all: Support syncing ownership (fixes #1329) (#8434)

This adds support for syncing ownership on Unixes and on Windows. The
scanner always picks up ownership information, but it is not applied
unless the new folder option "Sync Ownership" is set.

Ownership data is stored in a new FileInfo field called "platform data". This
is intended to hold further platform-specific data in the future
(specifically, extended attributes), which is why the whole design is a
bit overkill for just ownership.

lib/fs/basicfs_windows.go

@@ -17,6 +17,8 @@ import (
 	"strings"
 	"syscall"
 	"unsafe"
+
+	"golang.org/x/sys/windows"
 )
 
 var errNotSupported = errors.New("symlinks not supported")
@@ -152,6 +154,42 @@ func (f *BasicFilesystem) Roots() ([]string, error) {
 	return drives, nil
 }
 
+func (f *BasicFilesystem) Lchown(name, uid, gid string) error {
+	name, err := f.rooted(name)
+	if err != nil {
+		return err
+	}
+
+	hdl, err := windows.Open(name, windows.O_WRONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer windows.Close(hdl)
+
+	// Depending on whether we got an uid or a gid, we need to set the
+	// appropriate flag and parse the corresponding SID. The one we're not
+	// setting remains nil, which is what we want in the call to
+	// SetSecurityInfo.
+
+	var si windows.SECURITY_INFORMATION
+	var ownerSID, groupSID *syscall.SID
+	if uid != "" {
+		ownerSID, err = syscall.StringToSid(uid)
+		if err == nil {
+			si |= windows.OWNER_SECURITY_INFORMATION
+		}
+	} else if gid != "" {
+		groupSID, err = syscall.StringToSid(uid)
+		if err == nil {
+			si |= windows.GROUP_SECURITY_INFORMATION
+		}
+	} else {
+		return errors.New("neither uid nor gid specified")
+	}
+
+	return windows.SetSecurityInfo(hdl, windows.SE_FILE_OBJECT, si, (*windows.SID)(ownerSID), (*windows.SID)(groupSID), nil, nil)
+}
+
 // unrootedChecked returns the path relative to the folder root (same as
 // unrooted) or an error if the given path is not a subpath and handles the
 // special case when the given path is the folder root without a trailing