Fix handling of xattrs with symlinks

Fix Linux, Darwin, and other BSD (untested) to allow proper
handling of xattrs with symlinks. On Linux we cannot use the f*
syscalls for symlinks because symlinks cannot be opened.
File flags must be handled differently on darwin and other BSD due
to the lack of the LCHFLAGS syscall on darwin, and the fact that it
is emulated in libc. However, we do have O_SYMLINK on darwin.

src/duplicacy_utils_bsd_common.go

@@ -0,0 +1,90 @@
+// Copyright (c) Acrosync LLC. All rights reserved.
+// Free for personal use and commercial trial
+// Commercial use requires per-user licenses available from https://duplicacy.com
+
+//go:build freebsd || netbsd || darwin
+// +build freebsd netbsd darwin
+
+package duplicacy
+
+import (
+	"encoding/binary"
+	"os"
+	"path/filepath"
+	"bytes"
+	"syscall"
+
+	"github.com/pkg/xattr"
+)
+
+const bsdFileFlagsKey = "\x00bf"
+
+func (entry *Entry) ReadAttributes(top string) {
+	fullPath := filepath.Join(top, entry.Path)
+	fileInfo, err := os.Lstat(fullPath)
+	if err != nil {
+		return
+	}
+	attributes, _ := xattr.LList(fullPath)
+	if len(attributes) > 0 {
+		entry.Attributes = &map[string][]byte{}
+		for _, name := range attributes {
+			attribute, err := xattr.LGet(fullPath, name)
+			if err == nil {
+				(*entry.Attributes)[name] = attribute
+			}
+		}
+	}
+	if err := entry.readFileFlags(fileInfo); err != nil {
+		LOG_INFO("ATTR_BACKUP", "Could not backup flags for file %s: %v", fullPath, err)
+	}
+}
+
+func (entry *Entry) SetAttributesToFile(fullPath string) {
+	names, _ := xattr.LList(fullPath)
+	for _, name := range names {
+		newAttribute, found := (*entry.Attributes)[name]
+		if found {
+			oldAttribute, _ := xattr.LGet(fullPath, name)
+			if !bytes.Equal(oldAttribute, newAttribute) {
+				xattr.LSet(fullPath, name, newAttribute)
+			}
+			delete(*entry.Attributes, name)
+		} else {
+			xattr.LRemove(fullPath, name)
+		}
+	}
+
+	for name, attribute := range *entry.Attributes {
+		if len(name) > 0 && name[0] == '\x00' {
+			continue
+		}
+		xattr.LSet(fullPath, name, attribute)
+	}
+	if err := entry.restoreLateFileFlags(fullPath); err != nil {
+		LOG_DEBUG("ATTR_RESTORE", "Could not restore flags for file %s: %v", fullPath, err)
+	}
+}
+
+func (entry *Entry) readFileFlags(fileInfo os.FileInfo) error {
+	stat, ok := fileInfo.Sys().(*syscall.Stat_t)
+	if ok && stat.Flags != 0 {
+		if entry.Attributes == nil {
+			entry.Attributes = &map[string][]byte{}
+		}
+		v := make([]byte, 4)
+		binary.LittleEndian.PutUint32(v, stat.Flags)
+		(*entry.Attributes)[bsdFileFlagsKey] = v
+		LOG_DEBUG("ATTR_READ", "Read flags 0x%x for %s", stat.Flags, entry.Path)
+	}
+	return nil
+}
+
+func (entry *Entry) RestoreEarlyDirFlags(path string) error {
+	return nil
+}
+
+func (entry *Entry) RestoreEarlyFileFlags(f *os.File) error {
+	return nil
+}
+