perf: avoid unnecessary copy operation (#3376)

* optime: reduce unnecessary copy operations

* add a comment

* trigger CI without code changes, because the bug of docker

* add comments

Author: fukua95

ring.go

@@ -349,17 +349,16 @@ func (c *ringSharding) newRingShards(
 	return
 }
 
+// Warning: External exposure of `c.shards.list` may cause data races.
+// So keep internal or implement deep copy if exposed.
 func (c *ringSharding) List() []*ringShard {
-	var list []*ringShard
-
 	c.mu.RLock()
-	if !c.closed {
-		list = make([]*ringShard, len(c.shards.list))
-		copy(list, c.shards.list)
-	}
-	c.mu.RUnlock()
+	defer c.mu.RUnlock()
 
-	return list
+	if c.closed {
+		return nil
+	}
+	return c.shards.list
 }
 
 func (c *ringSharding) Hash(key string) string {
@@ -423,6 +422,7 @@ func (c *ringSharding) Heartbeat(ctx context.Context, frequency time.Duration) {
 		case <-ticker.C:
 			var rebalance bool
 
+			// note: `c.List()` return a shadow copy of `[]*ringShard`.
 			for _, shard := range c.List() {
 				err := shard.Client.Ping(ctx).Err()
 				isUp := err == nil || err == pool.ErrPoolTimeout
@@ -582,6 +582,7 @@ func (c *Ring) retryBackoff(attempt int) time.Duration {
 
 // PoolStats returns accumulated connection pool stats.
 func (c *Ring) PoolStats() *PoolStats {
+	// note: `c.List()` return a shadow copy of `[]*ringShard`.
 	shards := c.sharding.List()
 	var acc PoolStats
 	for _, shard := range shards {
@@ -651,6 +652,7 @@ func (c *Ring) ForEachShard(
 	ctx context.Context,
 	fn func(ctx context.Context, client *Client) error,
 ) error {
+	// note: `c.List()` return a shadow copy of `[]*ringShard`.
 	shards := c.sharding.List()
 	var wg sync.WaitGroup
 	errCh := make(chan error, 1)
@@ -682,6 +684,7 @@ func (c *Ring) ForEachShard(
 }
 
 func (c *Ring) cmdsInfo(ctx context.Context) (map[string]*CommandInfo, error) {
+	// note: `c.List()` return a shadow copy of `[]*ringShard`.
 	shards := c.sharding.List()
 	var firstErr error
 	for _, shard := range shards {
@@ -810,7 +813,7 @@ func (c *Ring) Watch(ctx context.Context, fn func(*Tx) error, keys ...string) er
 
 	for _, key := range keys {
 		if key != "" {
-			shard, err := c.sharding.GetByKey(hashtag.Key(key))
+			shard, err := c.sharding.GetByKey(key)
 			if err != nil {
 				return err
 			}