bugfix: logic of add/delete in round robin strategy

* Duplicates could be added to a list of connections, which then
  cannot be deleted.
* The delete function uses an address from a connection instead of
  argument value. It may lead to unexpected errors.

Part of #208

connection_pool/round_robin.go

@@ -46,8 +46,10 @@ func (r *RoundRobinStrategy) DeleteConnByAddr(addr string) *tarantool.Connection
 	r.conns = append(r.conns[:index], r.conns[index+1:]...)
 	r.size -= 1
 
-	for index, conn := range r.conns {
-		r.indexByAddr[conn.Addr()] = index
+	for k, v := range r.indexByAddr {
+		if v > index {
+			r.indexByAddr[k] = v - 1
+		}
 	}
 
 	return conn
@@ -94,9 +96,13 @@ func (r *RoundRobinStrategy) AddConn(addr string, conn *tarantool.Connection) {
 	r.mutex.Lock()
 	defer r.mutex.Unlock()
 
-	r.conns = append(r.conns, conn)
-	r.indexByAddr[addr] = r.size
-	r.size += 1
+	if idx, ok := r.indexByAddr[addr]; ok {
+		r.conns[idx] = conn
+	} else {
+		r.conns = append(r.conns, conn)
+		r.indexByAddr[addr] = r.size
+		r.size += 1
+	}
 }
 
 func (r *RoundRobinStrategy) nextIndex() int {

connection_pool/round_robin_test.go

@@ -0,0 +1,55 @@
+package connection_pool_test
+
+import (
+	"testing"
+
+	"github.com/tarantool/go-tarantool"
+	. "github.com/tarantool/go-tarantool/connection_pool"
+)
+
+const (
+	validAddr1 = "x"
+	validAddr2 = "y"
+)
+
+func TestRoundRobinAddDelete(t *testing.T) {
+	rr := NewEmptyRoundRobin(10)
+
+	addrs := []string{validAddr1, validAddr2}
+	conns := []*tarantool.Connection{&tarantool.Connection{}, &tarantool.Connection{}}
+
+	for i, addr := range addrs {
+		rr.AddConn(addr, conns[i])
+	}
+
+	for i, addr := range addrs {
+		if conn := rr.DeleteConnByAddr(addr); conn != conns[i] {
+			t.Errorf("Unexpected connection on address %s", addr)
+		}
+	}
+	if !rr.IsEmpty() {
+		t.Errorf("RoundRobin does not empty")
+	}
+}
+
+func TestRoundRobinAddDuplicateDelete(t *testing.T) {
+	rr := NewEmptyRoundRobin(10)
+
+	conn1 := &tarantool.Connection{}
+	conn2 := &tarantool.Connection{}
+
+	rr.AddConn(validAddr1, conn1)
+	rr.AddConn(validAddr1, conn2)
+
+	if rr.DeleteConnByAddr(validAddr1) != conn2 {
+		t.Errorf("Unexpected deleted connection")
+	}
+	if !rr.IsEmpty() {
+		t.Errorf("RoundRobin does not empty")
+	}
+	if rr.DeleteConnByAddr(validAddr1) != nil {
+		t.Errorf("Unexpected valued after second delition")
+	}
+}
+
+