hst: support ephemeral overlay mounts

This is useful for reusing a readonly template without autoroot.

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

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"},
 	})
 }