1 files changed, 175 insertions, 0 deletions
diff --git a/container/initoverlay.go b/container/initoverlay.go
new file mode 100644
index 00000000..718b965b
--- /dev/null
+++ b/container/initoverlay.go
@@ -0,0 +1,175 @@
+package container
+
+import (
+	"encoding/gob"
+	"fmt"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	. "syscall"
+)
+
+const (
+	// intermediate root file name pattern for [MountOverlayOp.Upper];
+	// remains after apply returns
+	intermediatePatternOverlayUpper = "overlay.upper.*"
+	// intermediate root file name pattern for [MountOverlayOp.Work];
+	// remains after apply returns
+	intermediatePatternOverlayWork = "overlay.work.*"
+)
+
+func init() { gob.Register(new(MountOverlayOp)) }
+
+// Overlay appends an [Op] that mounts the overlay pseudo filesystem on [MountOverlayOp.Target].
+func (f *Ops) Overlay(target, state, work *Absolute, layers ...*Absolute) *Ops {
+	*f = append(*f, &MountOverlayOp{
+		Target: target,
+		Lower:  layers,
+		Upper:  state,
+		Work:   work,
+	})
+	return f
+}
+
+// OverlayEphemeral appends an [Op] that mounts the overlay pseudo filesystem on [MountOverlayOp.Target]
+// with an ephemeral upperdir and workdir.
+func (f *Ops) OverlayEphemeral(target *Absolute, layers ...*Absolute) *Ops {
+	return f.Overlay(target, AbsFHSRoot, nil, layers...)
+}
+
+// OverlayReadonly appends an [Op] that mounts the overlay pseudo filesystem readonly on [MountOverlayOp.Target]
+func (f *Ops) OverlayReadonly(target *Absolute, layers ...*Absolute) *Ops {
+	return f.Overlay(target, nil, nil, layers...)
+}
+
+// MountOverlayOp mounts [FstypeOverlay] on container path Target.
+type MountOverlayOp struct {
+	Target *Absolute
+
+	// Any filesystem, does not need to be on a writable filesystem.
+	Lower []*Absolute
+	// formatted for [OptionOverlayLowerdir], resolved, prefixed and escaped during early
+	lower []string
+	// The upperdir is normally on a writable filesystem.
+	//
+	// If Work is nil and Upper holds the special value [FHSRoot],
+	// an ephemeral upperdir and workdir will be set up.
+	//
+	// If both Work and Upper are empty strings, upperdir and workdir is omitted and the overlay is mounted readonly.
+	Upper *Absolute
+	// formatted for [OptionOverlayUpperdir], resolved, prefixed and escaped during early
+	upper string
+	// The workdir needs to be an empty directory on the same filesystem as upperdir.
+	Work *Absolute
+	// formatted for [OptionOverlayWorkdir], resolved, prefixed and escaped during early
+	work string
+
+	ephemeral bool
+}
+
+func (o *MountOverlayOp) early(*setupState) error {
+	if o.Work == nil && o.Upper != nil {
+		switch o.Upper.String() {
+		case FHSRoot: // ephemeral
+			o.ephemeral = true // intermediate root not yet available
+
+		default:
+			return msg.WrapErr(EINVAL, fmt.Sprintf("upperdir has unexpected value %q", o.Upper))
+		}
+	}
+	// readonly handled in apply
+
+	if !o.ephemeral {
+		if o.Upper != o.Work && (o.Upper == nil || o.Work == nil) {
+			// unreachable
+			return msg.WrapErr(ENOTRECOVERABLE, "impossible overlay state reached")
+		}
+
+		if o.Upper != nil {
+			if v, err := filepath.EvalSymlinks(o.Upper.String()); err != nil {
+				return wrapErrSelf(err)
+			} else {
+				o.upper = EscapeOverlayDataSegment(toHost(v))
+			}
+		}
+
+		if o.Work != nil {
+			if v, err := filepath.EvalSymlinks(o.Work.String()); err != nil {
+				return wrapErrSelf(err)
+			} else {
+				o.work = EscapeOverlayDataSegment(toHost(v))
+			}
+		}
+	}
+
+	o.lower = make([]string, len(o.Lower))
+	for i, a := range o.Lower {
+		if a == nil {
+			return EBADE
+		}
+
+		if v, err := filepath.EvalSymlinks(a.String()); err != nil {
+			return wrapErrSelf(err)
+		} else {
+			o.lower[i] = EscapeOverlayDataSegment(toHost(v))
+		}
+	}
+	return nil
+}
+
+func (o *MountOverlayOp) apply(state *setupState) error {
+	if o.Target == nil {
+		return EBADE
+	}
+	target := toSysroot(o.Target.String())
+	if err := os.MkdirAll(target, state.ParentPerm); err != nil {
+		return wrapErrSelf(err)
+	}
+
+	if o.ephemeral {
+		var err error
+		// these directories are created internally, therefore early (absolute, symlink, prefix, escape) is bypassed
+		if o.upper, err = os.MkdirTemp(FHSRoot, intermediatePatternOverlayUpper); err != nil {
+			return wrapErrSelf(err)
+		}
+		if o.work, err = os.MkdirTemp(FHSRoot, intermediatePatternOverlayWork); err != nil {
+			return wrapErrSelf(err)
+		}
+	}
+
+	options := make([]string, 0, 4)
+
+	if o.upper == zeroString && o.work == zeroString { // readonly
+		if len(o.Lower) < 2 {
+			return msg.WrapErr(EINVAL, "readonly overlay requires at least two lowerdir")
+		}
+		// "upperdir=" and "workdir=" may be omitted. In that case the overlay will be read-only
+	} else {
+		if len(o.Lower) == 0 {
+			return msg.WrapErr(EINVAL, "overlay requires at least one lowerdir")
+		}
+		options = append(options,
+			OptionOverlayUpperdir+"="+o.upper,
+			OptionOverlayWorkdir+"="+o.work)
+	}
+	options = append(options,
+		OptionOverlayLowerdir+"="+strings.Join(o.lower, SpecialOverlayPath),
+		OptionOverlayUserxattr)
+
+	return wrapErrSuffix(Mount(SourceOverlay, target, FstypeOverlay, 0, strings.Join(options, SpecialOverlayOption)),
+		fmt.Sprintf("cannot mount overlay on %q:", o.Target))
+}
+
+func (o *MountOverlayOp) Is(op Op) bool {
+	vo, ok := op.(*MountOverlayOp)
+	return ok &&
+		o.Target == vo.Target &&
+		slices.Equal(o.Lower, vo.Lower) &&
+		o.Upper == vo.Upper &&
+		o.Work == vo.Work
+}
+func (*MountOverlayOp) prefix() string { return "mounting" }
+func (o *MountOverlayOp) String() string {
+	return fmt.Sprintf("overlay on %q with %d layers", o.Target, len(o.Lower))
+}