receiver: only receive uid/gid list if enabled

I did not notice this previously, because my transfers typically
use -a (archive), which enables preserveuid and preservegid.

Author: stapelberg

internal/receiver/flist.go

@@ -225,13 +225,15 @@ func (rt *Transfer) ReceiveFileList() ([]*File, error) {
 
 	sortFileList(fileList)
 
-	// receive the uid/gid list
-	users, groups, err := rt.RecvIdList()
-	if err != nil {
-		return nil, err
+	if rt.Opts.PreserveUid || rt.Opts.PreserveGid {
+		// receive the uid/gid list
+		users, groups, err := rt.RecvIdList()
+		if err != nil {
+			return nil, err
+		}
+		_ = users
+		_ = groups
 	}
-	_ = users
-	_ = groups
 
 	// read the i/o error flag
 	ioErrors, err := rt.Conn.ReadInt32()

internal/receiver/uidlist.go

@@ -38,26 +38,33 @@ func (rt *Transfer) recvIdMapping1(localId func(id int32, name string) int32) (m
 
 // rsync/uidlist.c:recv_id_list
 func (rt *Transfer) RecvIdList() (users map[int32]mapping, groups map[int32]mapping, _ error) {
-	var err error
-	users, err = rt.recvIdMapping1(func(remoteUid int32, remoteUsername string) int32 {
-		// TODO: look up local uid by username
-		return remoteUid
-	})
-	if err != nil {
-		return nil, nil, err
-	}
-	for remoteUid, mapping := range users {
-		log.Printf("remote uid %d(%s) maps to local uid %d", remoteUid, mapping.Name, mapping.LocalId)
-	}
-	groups, err = rt.recvIdMapping1(func(remoteGid int32, remoteGroupname string) int32 {
-		// TODO: look up local gid by groupname
-		return remoteGid
-	})
-	if err != nil {
-		return nil, nil, err
+	if rt.Opts.PreserveUid {
+		var err error
+		users, err = rt.recvIdMapping1(func(remoteUid int32, remoteUsername string) int32 {
+			// TODO: look up local uid by username
+			return remoteUid
+		})
+		if err != nil {
+			return nil, nil, err
+		}
+		for remoteUid, mapping := range users {
+			log.Printf("remote uid %d(%s) maps to local uid %d", remoteUid, mapping.Name, mapping.LocalId)
+		}
 	}
-	for remoteGid, mapping := range groups {
-		log.Printf("remote gid %d(%s) maps to local gid %d", remoteGid, mapping.Name, mapping.LocalId)
+
+	if rt.Opts.PreserveGid {
+		var err error
+		groups, err = rt.recvIdMapping1(func(remoteGid int32, remoteGroupname string) int32 {
+			// TODO: look up local gid by groupname
+			return remoteGid
+		})
+		if err != nil {
+			return nil, nil, err
+		}
+		for remoteGid, mapping := range groups {
+			log.Printf("remote gid %d(%s) maps to local gid %d", remoteGid, mapping.Name, mapping.LocalId)
+		}
 	}
+
 	return users, groups, nil
 }