You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 2fa.go
    path of file: ./repos/gitea/routers/web/auth
    
    the code of the file until where you have to start completion: // Copyright 2017 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package auth

import (
	"errors"
	"net/http"

	"code.gitea.io/gitea/models/auth"
	user_model "code.gitea.io/gitea/models/user"
	"code.gitea.io/gitea/modules/base"
	"code.gitea.io/gitea/modules/setting"
	"code.gitea.io/gitea/modules/web"
	"code.gitea.io/gitea/services/context"
	"code.gitea.io/gitea/services/externalaccount"
	"code.gitea.io/gitea/services/forms"
)

var (
	tplTwofa        base.TplName = "user/auth/twofa"
	tplTwofaScratch base.TplName = "user/auth/twofa_scratch"
)

// TwoFactor shows the user a two-factor authentication page.
func TwoFactor(ctx *context.Context) {
	ctx.Data["Title"] = ctx.Tr("twofa")

	if CheckAutoLogin(ctx) {
		return
	}

	// Ensure user is in a 2FA session.
	if ctx.Session.Get("twofaUid") == nil {
		ctx.ServerError("UserSignIn", errors.New("not in 2FA session"))
		return
	}

	ctx.HTML(http.StatusOK, tpl
    
func TwoFactorScratchPost(ctx *context.Context) {
	form := web.GetForm(ctx).(*forms.TwoFactorScratchAuthForm)
	ctx.Data["Title"] = ctx.Tr("twofa_scratch")

	// Ensure user is in a 2FA session.
	idSess := ctx.Session.Get("twofaUid")
	if idSess == nil {
		ctx.ServerError("UserSignIn", errors.New("not in 2FA session"))
		return
	}

	id := idSess.(int64)
	twofa, err := auth.GetTwoFactorByUID(ctx, id)
	if err != nil {
		ctx.ServerError("UserSignIn", err)
		return
	}

	// Validate the passcode with the stored TOTP secret.
	if twofa.VerifyScratchToken(form.Token) {
		// Invalidate the scratch token.
		_, err = twofa.GenerateScratchToken()
		if err != nil {
			ctx.ServerError("UserSignIn", err)
			return
		}
		if err = auth.UpdateTwoFactor(ctx, twofa); err != nil {
			ctx.ServerError("UserSignIn", err)
			return
		}

		remember := ctx.Session.Get("twofaRemember").(bool)
		u, err := user_model.GetUserByID(ctx, id)
		if err != nil {
			ctx.ServerError("UserSignIn", err)
			return
		}

		handleSignInFull(ctx, u, remember, false)
		if ctx.Written() {
			return
		}
		ctx.Flash.Info(ctx.Tr("auth.twofa_scratch_used"))
		ctx.Redirect(setting.AppSubURL + "/user/settings/security")
		return
	}

	ctx.RenderWithErr(ctx.Tr("auth.twofa_scratch_token_incorrect"), tplTwofaScratch, forms.TwoFactorScratchAuthForm{})
}