pool: fix race condition at GetNextConnection()

The `r.current` value can be changed by concurrent threads because
the change happens under read-lock. We could use the atomic counter
for a current connection number to avoid the race condition.

Closes #309

CHANGELOG.md

@@ -46,6 +46,7 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
 ### Fixed
 
 - Flaky decimal/TestSelect (#300)
+- Race condition at roundRobinStrategy.GetNextConnection() (#309)
 
 ## [1.12.0] - 2023-06-07
 

pool/connection_pool_test.go

@@ -2231,6 +2231,35 @@ func TestDo(t *testing.T) {
 	require.NotNilf(t, resp, "response is nil after Ping")
 }
 
+func TestDo_concurrent(t *testing.T) {
+	roles := []bool{true, true, false, true, false}
+
+	err := test_helpers.SetClusterRO(servers, connOpts, roles)
+	require.Nilf(t, err, "fail to set roles for cluster")
+
+	connPool, err := pool.Connect(servers, connOpts)
+	require.Nilf(t, err, "failed to connect")
+	require.NotNilf(t, connPool, "conn is nil after Connect")
+
+	defer connPool.Close()
+
+	req := tarantool.NewPingRequest()
+	const concurrency = 100
+	var wg sync.WaitGroup
+	wg.Add(concurrency)
+
+	for i := 0; i < concurrency; i++ {
+		go func() {
+			defer wg.Done()
+
+			_, err := connPool.Do(req, pool.ANY).Get()
+			assert.Nil(t, err)
+		}()
+	}
+
+	wg.Wait()
+}
+
 func TestNewPrepared(t *testing.T) {
 	test_helpers.SkipIfSQLUnsupported(t)
 

pool/round_robin.go

@@ -2,6 +2,7 @@ package pool
 
 import (
 	"sync"
+	"sync/atomic"
 
 	"github.com/tarantool/go-tarantool/v2"
 )
@@ -10,8 +11,8 @@ type roundRobinStrategy struct {
 	conns       []*tarantool.Connection
 	indexByAddr map[string]uint
 	mutex       sync.RWMutex
-	size        uint
-	current     uint
+	size        uint64
+	current     uint64
 }
 
 func newRoundRobinStrategy(size int) *roundRobinStrategy {
@@ -98,13 +99,12 @@ func (r *roundRobinStrategy) AddConn(addr string, conn *tarantool.Connection) {
 		r.conns[idx] = conn
 	} else {
 		r.conns = append(r.conns, conn)
-		r.indexByAddr[addr] = r.size
+		r.indexByAddr[addr] = uint(r.size)
 		r.size += 1
 	}
 }
 
-func (r *roundRobinStrategy) nextIndex() uint {
-	ret := r.current % r.size
-	r.current++
-	return ret
+func (r *roundRobinStrategy) nextIndex() uint64 {
+	next := atomic.AddUint64(&r.current, 1)
+	return (next - 1) % r.size
 }