hst: support ephemeral overlay mounts

This is useful for reusing a readonly template without autoroot.

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

hst/fs_test.go

@@ -3,6 +3,7 @@ package hst_test
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"os"
 	"reflect"
 	"strings"
@@ -283,11 +284,11 @@ func checkFs(t *testing.T, testCases []fsTestCase) {
 				if !reflect.DeepEqual(ops, &tc.ops) {
 					gotString := new(strings.Builder)
 					for _, op := range *ops {
-						gotString.WriteString("\n" + op.String())
+						gotString.WriteString("\n" + fmt.Sprintf("%#v", op))
 					}
 					wantString := new(strings.Builder)
 					for _, op := range tc.ops {
-						wantString.WriteString("\n" + op.String())
+						wantString.WriteString("\n" + fmt.Sprintf("%#v", op))
 					}
 					t.Errorf("Apply: %s, want %s", gotString, wantString)
 				}
@@ -339,6 +340,10 @@ func (p opsAdapter) Overlay(target, state, work *check.Absolute, layers ...*chec
 	return opsAdapter{p.Ops.Overlay(target, state, work, layers...)}
 }
 
+func (p opsAdapter) OverlayEphemeral(target *check.Absolute, layers ...*check.Absolute) hst.Ops {
+	return opsAdapter{p.Ops.OverlayEphemeral(target, layers...)}
+}
+
 func (p opsAdapter) OverlayReadonly(target *check.Absolute, layers ...*check.Absolute) hst.Ops {
 	return opsAdapter{p.Ops.OverlayReadonly(target, layers...)}
 }