hst: support ephemeral overlay mounts

This is useful for reusing a readonly template without autoroot.

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

hst/fsoverlay.go

@@ -2,6 +2,7 @@ package hst
 
 import (
 	"encoding/gob"
+	"slices"
 	"strings"
 
 	"hakurei.app/check"
@@ -40,7 +41,7 @@ func (o *FSOverlay) Valid() bool {
 	}
 
 	if o.Upper != nil { // rw
-		return o.Work != nil && len(o.Lower) > 0
+		return o.Work != nil || len(o.Lower) > 0
 	} else { // ro
 		return len(o.Lower) >= 2
 	}
@@ -58,8 +59,11 @@ func (o *FSOverlay) Host() []*check.Absolute {
 		return nil
 	}
 	p := make([]*check.Absolute, 0, 2+len(o.Lower))
-	if o.Upper != nil && o.Work != nil {
-		p = append(p, o.Upper, o.Work)
+	if o.Upper != nil {
+		p = append(p, o.Upper)
+		if o.Work != nil {
+			p = append(p, o.Work)
+		}
 	}
 	p = append(p, o.Lower...)
 	return p
@@ -70,11 +74,18 @@ func (o *FSOverlay) Apply(z *ApplyState) {
 		return
 	}
 
-	if o.Upper != nil && o.Work != nil {
-		z.Overlay(o.Target, o.Upper, o.Work, o.Lower...)
+	if o.Upper != nil {
 		if o.Target.Is(fhs.AbsRoot) {
 			z.NoRemountRoot = true
 		}
+		if o.Work != nil {
+			z.Overlay(o.Target, o.Upper, o.Work, o.Lower...)
+		} else {
+			z.OverlayEphemeral(o.Target, slices.Concat(
+				o.Lower,
+				[]*check.Absolute{o.Upper})...,
+			)
+		}
 	} else {
 		z.OverlayReadonly(o.Target, o.Lower...)
 	}
@@ -90,12 +101,19 @@ func (o *FSOverlay) String() string {
 		lower[i] = check.EscapeOverlayDataSegment(a.String())
 	}
 
-	if o.Upper != nil && o.Work != nil {
-		return "w*" + strings.Join(append([]string{
+	if o.Upper != nil {
+		if o.Work != nil {
+			return "w*" + strings.Join(append([]string{
+				check.EscapeOverlayDataSegment(o.Target.String()),
+				check.EscapeOverlayDataSegment(o.Upper.String()),
+				check.EscapeOverlayDataSegment(o.Work.String())},
+				lower...), check.SpecialOverlayPath)
+		}
+		return "e*" + strings.Join(append([]string{
 			check.EscapeOverlayDataSegment(o.Target.String()),
-			check.EscapeOverlayDataSegment(o.Upper.String()),
-			check.EscapeOverlayDataSegment(o.Work.String())},
+			check.EscapeOverlayDataSegment(o.Upper.String())},
 			lower...), check.SpecialOverlayPath)
+
 	} else {
 		return "*" + strings.Join(append([]string{
 			check.EscapeOverlayDataSegment(o.Target.String())},