cmd/app: alternative merged template layout

This implements an optional, alternative template layout to accommodate
ZFS and equivalent setups where each template occupies a different
filesystem. In such a setup, the default, separate workdir is unusable
as overlayfs requires the workdir to reside on the same filesystem as
the writable upperdir. The "merge" build tag enables this alternative
behaviour.

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

1 files changed, 22 insertions, 4 deletions

diff --git a/cmd/app/app.go b/cmd/app/app.go
index 9679413f..ca809334 100644
--- a/cmd/app/app.go
+++ b/cmd/app/app.go
@@ -14,6 +14,24 @@ import (
 	"hakurei.app/hst"
 )
 
+// base is the root of the persistent state directory.
+type base check.Absolute
+
+// append calls filepath.Join with b as the first element.
+func (b *base) append(elem ...string) *check.Absolute {
+	return (*check.Absolute)(b).Append(elem...)
+}
+
+// initial returns the pathname of the bottom layer.
+func (b *base) initial() *check.Absolute {
+	return b.append("initial")
+}
+
+// state returns the pathname of the home directory for id.
+func (b *base) state(id string) *check.Absolute {
+	return b.append("state", id)
+}
+
 // parsePair parses a NUL-delimited quoted paths pair.
 func parsePair(s string) (source, target *check.Absolute, err error) {
 	var p string
@@ -35,7 +53,7 @@ func parsePair(s string) (source, target *check.Absolute, err error) {
 // corresponding [hst.Config].
 func parse(
 	id string,
-	base *check.Absolute,
+	base *base,
 	r io.Reader,
 	templateP *string,
 ) (*hst.Config, error) {
@@ -44,7 +62,7 @@ func parse(
 
 	root := hst.FSOverlay{
 		Target: fhs.AbsRoot,
-		Lower:  []*check.Absolute{base.Append("initial")},
+		Lower:  []*check.Absolute{base.initial()},
 	}
 	c := hst.Config{
 		ID:          id,
@@ -65,7 +83,7 @@ func parse(
 				{FilesystemConfig: &root},
 				{FilesystemConfig: &hst.FSBind{
 					Target: home,
-					Source: base.Append("state", id),
+					Source: base.state(id),
 					Write:  true,
 					Ensure: true,
 				}},
@@ -110,7 +128,7 @@ func parse(
 			*templateP = template
 		}
 		c.Identity = v
-		root.Upper = base.Append("template", template)
+		root.Upper = base.template(template)
 	}
 
 	if err := scanOnce(); err != nil {