randomWRR: remove lock for accessing WRR.items (#6666)

internal/wrr/random.go

@@ -20,7 +20,6 @@ package wrr
 import (
 	"fmt"
 	"sort"
-	"sync"
 
 	"google.golang.org/grpc/internal/grpcrand"
 )
@@ -38,7 +37,6 @@ func (w *weightedItem) String() string {
 
 // randomWRR is a struct that contains weighted items implement weighted random algorithm.
 type randomWRR struct {
-	mu    sync.RWMutex
 	items []*weightedItem
 	// Are all item's weights equal
 	equalWeights bool
@@ -52,8 +50,6 @@ func NewRandom() WRR {
 var grpcrandInt63n = grpcrand.Int63n
 
 func (rw *randomWRR) Next() (item any) {
-	rw.mu.RLock()
-	defer rw.mu.RUnlock()
 	if len(rw.items) == 0 {
 		return nil
 	}
@@ -72,8 +68,6 @@ func (rw *randomWRR) Next() (item any) {
 }
 
 func (rw *randomWRR) Add(item any, weight int64) {
-	rw.mu.Lock()
-	defer rw.mu.Unlock()
 	accumulatedWeight := weight
 	equalWeights := true
 	if len(rw.items) > 0 {

internal/wrr/wrr.go

@@ -21,12 +21,12 @@ package wrr
 
 // WRR defines an interface that implements weighted round robin.
 type WRR interface {
-	// Add adds an item with weight to the WRR set.
-	//
-	// Add and Next need to be thread safe.
+	// Add adds an item with weight to the WRR set. Add must be only called
+	// before any calls to Next.
 	Add(item any, weight int64)
 	// Next returns the next picked item.
 	//
-	// Add and Next need to be thread safe.
+	// Next needs to be thread safe. Add may not be called after any call to
+	// Next.
 	Next() any
 }