moving_filter: fix data race with cache result (#6080)

ref #5798, close #6069

Signed-off-by: lhy1024 <admin@liudos.us>

Co-authored-by: Ti Chi Robot <ti-community-prow-bot@tidb.io>

Author: lhy1024

pkg/movingaverage/median_filter.go

@@ -23,58 +23,48 @@ type MedianFilter struct {
 	// It is not thread safe to read and write records at the same time.
 	// If there are concurrent read and write, the read may get an old value.
 	// And we should avoid concurrent write.
-	records   []float64
-	size      uint64
-	count     uint64
-	isUpdated bool
-	result    float64
+	records []float64
+	size    uint64
+	count   uint64
+	result  float64
 }
 
 // NewMedianFilter returns a MedianFilter.
 func NewMedianFilter(size int) *MedianFilter {
 	return &MedianFilter{
-		records:   make([]float64, size),
-		size:      uint64(size),
-		isUpdated: false,
-		result:    0,
+		records: make([]float64, size),
+		size:    uint64(size),
+		result:  0,
 	}
 }
 
 // Add adds a data point.
 func (r *MedianFilter) Add(n float64) {
 	r.records[r.count%r.size] = n
 	r.count++
-	r.isUpdated = true
-}
-
-// Get returns the median of the data set.
-func (r *MedianFilter) Get() float64 {
-	if !r.isUpdated {
-		return r.result
-	}
-	if r.count == 0 {
-		return 0
-	}
 	records := r.records
 	if r.count < r.size {
 		records = r.records[:r.count]
 	}
 	r.result = pie.Median(records)
-	r.isUpdated = false
+}
+
+// Get returns the median of the data set.
+func (r *MedianFilter) Get() float64 {
 	return r.result
 }
 
 // Reset cleans the data set.
 func (r *MedianFilter) Reset() {
 	r.count = 0
-	r.isUpdated = true
+	r.result = 0
 }
 
 // Set = Reset + Add.
 func (r *MedianFilter) Set(n float64) {
 	r.records[0] = n
 	r.count = 1
-	r.isUpdated = true
+	r.result = n
 }
 
 // GetInstantaneous returns the value just added.
@@ -87,10 +77,9 @@ func (r *MedianFilter) Clone() *MedianFilter {
 	records := make([]float64, len(r.records))
 	copy(records, r.records)
 	return &MedianFilter{
-		records:   records,
-		size:      r.size,
-		count:     r.count,
-		isUpdated: r.isUpdated,
-		result:    r.result,
+		records: records,
+		size:    r.size,
+		count:   r.count,
+		result:  r.result,
 	}
 }