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/basicfs_test.go

@@ -7,6 +7,9 @@
 package fs
 
 import (
+	"bytes"
+	"errors"
+	"fmt"
 	"os"
 	"path/filepath"
 	"sort"
@@ -16,6 +19,7 @@ import (
 	"time"
 
 	"github.com/syncthing/syncthing/lib/build"
+	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/rand"
 )
 
@@ -565,6 +569,87 @@ func TestRel(t *testing.T) {
 	}
 }
 
+func TestXattr(t *testing.T) {
+	tfs, _ := setup(t)
+	if err := tfs.Mkdir("/test", 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	xattrSize := func() int { return 20 + rand.Intn(20) }
+
+	// Create a set of random attributes that we will set and read back
+	var attrs []protocol.Xattr
+	for i := 0; i < 10; i++ {
+		key := fmt.Sprintf("user.test-%d", i)
+		value := make([]byte, xattrSize())
+		rand.Read(value)
+		attrs = append(attrs, protocol.Xattr{
+			Name:  key,
+			Value: value,
+		})
+	}
+
+	// Set the xattrs, read them back and compare
+	if err := tfs.SetXattr("/test", attrs, noopXattrFilter{}); errors.Is(err, ErrXattrsNotSupported) {
+		t.Skip("xattrs not supported")
+	} else if err != nil {
+		t.Fatal(err)
+	}
+	res, err := tfs.GetXattr("/test", noopXattrFilter{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res) != len(attrs) {
+		t.Fatalf("length of returned xattrs does not match (%d != %d)", len(res), len(attrs))
+	}
+	for i, xa := range res {
+		if xa.Name != attrs[i].Name {
+			t.Errorf("xattr name %q != %q", xa.Name, attrs[i].Name)
+		}
+		if !bytes.Equal(xa.Value, attrs[i].Value) {
+			t.Errorf("xattr value %q != %q", xa.Value, attrs[i].Value)
+		}
+	}
+
+	// Remove a couple, change a couple, and add another couple of
+	// attributes. Replacing the xattrs again should work.
+	attrs = attrs[2:]
+	attrs[1].Value = make([]byte, xattrSize())
+	rand.Read(attrs[1].Value)
+	attrs[3].Value = make([]byte, xattrSize())
+	rand.Read(attrs[3].Value)
+	for i := 10; i < 12; i++ {
+		key := fmt.Sprintf("user.test-%d", i)
+		value := make([]byte, xattrSize())
+		rand.Read(value)
+		attrs = append(attrs, protocol.Xattr{
+			Name:  key,
+			Value: value,
+		})
+	}
+	sort.Slice(attrs, func(i, j int) bool { return attrs[i].Name < attrs[j].Name })
+
+	// Set the xattrs, read them back and compare
+	if err := tfs.SetXattr("/test", attrs, noopXattrFilter{}); err != nil {
+		t.Fatal(err)
+	}
+	res, err = tfs.GetXattr("/test", noopXattrFilter{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res) != len(attrs) {
+		t.Fatalf("length of returned xattrs does not match (%d != %d)", len(res), len(attrs))
+	}
+	for i, xa := range res {
+		if xa.Name != attrs[i].Name {
+			t.Errorf("xattr name %q != %q", xa.Name, attrs[i].Name)
+		}
+		if !bytes.Equal(xa.Value, attrs[i].Value) {
+			t.Errorf("xattr value %q != %q", xa.Value, attrs[i].Value)
+		}
+	}
+}
+
 func TestBasicWalkSkipSymlink(t *testing.T) {
 	_, dir := setup(t)
 	testWalkSkipSymlink(t, FilesystemTypeBasic, dir)
@@ -579,3 +664,9 @@ func TestWalkInfiniteRecursion(t *testing.T) {
 	_, dir := setup(t)
 	testWalkInfiniteRecursion(t, FilesystemTypeBasic, dir)
 }
+
+type noopXattrFilter struct{}
+
+func (noopXattrFilter) Permit(string) bool         { return true }
+func (noopXattrFilter) GetMaxSingleEntrySize() int { return 0 }
+func (noopXattrFilter) GetMaxTotalSize() int       { return 0 }