Replace CSRF cookie with CrossOriginProtection (#36183)

Removes the CSRF cookie in favor of
[`CrossOriginProtection`](https://pkg.go.dev/net/http#CrossOriginProtection)
which relies purely on HTTP headers.

Fixes: https://github.com/go-gitea/gitea/issues/11188
Fixes: https://github.com/go-gitea/gitea/issues/30333
Helps: https://github.com/go-gitea/gitea/issues/35107

TODOs:

- [x] Fix tests
- [ ] Ideally add tests to validates the protection

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/context/context.go

@@ -6,14 +6,12 @@ package context
 
 import (
 	"context"
-	"encoding/hex"
 	"fmt"
 	"html/template"
 	"io"
 	"net/http"
 	"net/url"
 	"strings"
-	"time"
 
 	"code.gitea.io/gitea/models/unit"
 	user_model "code.gitea.io/gitea/models/user"
@@ -48,7 +46,6 @@ type Context struct {
 	PageData map[string]any // data used by JavaScript modules in one page, it's `window.config.pageData`
 
 	Cache   cache.StringCache
-	Csrf    CSRFProtector
 	Flash   *middleware.Flash
 	Session session.Store
 
@@ -143,18 +140,6 @@ func NewWebContext(base *Base, render Render, session session.Store) *Context {
 // Contexter initializes a classic context for a request.
 func Contexter() func(next http.Handler) http.Handler {
 	rnd := templates.HTMLRenderer()
-	csrfOpts := CsrfOptions{
-		Secret:         hex.EncodeToString(setting.GetGeneralTokenSigningSecret()),
-		Cookie:         setting.CSRFCookieName,
-		Secure:         setting.SessionConfig.Secure,
-		CookieHTTPOnly: setting.CSRFCookieHTTPOnly,
-		CookieDomain:   setting.SessionConfig.Domain,
-		CookiePath:     setting.SessionConfig.CookiePath,
-		SameSite:       setting.SessionConfig.SameSite,
-	}
-	if !setting.IsProd {
-		CsrfTokenRegenerationInterval = 5 * time.Second // in dev, re-generate the tokens more aggressively for debug purpose
-	}
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
 			base := NewBaseContext(resp, req)
@@ -167,8 +152,6 @@ func Contexter() func(next http.Handler) http.Handler {
 			ctx.PageData = map[string]any{}
 			ctx.Data["PageData"] = ctx.PageData
 
-			ctx.Csrf = NewCSRFProtector(csrfOpts)
-
 			// get the last flash message from cookie
 			lastFlashCookie, lastFlashMsg := middleware.GetSiteCookieFlashMessage(ctx, ctx.Req, CookieNameFlash)
 			if vals, _ := url.ParseQuery(lastFlashCookie); len(vals) > 0 {
@@ -184,7 +167,10 @@ func Contexter() func(next http.Handler) http.Handler {
 				}
 			})
 
-			// If request sends files, parse them here otherwise the Query() can't be parsed and the CsrfToken will be invalid.
+			// FIXME: GLOBAL-PARSE-FORM: this ParseMultipartForm was used for parsing the csrf token from multipart/form-data
+			// We have dropped the csrf token, so ideally this global ParseMultipartForm should be removed.
+			// When removing this, we need to avoid regressions in the handler functions because Golang's http framework is quite fragile
+			// and developers sometimes need to manually parse the form before accessing some values.
 			if ctx.Req.Method == http.MethodPost && strings.Contains(ctx.Req.Header.Get("Content-Type"), "multipart/form-data") {
 				if !ctx.ParseMultipartForm() {
 					return