Add a config option to block "expensive" pages for anonymous users (#34024)

Fix #33966

```
;; User must sign in to view anything.
;; It could be set to "expensive" to block anonymous users accessing some pages which consume a lot of resources,
;; for example: block anonymous AI crawlers from accessing repo code pages.
;; The "expensive" mode is experimental and subject to change.
;REQUIRE_SIGNIN_VIEW = false
```

routers/common/blockexpensive.go

@@ -0,0 +1,90 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"net/http"
+	"strings"
+
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/reqctx"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/web/middleware"
+
+	"github.com/go-chi/chi/v5"
+)
+
+func BlockExpensive() func(next http.Handler) http.Handler {
+	if !setting.Service.BlockAnonymousAccessExpensive {
+		return nil
+	}
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+			ret := determineRequestPriority(reqctx.FromContext(req.Context()))
+			if !ret.SignedIn {
+				if ret.Expensive || ret.LongPolling {
+					http.Redirect(w, req, setting.AppSubURL+"/user/login", http.StatusSeeOther)
+					return
+				}
+			}
+			next.ServeHTTP(w, req)
+		})
+	}
+}
+
+func isRoutePathExpensive(routePattern string) bool {
+	if strings.HasPrefix(routePattern, "/user/") || strings.HasPrefix(routePattern, "/login/") {
+		return false
+	}
+
+	expensivePaths := []string{
+		// code related
+		"/{username}/{reponame}/archive/",
+		"/{username}/{reponame}/blame/",
+		"/{username}/{reponame}/commit/",
+		"/{username}/{reponame}/commits/",
+		"/{username}/{reponame}/graph",
+		"/{username}/{reponame}/media/",
+		"/{username}/{reponame}/raw/",
+		"/{username}/{reponame}/src/",
+
+		// issue & PR related (no trailing slash)
+		"/{username}/{reponame}/issues",
+		"/{username}/{reponame}/{type:issues}",
+		"/{username}/{reponame}/pulls",
+		"/{username}/{reponame}/{type:pulls}",
+
+		// wiki
+		"/{username}/{reponame}/wiki/",
+
+		// activity
+		"/{username}/{reponame}/activity/",
+	}
+	for _, path := range expensivePaths {
+		if strings.HasPrefix(routePattern, path) {
+			return true
+		}
+	}
+	return false
+}
+
+func isRoutePathForLongPolling(routePattern string) bool {
+	return routePattern == "/user/events"
+}
+
+func determineRequestPriority(reqCtx reqctx.RequestContext) (ret struct {
+	SignedIn    bool
+	Expensive   bool
+	LongPolling bool
+},
+) {
+	chiRoutePath := chi.RouteContext(reqCtx).RoutePattern()
+	if _, ok := reqCtx.GetData()[middleware.ContextDataKeySignedUser].(*user_model.User); ok {
+		ret.SignedIn = true
+	} else {
+		ret.Expensive = isRoutePathExpensive(chiRoutePath)
+		ret.LongPolling = isRoutePathForLongPolling(chiRoutePath)
+	}
+	return ret
+}