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/testdata/main.go

@@ -39,11 +39,19 @@ func main() {
 	if !slices.Equal(wantEnv, os.Environ()) {
 		log.Fatalf("Environ: %q, want %q", os.Environ(), wantEnv)
 	}
-	const wantExec = "/opt/bin/testtool"
+
+	var overlayTmp bool
+	const (
+		wantExec        = "/opt/bin/testtool"
+		wantExecOverlay = "/tmp/bin/testtool"
+	)
 	if got, err := os.Executable(); err != nil {
 		log.Fatalf("Executable: error = %v", err)
 	} else if got != wantExec {
-		log.Fatalf("Executable: %q, want %q", got, wantExec)
+		if got != wantExecOverlay {
+			log.Fatalf("Executable: %q, want %q", got, wantExec)
+		}
+		overlayTmp = true
 	}
 
 	wantHostname := "cure"
@@ -105,7 +113,6 @@ func main() {
 		if path.Base(lowerdir) != checksumEmptyDir {
 			log.Fatal("unexpected artifact checksum")
 		}
-
 	} else {
 		if hostNet {
 			ident = "I3T53NtN6HPAyyodHtq2B0clcsoS1nPdvCEb-Zc5K-hoqFGL2od1mftHhwG7gX1S"
@@ -126,7 +133,16 @@ func main() {
 		}
 	}
 
-	next() // testtool artifact
+	if !overlayTmp {
+		next() // testtool artifact
+	} else {
+		ident = "3ELJ8l42g6XeIoLGR9LheVhMIwSIleD6VrhsliBuon5DAdBOwFSMqd7aiUI4fll7"
+
+		next()
+		if path.Base(m.Root) != checksumEmptyDir || m.Target != "/tmp" {
+			log.Fatal("unexpected artifact checksum")
+		}
+	}
 
 	next()
 	if path.Base(m.Root) != ident || m.Target != "/work" {
@@ -134,8 +150,16 @@ func main() {
 	}
 
 	next()
-	if path.Base(m.Root) != ident || m.Target != "/tmp" {
-		log.Fatal("unexpected temp mount entry")
+	if !overlayTmp {
+		if path.Base(m.Root) != ident || m.Target != "/tmp" {
+			log.Fatal("unexpected temp mount entry")
+		}
+	} else {
+		// testtool artifact
+		if m.Root != "/" || m.Target != "/tmp" ||
+			m.Source != "overlay" || m.FsType != "overlay" {
+			log.Fatal("unexpected temp mount entry")
+		}
 	}
 
 	next()