Add middleware for request prioritization (#33951)

This adds a middleware for overload protection that is intended to help protect against malicious scrapers.
It does this via [`codel`](https://github.com/bohde/codel), which will perform the following:

1. Limit the number of in-flight requests to some user-defined max
2. When in-flight requests have reached their begin queuing requests.
    Logged-in requests having priority above logged-out requests
3. Once a request has been queued for too long,
    it has a probabilistic chance to be rejected based on how overloaded the entire system is.

When a server experiences more traffic than it can handle,
this keeps latency low for logged-in users and rejects just
enough requests from logged-out users to not overload the service.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/setting/service.go

@@ -5,6 +5,7 @@ package setting
 
 import (
 	"regexp"
+	"runtime"
 	"strings"
 	"time"
 
@@ -98,6 +99,13 @@ var Service = struct {
 		DisableOrganizationsPage bool `ini:"DISABLE_ORGANIZATIONS_PAGE"`
 		DisableCodePage          bool `ini:"DISABLE_CODE_PAGE"`
 	} `ini:"service.explore"`
+
+	QoS struct {
+		Enabled             bool
+		MaxInFlightRequests int
+		MaxWaitingRequests  int
+		TargetWaitTime      time.Duration
+	}
 }{
 	AllowedUserVisibilityModesSlice: []bool{true, true, true},
 }
@@ -255,6 +263,7 @@ func loadServiceFrom(rootCfg ConfigProvider) {
 	mustMapSetting(rootCfg, "service.explore", &Service.Explore)
 
 	loadOpenIDSetting(rootCfg)
+	loadQosSetting(rootCfg)
 }
 
 func loadOpenIDSetting(rootCfg ConfigProvider) {
@@ -276,3 +285,11 @@ func loadOpenIDSetting(rootCfg ConfigProvider) {
 		}
 	}
 }
+
+func loadQosSetting(rootCfg ConfigProvider) {
+	sec := rootCfg.Section("qos")
+	Service.QoS.Enabled = sec.Key("ENABLED").MustBool(false)
+	Service.QoS.MaxInFlightRequests = sec.Key("MAX_INFLIGHT").MustInt(4 * runtime.NumCPU())
+	Service.QoS.MaxWaitingRequests = sec.Key("MAX_WAITING").MustInt(100)
+	Service.QoS.TargetWaitTime = sec.Key("TARGET_WAIT_TIME").MustDuration(250 * time.Millisecond)
+}