ndb: add read lock to prevent NDB blob magic error

This error can happen if the ndb packages file is being updated
(e.g. zypper up) while we are reading it. The rpm binary uses a
shared flock when running `rpm -qa` on the ndb backend. This
commit does the same for go-rpmdb.

pkg/ndb/ndb.go

@@ -26,6 +26,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"syscall"
 	"unsafe"
 
 	dbi "github.com/knqyf263/go-rpmdb/pkg/db"
@@ -96,6 +97,11 @@ func Open(path string) (*RpmNDB, error) {
 		return nil, err
 	}
 
+	err = syscall.Flock(int(file.Fd()), syscall.LOCK_SH)
+	if err != nil {
+	    return nil, err
+	}
+
 	hdrBuff := ndbHeader{}
 	err = binary.Read(file, binary.LittleEndian, &hdrBuff)
 	if err != nil {
@@ -127,6 +133,7 @@ func Open(path string) (*RpmNDB, error) {
 }
 
 func (db *RpmNDB) Close() error {
+	_ = syscall.Flock(int(db.file.Fd()), syscall.LOCK_UN)
 	return db.file.Close()
 }