hst: support ephemeral overlay mounts

This is useful for reusing a readonly template without autoroot.

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

check/absolute.go

@@ -31,6 +31,8 @@ func (e AbsoluteError) Is(target error) bool {
 type Absolute struct{ pathname unique.Handle[string] }
 
 var (
+	_ fmt.GoStringer = new(Absolute)
+
 	_ encoding.TextAppender    = new(Absolute)
 	_ encoding.TextMarshaler   = new(Absolute)
 	_ encoding.TextUnmarshaler = new(Absolute)
@@ -40,6 +42,10 @@ var (
 	_ encoding.BinaryUnmarshaler = new(Absolute)
 )
 
+func (a *Absolute) GoString() string {
+	return fmt.Sprintf("check.MustAbs(%q)", a.String())
+}
+
 // ok returns whether [Absolute] is not the zero value.
 func (a *Absolute) ok() bool { return a != nil && *a != (Absolute{}) }
 

hst/fs.go

@@ -37,6 +37,8 @@ type Ops interface {
 	Bind(source, target *check.Absolute, flags int) Ops
 	// Overlay appends an op that mounts the overlay pseudo filesystem.
 	Overlay(target, state, work *check.Absolute, layers ...*check.Absolute) Ops
+	// OverlayEphemeral appends a MountOverlayOp with an ephemeral upperdir and workdir.
+	OverlayEphemeral(target *check.Absolute, layers ...*check.Absolute) Ops
 	// OverlayReadonly appends an op that mounts the overlay pseudo filesystem readonly.
 	OverlayReadonly(target *check.Absolute, layers ...*check.Absolute) Ops
 

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...)}
 }

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 {
+	if o.Upper != nil {
-		p = append(p, o.Upper, o.Work)
+		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 {
+	if o.Upper != nil {
-		z.Overlay(o.Target, o.Upper, o.Work, o.Lower...)
 		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 {
+	if o.Upper != nil {
-		return "w*" + strings.Join(append([]string{
+		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.Upper.String())},
-			check.EscapeOverlayDataSegment(o.Work.String())},
 			lower...), check.SpecialOverlayPath)
+
 	} else {
 		return "*" + strings.Join(append([]string{
 			check.EscapeOverlayDataSegment(o.Target.String())},

hst/fsoverlay_test.go

@@ -5,6 +5,7 @@ import (
 
 	"hakurei.app/check"
 	"hakurei.app/container"
+	"hakurei.app/fhs"
 	"hakurei.app/hst"
 )
 
@@ -14,7 +15,7 @@ func TestFSOverlay(t *testing.T) {
 	checkFs(t, []fsTestCase{
 		{"nil", (*hst.FSOverlay)(nil), false, nil, nil, nil, "<invalid>"},
 		{"nil lower", &hst.FSOverlay{Target: m("/etc"), Lower: []*check.Absolute{nil}}, false, nil, nil, nil, "<invalid>"},
-		{"zero lower", &hst.FSOverlay{Target: m("/etc"), Upper: m("/"), Work: m("/")}, false, nil, nil, nil, "<invalid>"},
+		{"zero lower", &hst.FSOverlay{Target: m("/etc"), Work: m("/")}, false, nil, nil, nil, "<invalid>"},
 		{"zero lower ro", &hst.FSOverlay{Target: m("/etc")}, false, nil, nil, nil, "<invalid>"},
 		{"short lower", &hst.FSOverlay{Target: m("/etc"), Lower: ms("/etc")}, false, nil, nil, nil, "<invalid>"},
 
@@ -62,5 +63,16 @@ func TestFSOverlay(t *testing.T) {
 			Work:   m("/tmp/work"),
 		}}, m("/"), ms("/tmp/upper", "/tmp/work", "/tmp/.src0", "/tmp/.src1"),
 			"w*/:/tmp/upper:/tmp/work:/tmp/.src0:/tmp/.src1"},
+
+		{"ephemeral", &hst.FSOverlay{
+			Target: m("/"),
+			Lower:  ms("/tmp/.src0", "/tmp/.src1"),
+			Upper:  m("/tmp/upper"),
+		}, true, container.Ops{&container.MountOverlayOp{
+			Target: m("/"),
+			Lower:  ms("/tmp/.src0", "/tmp/.src1", "/tmp/upper"),
+			Upper:  fhs.AbsRoot,
+		}}, m("/"), ms("/tmp/upper", "/tmp/.src0", "/tmp/.src1"),
+			"e*/:/tmp/upper:/tmp/.src0:/tmp/.src1"},
 	})
 }

internal/outcome/spcontainer.go

@@ -382,6 +382,10 @@ 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...)}
 }