Transaction rollback does not close COPY IN prepared statement and hangs as result

[yurkovoznyak]

In the transaction, I use a prepared statement with COPY IN query.
I have some function which runs ~5-10 sec and may return an error. If I receive error I need to rollback transaction.
According to database/sql doc prepared statement should be closed on transaction Commit or Rollback

Test which describes the behavior below:

func TestCopyInPrepareRollback(t *testing.T) {
	db := openTestConn(t)
	defer db.Close()

	txn, err := db.Begin()
	if err != nil {
		txn.Rollback()
		t.Fatal(err)
	}

	_, err = txn.Exec("CREATE TEMP TABLE temp (num INTEGER)")
	if err != nil {
		txn.Rollback()
		t.Fatal(err)
	}

	stmt, err := txn.Prepare(CopyIn("temp", "num"))
	if err != nil {
		txn.Rollback()
		t.Fatal(err)
	}
	defer stmt.Close()

	someLongRunningErrorFunc := func() error {
		time.Sleep(time.Second * 5)  // without sleep works OK
		return fmt.Errorf("Some external error")
	}

	if err = someLongRunningErrorFunc(); err != nil {
                // stmt.Close()  // this fixes hanged Rollback
		txn.Rollback()  // on this statement test hangs
	}
}

Go version:
go version go1.12.5 linux/amd64

lib/pq version v1.2.0