lib/model, lib/scanner: Properly ignore symlinks on Windows (fixes #4035)

Adds a unit test to ensure we don't scan symlinks on Windows. For the
rwfolder, trusts that the logic in the invalid check is correct and that
the check is actually called from the need loop.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4042

lib/model/rwfolder.go

@@ -53,8 +53,9 @@ type copyBlocksState struct {
 const retainBits = os.ModeSetgid | os.ModeSetuid | os.ModeSticky
 
 var (
-	activity    = newDeviceActivity()
-	errNoDevice = errors.New("peers who had this file went away, or the file has changed while syncing. will retry later")
+	activity               = newDeviceActivity()
+	errNoDevice            = errors.New("peers who had this file went away, or the file has changed while syncing. will retry later")
+	errSymlinksUnsupported = errors.New("symlinks not supported")
 )
 
 const (
@@ -1759,6 +1760,9 @@ func fileValid(file db.FileIntf) error {
 		// We don't care about file validity if we're not supposed to have it
 		return nil
 
+	case runtime.GOOS == "windows" && file.IsSymlink():
+		return errSymlinksUnsupported
+
 	case runtime.GOOS == "windows" && windowsInvalidFilename(file.FileName()):
 		return errInvalidFilename
 	}