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/exec_test.go

@@ -26,6 +26,10 @@ var testtoolBin []byte
 func TestExec(t *testing.T) {
 	t.Parallel()
 
+	wantChecksumOffline := pkg.MustDecode(
+		"GPa4aBakdSJd7Tz7LYj_VJFoojzyZinmVcG3k6M5xI6CZ821J5sXLhLDDuS47gi9",
+	)
+
 	checkWithCache(t, []cacheTestCase{
 		{"offline", nil, func(t *testing.T, base *check.Absolute, c *pkg.Cache) {
 			c.SetStrict(true)
@@ -59,9 +63,7 @@ func TestExec(t *testing.T) {
 						},
 					}),
 					pkg.MustPath("/opt", testtool),
-				), ignorePathname, pkg.MustDecode(
-					"GPa4aBakdSJd7Tz7LYj_VJFoojzyZinmVcG3k6M5xI6CZ821J5sXLhLDDuS47gi9",
-				), nil},
+				), ignorePathname, wantChecksumOffline, nil},
 
 				{"error passthrough", pkg.NewExec(
 					t.Context(),
@@ -155,6 +157,38 @@ func TestExec(t *testing.T) {
 
 			testtoolDestroy(t, base, c)
 		}, pkg.MustDecode("bBQVFIt0FnOulljgpLnGtuzHSFgwiCMjc4pmc4rHRqXKQ60Q5aBVYp5f6aH9VdZi")},
+
+		{"overlay root", nil, func(t *testing.T, base *check.Absolute, c *pkg.Cache) {
+			c.SetStrict(true)
+			testtool, testtoolDestroy := newTesttool()
+
+			msg := message.New(log.New(os.Stderr, "container: ", 0))
+			msg.SwapVerbose(testing.Verbose())
+
+			cureMany(t, c, []cureStep{
+				{"container", pkg.NewExec(
+					t.Context(),
+					msg,
+					0,
+					nil,
+					check.MustAbs("/work"),
+					[]string{"HAKUREI_TEST=1", "HAKUREI_ROOT=1"},
+					check.MustAbs("/opt/bin/testtool"),
+					[]string{"testtool"},
+
+					pkg.MustPath("/", stubArtifact{
+						kind:   pkg.KindTar,
+						params: []byte("empty directory"),
+						cure: func(c *pkg.CureContext) error {
+							return os.MkdirAll(c.GetWorkDir().String(), 0700)
+						},
+					}),
+					pkg.MustPath("/opt", testtool),
+				), ignorePathname, wantChecksumOffline, nil},
+			})
+
+			testtoolDestroy(t, base, c)
+		}, pkg.MustDecode("gFT9kprYBqEJKifJIl2sHn_3TgULWVLTU4DrYAHiGcRmcdFRZ0YtjiROW820cAEc")},
 	})
 }