all: Support syncing extended attributes (fixes #2698) (#8513)

This adds support for syncing extended attributes on supported
filesystem on Linux, macOS, FreeBSD and NetBSD. Windows is currently
excluded because the APIs seem onerous and annoying and frankly the uses
cases seem few and far between. On Unixes this also covers ACLs as those
are stored as extended attributes.

Similar to ownership syncing this will optional & opt-in, which two
settings controlling the main behavior: one to "sync" xattrs (read &
write) and another one to "scan" xattrs (only read them so other devices
can "sync" them, but not apply any locally).

Co-authored-by: Tomasz Wilczyński <twilczynski@naver.com>

lib/fs/filesystem.go

@@ -29,6 +29,12 @@ const (
 	filesystemWrapperTypeLog
 )
 
+type XattrFilter interface {
+	Permit(string) bool
+	GetMaxSingleEntrySize() int
+	GetMaxTotalSize() int
+}
+
 // The Filesystem interface abstracts access to the file system.
 type Filesystem interface {
 	Chmod(name string, mode FileMode) error
@@ -62,7 +68,9 @@ type Filesystem interface {
 	URI() string
 	Options() []Option
 	SameFile(fi1, fi2 FileInfo) bool
-	PlatformData(name string) (protocol.PlatformData, error)
+	PlatformData(name string, withOwnership, withXattrs bool, xattrFilter XattrFilter) (protocol.PlatformData, error)
+	GetXattr(name string, xattrFilter XattrFilter) ([]protocol.Xattr, error)
+	SetXattr(path string, xattrs []protocol.Xattr, xattrFilter XattrFilter) error
 
 	// Used for unwrapping things
 	underlying() (Filesystem, bool)
@@ -94,11 +102,13 @@ type FileInfo interface {
 	Size() int64
 	ModTime() time.Time
 	IsDir() bool
+	Sys() interface{}
 	// Extensions
 	IsRegular() bool
 	IsSymlink() bool
 	Owner() int
 	Group() int
+	InodeChangeTime() time.Time // may be zero if not supported
 }
 
 // FileMode is similar to os.FileMode
@@ -154,7 +164,10 @@ func (evType EventType) String() string {
 	}
 }
 
-var ErrWatchNotSupported = errors.New("watching is not supported")
+var (
+	ErrWatchNotSupported  = errors.New("watching is not supported")
+	ErrXattrsNotSupported = errors.New("extended attributes are not supported on this platform")
+)
 
 // Equivalents from os package.