engine: resources: Allow symbolic modes for missing files

In 83a747794e a bug was introduced with
the implementation of symbolic modes, that would prevent a file resource
from passing the Validate step if you were using a symbolic mode, and
the file didn't already exist. If you didn't use symbolic modes and
those files weren't absent, then you wouldn't have noticed.

It might be worth looking into the API for symbolic parsing as well.

engine/resources/file.go

@@ -239,18 +239,19 @@ func (obj *FileRes) isDir() bool {
 // the case where the mode is not specified. The caller should check obj.Mode is
 // not empty.
 func (obj *FileRes) mode() (os.FileMode, error) {
+	// First check if this is an octal number.
 	if n, err := strconv.ParseInt(obj.Mode, 8, 32); err == nil {
 		return os.FileMode(n), nil
 	}
 
 	// Try parsing symbolically by first getting the files current mode.
-	stat, err := os.Stat(obj.getPath())
-	if err != nil {
-		return os.FileMode(0), errwrap.Wrapf(err, "failed to get the current file mode")
+	from := os.FileMode(0) // default
+	if stat, err := os.Stat(obj.getPath()); err == nil {
+		from = stat.Mode()
 	}
 
 	modes := strings.Split(obj.Mode, ",")
-	m, err := engineUtil.ParseSymbolicModes(modes, stat.Mode(), FileModeAllowAssign)
+	m, err := engineUtil.ParseSymbolicModes(modes, from, FileModeAllowAssign)
 	if err != nil {
 		return os.FileMode(0), errwrap.Wrapf(err, "mode should be an octal number or symbolic mode (%s)", obj.Mode)
 	}