.Subject
		}
		msg.InReplyTo = source.Envelope.InReplyTo

		attachments := source.Attachments()
		for i := range attachments {
			// No need to populate attachment body here, we just need the
			// metadata
			msg.Attachments = append(msg.Attachments, &imapAttachment{
				Mailbox: sourcePath.Mailbox,
				Uid:     sourcePath.Uid,
				Node:    &attachments[i],
			})
		}
	}

	return handleCompose(ctx, &msg, &composeOptions{Forward: &sourcePath})
}

func handleEdit(ctx *koushin.Context) error {
	var sourcePath messagePath
	var err error
	sourcePath.Mailbox, sourcePath.Uid, err = parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}

	// TODO: somehow get the path to the In-Reply-To message (with a search?)

	var msg OutgoingMessage
	if ctx.Request().Method == http.MethodGet {
		// Populate fields from source message
		partPath, err := parsePartPath(ctx.QueryParam("part"))
		if err != nil {
			return echo.NewHTTPError(http.StatusBadRequest, err)
		}

		var source *IMAPMessage
		var part *message.Entity
		err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
			var err error
			source, part, err = getMessagePart(c, sourcePath.Mailbox, sourcePath.Uid, partPath)
			return err
		})
		if err != nil {
			return err
		}

		mimeType, _, err := part.Header.ContentType()
		if err != nil {
			return fmt.Errorf("failed to parse part Content-Type: %v", err)
		}

		if !strings.EqualFold(mimeType, "text/plain") {
			err := fmt.Errorf("cannot edit %q part", mimeType)
			return echo.NewHTTPError(http.StatusBadRequest, err)
		}

		b, err := ioutil.ReadAll(part.Body)
		if err != nil {
			return fmt.Errorf("failed to read part body: %v", err)
		}
		msg.Text = string(b)

		if len(source.Envelope.From) > 0 {
			msg.From = source.Envelope.From[0].Address()
		}
		msg.To = unwrapIMAPAddressList(source.Envelope.To)
		msg.Subject = source.Envelope.Subject
		msg.InReplyTo = source.Envelope.InReplyTo
		// TODO: preserve Message-Id

		attachments := source.Attachments()
		for i := range attachments {
			// No need to populate attachment body here, we just need the
			// metadata
			msg.Attachments = append(msg.Attachments, &imapAttachment{
				Mailbox: sourcePath.Mailbox,
				Uid:     sourcePath.Uid,
				Node:    &attachments[i],
			})
		}
	}

	return handleCompose(ctx, &msg, &composeOptions{Draft: &sourcePath})
}

func handleMove(ctx *koushin.Context) error {
	mboxName, uid, err := parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}

	to := ctx.FormValue("to")

	err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
		mc := imapmove.NewClient(c)

		if err := ensureMailboxSelected(c, mboxName); err != nil {
			return err
		}

		var seqSet imap.SeqSet
		seqSet.AddNum(uid)
		if err := mc.UidMoveWithFallback(&seqSet, to); err != nil {
			return fmt.Errorf("failed to move message: %v", err)
		}

		// TODO: get the UID of the message in the destination mailbox with UIDPLUS
		return nil
	})
	if err != nil {
		return err
	}

	return ctx.Redirect(http.StatusFound, fmt.Sprintf("/mailbox/%v", url.PathEscape(to)))
}

func handleDelete(ctx *koushin.Context) error {
	mboxName, uid, err := parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}

	err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
		if err := ensureMailboxSelected(c, mboxName); err != nil {
			return err
		}

		var seqSet imap.SeqSet
		seqSet.AddNum(uid)

		item := imap.FormatFlagsOp(imap.AddFlags, true)
		flags := []interface{}{imap.DeletedFlag}
		if err := c.UidStore(&seqSet, item, flags, nil); err != nil {
			return fmt.Errorf("failed to add deleted flag: %v", err)
		}

		if err := c.Expunge(nil); err != nil {
			return fmt.Errorf("failed to expunge mailbox: %v", err)
		}

