internal/pkg: automatic overlay mount on root

This makes it possible to use an Artifact as root without arranging for directory creation in the Artifact ahead of time.

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

internal/pkg/testdata/main.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"path"
 	"slices"
+	"strings"
 	"syscall"
 
 	"hakurei.app/container/fhs"
@@ -28,9 +29,14 @@ func main() {
 	if !slices.Equal(os.Args, wantArgs) {
 		log.Fatalf("Args: %q, want %q", os.Args, wantArgs)
 	}
-	if wantEnv := []string{
-		"HAKUREI_TEST=1",
-	}; !slices.Equal(wantEnv, os.Environ()) {
+
+	var overlayRoot bool
+	wantEnv := []string{"HAKUREI_TEST=1"}
+	if len(os.Environ()) == 2 {
+		overlayRoot = true
+		wantEnv = []string{"HAKUREI_TEST=1", "HAKUREI_ROOT=1"}
+	}
+	if !slices.Equal(wantEnv, os.Environ()) {
 		log.Fatalf("Environ: %q, want %q", os.Environ(), wantEnv)
 	}
 	const wantExec = "/opt/bin/testtool"
@@ -51,11 +57,6 @@ func main() {
 		log.Fatalf("Hostname: %q, want %q", hostname, wantHostname)
 	}
 
-	ident := "oHuqV7p0v1Vd8IdAzjyYM8sfCS0P2LR5tfv5cb6Gbf2ZWUm8Ec-7hYPJ_qr183m7"
-	if hostNet {
-		ident = "I3T53NtN6HPAyyodHtq2B0clcsoS1nPdvCEb-Zc5K-hoqFGL2od1mftHhwG7gX1S"
-	}
-
 	var m *vfs.MountInfo
 	if f, err := os.Open(fhs.Proc + "self/mountinfo"); err != nil {
 		log.Fatalf("Open: error = %v", err)
@@ -82,20 +83,47 @@ func main() {
 		}
 	}
 
+	const checksumEmptyDir = "MGWmEfjut2QE2xPJwTsmUzpff4BN_FEnQ7T0j7gvUCCiugJQNwqt9m151fm9D1yU"
+	ident := "oHuqV7p0v1Vd8IdAzjyYM8sfCS0P2LR5tfv5cb6Gbf2ZWUm8Ec-7hYPJ_qr183m7"
 	log.Println(m)
-	if m.Root != "/sysroot" || m.Target != "/" {
-		log.Fatal("unexpected root mount entry")
-	}
 	next := func() { m = m.Next; log.Println(m) }
 
-	next()
-	if path.Base(m.Root) != "OLBgp1GsljhM2TJ-sbHjaiH9txEUvgdDTAzHv2P24donTt6_529l-9Ua0vFImLlb" {
-		log.Fatal("unexpected file artifact checksum")
-	}
+	if overlayRoot {
+		ident = "cIjP14zs5el6W_BQhufL_c0vWg-V6Z6pDpsbEa3sYtZ1381u1bKnH3N16RIrw-1S"
 
-	next()
-	if path.Base(m.Root) != "MGWmEfjut2QE2xPJwTsmUzpff4BN_FEnQ7T0j7gvUCCiugJQNwqt9m151fm9D1yU" {
-		log.Fatal("unexpected artifact checksum")
+		if m.Root != "/" || m.Target != "/" ||
+			m.Source != "overlay" || m.FsType != "overlay" {
+			log.Fatal("unexpected root mount entry")
+		}
+		var lowerdir string
+		for _, o := range strings.Split(m.FsOptstr, ",") {
+			const lowerdirKey = "lowerdir="
+			if strings.HasPrefix(o, lowerdirKey) {
+				lowerdir = o[len(lowerdirKey):]
+			}
+		}
+		if path.Base(lowerdir) != checksumEmptyDir {
+			log.Fatal("unexpected artifact checksum")
+		}
+
+	} else {
+		if hostNet {
+			ident = "I3T53NtN6HPAyyodHtq2B0clcsoS1nPdvCEb-Zc5K-hoqFGL2od1mftHhwG7gX1S"
+		}
+
+		if m.Root != "/sysroot" || m.Target != "/" {
+			log.Fatal("unexpected root mount entry")
+		}
+
+		next()
+		if path.Base(m.Root) != "OLBgp1GsljhM2TJ-sbHjaiH9txEUvgdDTAzHv2P24donTt6_529l-9Ua0vFImLlb" {
+			log.Fatal("unexpected file artifact checksum")
+		}
+
+		next()
+		if path.Base(m.Root) != checksumEmptyDir {
+			log.Fatal("unexpected artifact checksum")
+		}
 	}
 
 	next() // testtool artifact