cmd/app: enforce mutable instance exclusion

This avoids invoking undefined behaviour in the underlying overlay filesystem implementation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/app/app.go

@@ -31,7 +31,12 @@ func parsePair(s string) (source, target *check.Absolute, err error) {
 
 // parse decodes a high-level configuration stream and returns its
 // corresponding [hst.Config].
-func parse(id string, base *check.Absolute, r io.Reader) (*hst.Config, error) {
+func parse(
+	id string,
+	base *check.Absolute,
+	r io.Reader,
+	templateP *string,
+) (*hst.Config, error) {
 	shell := fhs.AbsRoot.Append("bin", "zsh")
 	home := hst.AbsPrivateTmp.Append("home")
 
@@ -99,6 +104,9 @@ func parse(id string, base *check.Absolute, r io.Reader) (*hst.Config, error) {
 	} else if v, err := strconv.Atoi(identity); err != nil {
 		return nil, err
 	} else {
+		if templateP != nil {
+			*templateP = template
+		}
 		c.Identity = v
 		root.Upper = base.Append("template", template)
 	}