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_linux.go

@@ -5,10 +5,14 @@
 package duplicacy
 
 import (
+	"bytes"
 	"encoding/binary"
 	"os"
 	"syscall"
 	"unsafe"
+	"path/filepath"
+
+	"github.com/pkg/xattr"
 )
 
 const (
@@ -47,10 +51,116 @@ func ioctl(f *os.File, request uintptr, attrp *uint32) error {
 	return nil
 }
 
-func (entry *Entry) ReadFileFlags(f *os.File) error {
-	if entry.IsSpecial() {
-		return nil
+type xattrHandle struct {
+	f *os.File
+	fullPath string
+}
+
+func (x xattrHandle) list() ([]string, error) {
+	if x.f != nil {
+		return xattr.FList(x.f)
+	} else {
+		return xattr.LList(x.fullPath)
 	}
+}
+
+func (x xattrHandle) get(name string) ([]byte, error) {
+	if x.f != nil {
+		return xattr.FGet(x.f, name)
+	} else {
+		return xattr.LGet(x.fullPath, name)
+	}
+}
+
+func (x xattrHandle) set(name string, value []byte) error {
+	if x.f != nil {
+		return xattr.FSet(x.f, name, value)
+	} else {
+		return xattr.LSet(x.fullPath, name, value)
+	}
+}
+
+func (x xattrHandle) remove(name string) error {
+	if x.f != nil {
+		return xattr.FRemove(x.f, name)
+	} else {
+		return xattr.LRemove(x.fullPath, name)
+	}
+}
+
+func (entry *Entry) ReadAttributes(top string) {
+	fullPath := filepath.Join(top, entry.Path)
+	x := xattrHandle{nil, fullPath}
+
+	if !entry.IsLink() {
+		var err error
+		x.f, err = os.OpenFile(fullPath, os.O_RDONLY|syscall.O_NOFOLLOW|syscall.O_NONBLOCK, 0)
+		if err != nil {
+			// FIXME: We really should return errors for failure to read
+			return
+		}
+	}
+
+	attributes, _ := x.list()
+
+	if len(attributes) > 0 {
+		entry.Attributes = &map[string][]byte{}
+	}
+	for _, name := range attributes {
+		attribute, err := x.get(name)
+		if err == nil {
+			(*entry.Attributes)[name] = attribute
+		}
+	}
+
+	if entry.IsFile() || entry.IsDir() {
+		if err := entry.readFileFlags(x.f); err != nil {
+			LOG_INFO("ATTR_BACKUP", "Could not backup flags for file %s: %v", fullPath, err)
+		}
+	}
+	x.f.Close()
+}
+
+func (entry *Entry) SetAttributesToFile(fullPath string) {
+	x := xattrHandle{nil, fullPath}
+	if !entry.IsLink() {
+		var err error
+		x.f, err = os.OpenFile(fullPath, os.O_RDONLY|syscall.O_NOFOLLOW, 0)
+		if err != nil {
+			return
+		}
+	}
+
+	names, _ := x.list()
+
+	for _, name := range names {
+		newAttribute, found := (*entry.Attributes)[name]
+		if found {
+			oldAttribute, _ := x.get(name)
+			if !bytes.Equal(oldAttribute, newAttribute) {
+				x.set(name, newAttribute)
+			}
+			delete(*entry.Attributes, name)
+		} else {
+			x.remove(name)
+		}
+	}
+
+	for name, attribute := range *entry.Attributes {
+		if len(name) > 0 && name[0] == '\x00' {
+			continue
+		}
+		x.set(name, attribute)
+	}
+	if entry.IsFile() || entry.IsDir() {
+		if err := entry.restoreLateFileFlags(x.f); err != nil {
+			LOG_DEBUG("ATTR_RESTORE", "Could not restore flags for file %s: %v", fullPath, err)
+		}
+	}
+	x.f.Close()
+}
+
+func (entry *Entry) readFileFlags(f *os.File) error {
 	var flags uint32
 	if err := ioctl(f, linux_FS_IOC_GETFLAGS, &flags); err != nil {
 		return err
@@ -97,7 +207,7 @@ func (entry *Entry) RestoreEarlyFileFlags(f *os.File) error {
 	return nil
 }
 
-func (entry *Entry) RestoreLateFileFlags(f *os.File) error {
+func (entry *Entry) restoreLateFileFlags(f *os.File) error {
 	if entry.Attributes == nil {
 		return nil
 	}