Least load balancer (#2999)

* v5: Health Check & LeastLoad Strategy (rebased from 2c5a71490368500a982018a74a6d519c7e121816)

Some changes will be necessary to integrate it into V2Ray

* Update proto

* parse duration conf with time.Parse()

* moving health ping to observatory as a standalone component

* moving health ping to observatory as a standalone component: auto generated file

* add initialization for health ping

* incorporate changes in router implementation

* support principle target output

* add v4 json support for BurstObservatory & fix balancer reference

* update API command

* remove cancelled API

* return zero length value when observer is not found

* remove duplicated targeted dispatch

* adjust test with updated structure

* bug fix for observer

* fix strategy selector

* fix strategy least load

* Fix ticker usage

ticker.Close does not close ticker.C

* feat: Replace default Health Ping URL to HTTPS (#1991)

* fix selectLeastLoad() returns wrong number of nodes (#2083)

* Test: fix leastload strategy unit test

* fix(router): panic caused by concurrent map read and write (#2678)

* Clean up code

---------

Co-authored-by: Jebbs <qjebbs@gmail.com>
Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>
Co-authored-by: 世界 <i@sekai.icu>
Co-authored-by: Bernd Eichelberger <46166740+4-FLOSS-Free-Libre-Open-Source-Software@users.noreply.github.com>
Co-authored-by: 秋のかえで <autmaple@protonmail.com>
Co-authored-by: Rinka <kujourinka@gmail.com>

app/observatory/burst/healthping_result.go

@@ -0,0 +1,143 @@
+package burst
+
+import (
+	"math"
+	"time"
+)
+
+// HealthPingStats is the statistics of HealthPingRTTS
+type HealthPingStats struct {
+	All       int
+	Fail      int
+	Deviation time.Duration
+	Average   time.Duration
+	Max       time.Duration
+	Min       time.Duration
+}
+
+// HealthPingRTTS holds ping rtts for health Checker
+type HealthPingRTTS struct {
+	idx      int
+	cap      int
+	validity time.Duration
+	rtts     []*pingRTT
+
+	lastUpdateAt time.Time
+	stats        *HealthPingStats
+}
+
+type pingRTT struct {
+	time  time.Time
+	value time.Duration
+}
+
+// NewHealthPingResult returns a *HealthPingResult with specified capacity
+func NewHealthPingResult(cap int, validity time.Duration) *HealthPingRTTS {
+	return &HealthPingRTTS{cap: cap, validity: validity}
+}
+
+// Get gets statistics of the HealthPingRTTS
+func (h *HealthPingRTTS) Get() *HealthPingStats {
+	return h.getStatistics()
+}
+
+// GetWithCache get statistics and write cache for next call
+// Make sure use Mutex.Lock() before calling it, RWMutex.RLock()
+// is not an option since it writes cache
+func (h *HealthPingRTTS) GetWithCache() *HealthPingStats {
+	lastPutAt := h.rtts[h.idx].time
+	now := time.Now()
+	if h.stats == nil || h.lastUpdateAt.Before(lastPutAt) || h.findOutdated(now) >= 0 {
+		h.stats = h.getStatistics()
+		h.lastUpdateAt = now
+	}
+	return h.stats
+}
+
+// Put puts a new rtt to the HealthPingResult
+func (h *HealthPingRTTS) Put(d time.Duration) {
+	if h.rtts == nil {
+		h.rtts = make([]*pingRTT, h.cap)
+		for i := 0; i < h.cap; i++ {
+			h.rtts[i] = &pingRTT{}
+		}
+		h.idx = -1
+	}
+	h.idx = h.calcIndex(1)
+	now := time.Now()
+	h.rtts[h.idx].time = now
+	h.rtts[h.idx].value = d
+}
+
+func (h *HealthPingRTTS) calcIndex(step int) int {
+	idx := h.idx
+	idx += step
+	if idx >= h.cap {
+		idx %= h.cap
+	}
+	return idx
+}
+
+func (h *HealthPingRTTS) getStatistics() *HealthPingStats {
+	stats := &HealthPingStats{}
+	stats.Fail = 0
+	stats.Max = 0
+	stats.Min = rttFailed
+	sum := time.Duration(0)
+	cnt := 0
+	validRTTs := make([]time.Duration, 0)
+	for _, rtt := range h.rtts {
+		switch {
+		case rtt.value == 0 || time.Since(rtt.time) > h.validity:
+			continue
+		case rtt.value == rttFailed:
+			stats.Fail++
+			continue
+		}
+		cnt++
+		sum += rtt.value
+		validRTTs = append(validRTTs, rtt.value)
+		if stats.Max < rtt.value {
+			stats.Max = rtt.value
+		}
+		if stats.Min > rtt.value {
+			stats.Min = rtt.value
+		}
+	}
+	stats.All = cnt + stats.Fail
+	if cnt == 0 {
+		stats.Min = 0
+		return stats
+	}
+	stats.Average = time.Duration(int(sum) / cnt)
+	var std float64
+	if cnt < 2 {
+		// no enough data for standard deviation, we assume it's half of the average rtt
+		// if we don't do this, standard deviation of 1 round tested nodes is 0, will always
+		// selected before 2 or more rounds tested nodes
+		std = float64(stats.Average / 2)
+	} else {
+		variance := float64(0)
+		for _, rtt := range validRTTs {
+			variance += math.Pow(float64(rtt-stats.Average), 2)
+		}
+		std = math.Sqrt(variance / float64(cnt))
+	}
+	stats.Deviation = time.Duration(std)
+	return stats
+}
+
+func (h *HealthPingRTTS) findOutdated(now time.Time) int {
+	for i := h.cap - 1; i < 2*h.cap; i++ {
+		// from oldest to latest
+		idx := h.calcIndex(i)
+		validity := h.rtts[idx].time.Add(h.validity)
+		if h.lastUpdateAt.After(validity) {
+			return idx
+		}
+		if validity.Before(now) {
+			return idx
+		}
+	}
+	return -1
+}