hst/fs: implement overlay fstype

This finally exposes overlay mounts in the high level hakurei API.

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

hst/fsoverlay.go

@@ -0,0 +1,98 @@
+package hst
+
+import (
+	"encoding/gob"
+	"strings"
+
+	"hakurei.app/container"
+)
+
+func init() { gob.Register(new(FSOverlay)) }
+
+// FilesystemOverlay is the [FilesystemConfig.Type] name of an overlay mount point.
+const FilesystemOverlay = "overlay"
+
+// FSOverlay represents an overlay mount point.
+type FSOverlay struct {
+	// mount point in container
+	Dst *container.Absolute `json:"dst"`
+
+	// any filesystem, does not need to be on a writable filesystem, must not be nil
+	Lower []*container.Absolute `json:"lower"`
+	// the upperdir is normally on a writable filesystem, leave as nil to mount Lower readonly
+	Upper *container.Absolute `json:"upper,omitempty"`
+	// the workdir needs to be an empty directory on the same filesystem as Upper, must not be nil if Upper is populated
+	Work *container.Absolute `json:"work,omitempty"`
+}
+
+func (o *FSOverlay) Valid() bool {
+	if o == nil || o.Dst == nil {
+		return false
+	}
+
+	for _, a := range o.Lower {
+		if a == nil {
+			return false
+		}
+	}
+
+	if o.Upper != nil { // rw
+		return o.Work != nil && len(o.Lower) > 0
+	} else { // ro
+		return len(o.Lower) >= 2
+	}
+}
+
+func (o *FSOverlay) Target() *container.Absolute {
+	if !o.Valid() {
+		return nil
+	}
+	return o.Dst
+}
+
+func (o *FSOverlay) Host() []*container.Absolute {
+	if !o.Valid() {
+		return nil
+	}
+	p := make([]*container.Absolute, 0, 2+len(o.Lower))
+	if o.Upper != nil && o.Work != nil {
+		p = append(p, o.Upper, o.Work)
+	}
+	p = append(p, o.Lower...)
+	return p
+}
+
+func (o *FSOverlay) Apply(op *container.Ops) {
+	if !o.Valid() {
+		return
+	}
+
+	if o.Upper != nil && o.Work != nil { // rw
+		op.Overlay(o.Dst, o.Upper, o.Work, o.Lower...)
+	} else { // ro
+		op.OverlayReadonly(o.Dst, o.Lower...)
+	}
+}
+
+func (o *FSOverlay) String() string {
+	if !o.Valid() {
+		return "<invalid>"
+	}
+
+	lower := make([]string, len(o.Lower))
+	for i, a := range o.Lower {
+		lower[i] = container.EscapeOverlayDataSegment(a.String())
+	}
+
+	if o.Upper != nil && o.Work != nil {
+		return "w*" + strings.Join(append([]string{
+			container.EscapeOverlayDataSegment(o.Dst.String()),
+			container.EscapeOverlayDataSegment(o.Upper.String()),
+			container.EscapeOverlayDataSegment(o.Work.String())},
+			lower...), container.SpecialOverlayPath)
+	} else {
+		return "*" + strings.Join(append([]string{
+			container.EscapeOverlayDataSegment(o.Dst.String())},
+			lower...), container.SpecialOverlayPath)
+	}
+}