internal/pkg: automatic overlay mount on tmp

This sets up the last Artifact to target /tmp as a writable overlay mount backed by the host side temp directory. This is useful for an Artifact containing source code to be built for another Artifact for example.

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

internal/pkg/exec.go

@@ -89,7 +89,9 @@ func (a *execNetArtifact) Cure(c *CureContext) error {
 // A private instance of /proc and /dev is made available to the container.
 //
 // The working and temporary directories are both created and mounted writable
-// on /work and /tmp respectively.
+// on /work and /tmp respectively. If one or more paths target [fhs.AbsTmp], the
+// final entry is set up as a writable overlay mount on /tmp backed by the host
+// side temporary directory.
 //
 // If the first path targets [fhs.AbsRoot], it is made writable via an overlay
 // mount with writes going to an ephemeral tmpfs bound to the lifetime of the
@@ -273,18 +275,46 @@ func (a *execArtifact) cure(c *CureContext, hostNet bool) (err error) {
 		z.OverlayEphemeral(fhs.AbsRoot, paths[0][0])
 		paths = paths[1:]
 	}
+	var overlayTemp *check.Absolute
 	for _, b := range paths {
+		if b[1].Is(fhs.AbsTmp) {
+			if overlayTemp != nil {
+				z.Bind(overlayTemp, fhs.AbsTmp, 0)
+			}
+			overlayTemp = b[0]
+			continue
+		}
 		z.Bind(b[0], b[1], 0)
 	}
 	z.Bind(
 		c.GetWorkDir(),
 		fhs.AbsRoot.Append("work"),
 		std.BindWritable|std.BindEnsure,
-	).Bind(
-		c.GetTempDir(),
-		fhs.AbsTmp,
-		std.BindWritable|std.BindEnsure,
-	).Proc(fhs.AbsProc).Dev(fhs.AbsDev, true)
+	)
+	if overlayTemp == nil {
+		z.Bind(
+			c.GetTempDir(),
+			fhs.AbsTmp,
+			std.BindWritable|std.BindEnsure,
+		)
+	} else {
+		temp := c.GetTempDir()
+		tempUpper := temp.Append("upper")
+		if err = os.MkdirAll(tempUpper.String(), 0700); err != nil {
+			return
+		}
+		tempWork := temp.Append("work")
+		if err = os.MkdirAll(tempWork.String(), 0700); err != nil {
+			return
+		}
+		z.Overlay(
+			fhs.AbsTmp,
+			tempUpper,
+			tempWork,
+			overlayTemp,
+		)
+	}
+	z.Proc(fhs.AbsProc).Dev(fhs.AbsDev, true)
 
 	if err = z.Start(); err != nil {
 		return