api: add connection_pool.ErrExists

ConnectionPool.Add() returns the error if an endpoint exists in
a connection pool.

Author: oleg-jukovec

connection_pool/connection_pool.go

@@ -30,6 +30,7 @@ var (
 	ErrNoRwInstance      = errors.New("can't find rw instance in pool")
 	ErrNoRoInstance      = errors.New("can't find ro instance in pool")
 	ErrNoHealthyInstance = errors.New("can't find healthy instance in pool")
+	ErrExists            = errors.New("endpoint exists")
 	ErrClosed            = errors.New("pool is closed")
 )
 
@@ -233,7 +234,7 @@ func (pool *ConnectionPool) Add(addr string) error {
 	}
 	if _, ok := pool.addrs[addr]; ok {
 		pool.addrsMutex.Unlock()
-		return errors.New("endpoint exist")
+		return ErrExists
 	}
 	pool.addrs[addr] = e
 	pool.addrsMutex.Unlock()

connection_pool/connection_pool_test.go

@@ -282,7 +282,7 @@ func TestAdd_exist(t *testing.T) {
 	defer connPool.Close()
 
 	err = connPool.Add(servers[0])
-	require.ErrorContains(t, err, "endpoint exist")
+	require.Equal(t, connection_pool.ErrExists, err)
 
 	args := test_helpers.CheckStatusesArgs{
 		ConnPool:           connPool,