		// Deleting a message invalidates our cached message count
		// TODO: listen to async updates instead
		if _, err := c.Select(mboxName, false); err != nil {
			return fmt.Errorf("failed to select mailbox: %v", err)
		}

		return nil
	})
	if err != nil {
		return err
	}

	return ctx.Redirect(http.StatusFound, fmt.Sprintf("/mailbox/%v", url.PathEscape(mboxName)))
}

func handleSetFlags(ctx *koushin.Context) error {
	mboxName, uid, err := parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}

	form, err := ctx.FormParams()
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, err)
	}
	flags, ok := form["flags"]
	if !ok {
		return echo.NewHTTPError(http.StatusBadRequest, "missing 'flags' form values")
	}

	var op imap.FlagsOp
	switch ctx.FormValue("action") {
	case "", "set":
		op = imap.SetFlags
	case "add":
		op = imap.AddFlags
	case "remove":
		op = imap.RemoveFlags
	default:
		return echo.NewHTTPError(http.StatusBadRequest, "invalid 'action' value")
	}

	err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
		if err := ensureMailboxSelected(c, mboxName); err != nil {
			return err
		}

		var seqSet imap.SeqSet
		seqSet.AddNum(uid)

		storeItems := make([]interface{}, len(flags))
		for i, f := range flags {
			storeItems[i] = f
		}

		item := imap.FormatFlagsOp(op, true)
		if err := c.UidStore(&seqSet, item, storeItems, nil); err != nil {
			return fmt.Errorf("failed to add deleted flag: %v", err)
		}

		return nil
	})
	if err != nil {
		return err
	}

	if op == imap.RemoveFlags && len(flags) == 1 && flags[0] == "\\Seen" {
		// Redirecting to the message view would mark the message as read again
		return ctx.Redirect(http.StatusFound, fmt.Sprintf("/mailbox/%v", url.PathEscape(mboxName)))
	}
	return ctx.Redirect(http.StatusFound, fmt.Sprintf("/message/%v/%v", url.PathEscape(mboxName), uid))
}

const settingsKey = "base.settings"
const maxMessagesPerPage = 100

type Settings struct {
	MessagesPerPage int
}

func loadSettings(s koushin.Store) (*Settings, error) {
	settings := &Settings{
		MessagesPerPage: 50,
	}
	if err := s.Get(settingsKey, settings); err != nil && err != koushin.ErrNoStoreEntry {
		return nil, err
	}
	if err := settings.check(); err != nil {
		return nil, err
	}
	return settings, nil
}

func (s *Settings) check() error {
	if s.MessagesPerPage <= 0 || s.MessagesPerPage > maxMessagesPerPage {
		return fmt.Errorf("messages per page out of bounds: %v", s.MessagesPerPage)
	}
	return nil
}

type SettingsRenderData struct {
	koushin.BaseRenderData
	Settings *Settings
}

func handleSettings(ctx *koushin.Context) error {
	settings, err := loadSettings(ctx.Session.Store())
	if err != nil {
		return fmt.Errorf("failed to load settings: %v", err)
	}

	if ctx.Request().Method == http.MethodPost {
		settings.MessagesPerPage, err = strconv.Atoi(ctx.FormValue("messages_per_page"))
		if err != nil {
			return echo.NewHTTPError(http.StatusBadRequest, "invalid messages per page: %v", err)
		}

		if err := settings.check(); err != nil {
			return echo.NewHTTPError(http.StatusBadRequest, err)
		}
		if err := ctx.Session.Store().Put(settingsKey, settings); err != nil {
			return fmt.Errorf("failed to save settings: %v", err)
		}

		return ctx.Redirect(http.StatusFound, "/mailbox/INBOX")
	}

	return ctx.Render(http.StatusOK, "settings.html", &SettingsRenderData{
		BaseRenderData: *koushin.NewBaseRenderData(ctx),
		Settings:       settings,
	})
}