fix: reduce `SetAddrs` shards lock contention

Introduces a new lock to make `SetAddrs` calls exclusive.
This allows release of shards lock for the duration of potentially long `newRingShards` call.

`TestRingSetAddrsContention` observes number of pings increased from <1000 to ~40_000.

See https://github.com/go-redis/redis/pull/2190#discussion_r953040289

Updates #2077

ring.go

@@ -219,6 +219,10 @@ type ringSharding struct {
 	hash      ConsistentHash
 	numShard  int
 	onNewNode []func(rdb *Client)
+
+	// ensures exclusive access to SetAddrs so there is no need
+	// to hold mu for the duration of potentially long shard creation
+	setAddrsMu sync.Mutex
 }
 
 type ringShards struct {
@@ -245,46 +249,62 @@ func (c *ringSharding) OnNewNode(fn func(rdb *Client)) {
 // decrease number of shards, that you use. It will reuse shards that
 // existed before and close the ones that will not be used anymore.
 func (c *ringSharding) SetAddrs(addrs map[string]string) {
-	c.mu.Lock()
+	c.setAddrsMu.Lock()
+	defer c.setAddrsMu.Unlock()
 
+	cleanup := func(shards map[string]*ringShard) {
+		for addr, shard := range shards {
+			if err := shard.Client.Close(); err != nil {
+				internal.Logger.Printf(context.Background(), "shard.Close %s failed: %s", addr, err)
+			}
+		}
+	}
+
+	c.mu.RLock()
 	if c.closed {
+		c.mu.RUnlock()
+		return
+	}
+	existing := c.shards
+	c.mu.RUnlock()
+
+	shards, created, unused := c.newRingShards(addrs, existing)
+
+	c.mu.Lock()
+	if c.closed {
+		cleanup(created)
 		c.mu.Unlock()
 		return
 	}
-
-	shards, cleanup := c.newRingShards(addrs, c.shards)
 	c.shards = shards
 	c.rebalanceLocked()
 	c.mu.Unlock()
 
-	cleanup()
+	cleanup(unused)
 }
 
 func (c *ringSharding) newRingShards(
-	addrs map[string]string, existingShards *ringShards,
-) (*ringShards, func()) {
-	shardMap := make(map[string]*ringShard)     // indexed by addr
-	unusedShards := make(map[string]*ringShard) // indexed by addr
+	addrs map[string]string, existing *ringShards,
+) (shards *ringShards, created, unused map[string]*ringShard) {
 
-	if existingShards != nil {
-		for _, shard := range existingShards.list {
-			addr := shard.Client.opt.Addr
-			shardMap[addr] = shard
-			unusedShards[addr] = shard
-		}
-	}
+	shards = &ringShards{m: make(map[string]*ringShard, len(addrs))}
+	created = make(map[string]*ringShard) // indexed by addr
+	unused = make(map[string]*ringShard)  // indexed by addr
 
-	shards := &ringShards{
-		m: make(map[string]*ringShard),
+	if existing != nil {
+		for _, shard := range existing.list {
+			unused[shard.addr] = shard
+		}
 	}
 
 	for name, addr := range addrs {
-		if shard, ok := shardMap[addr]; ok {
+		if shard, ok := unused[addr]; ok {
 			shards.m[name] = shard
-			delete(unusedShards, addr)
+			delete(unused, addr)
 		} else {
 			shard := newRingShard(c.opt, addr)
 			shards.m[name] = shard
+			created[addr] = shard
 
 			for _, fn := range c.onNewNode {
 				fn(shard.Client)
@@ -296,13 +316,7 @@ func (c *ringSharding) newRingShards(
 		shards.list = append(shards.list, shard)
 	}
 
-	return shards, func() {
-		for addr, shard := range unusedShards {
-			if err := shard.Client.Close(); err != nil {
-				internal.Logger.Printf(context.Background(), "shard.Close %s failed: %s", addr, err)
-			}
-		}
-	}
+	return
 }
 
 func (c *ringSharding) List() []*ringShard {

ring_test.go

@@ -124,7 +124,7 @@ var _ = Describe("Redis Ring", func() {
 			})
 			Expect(ring.Len(), 1)
 			gotShard := ring.ShardByName("ringShardOne")
-			Expect(gotShard).To(Equal(wantShard))
+			Expect(gotShard).To(BeIdenticalTo(wantShard))
 
 			ring.SetAddrs(map[string]string{
 				"ringShardOne": ":" + ringShard1Port,
@@ -132,7 +132,7 @@ var _ = Describe("Redis Ring", func() {
 			})
 			Expect(ring.Len(), 2)
 			gotShard = ring.ShardByName("ringShardOne")
-			Expect(gotShard).To(Equal(wantShard))
+			Expect(gotShard).To(BeIdenticalTo(wantShard))
 		})
 
 		It("uses 3 shards after setting it to 3 shards", func() {
@@ -156,8 +156,8 @@ var _ = Describe("Redis Ring", func() {
 			gotShard1 := ring.ShardByName(shardName1)
 			gotShard2 := ring.ShardByName(shardName2)
 			gotShard3 := ring.ShardByName(shardName3)
-			Expect(gotShard1).To(Equal(wantShard1))
-			Expect(gotShard2).To(Equal(wantShard2))
+			Expect(gotShard1).To(BeIdenticalTo(wantShard1))
+			Expect(gotShard2).To(BeIdenticalTo(wantShard2))
 			Expect(gotShard3).ToNot(BeNil())
 
 			ring.SetAddrs(map[string]string{
@@ -168,8 +168,8 @@ var _ = Describe("Redis Ring", func() {
 			gotShard1 = ring.ShardByName(shardName1)
 			gotShard2 = ring.ShardByName(shardName2)
 			gotShard3 = ring.ShardByName(shardName3)
-			Expect(gotShard1).To(Equal(wantShard1))
-			Expect(gotShard2).To(Equal(wantShard2))
+			Expect(gotShard1).To(BeIdenticalTo(wantShard1))
+			Expect(gotShard2).To(BeIdenticalTo(wantShard2))
 			Expect(gotShard3).To(BeNil())
 		})
 	})