internal/pkg: tar optional file

This allows tar to take a single-file directory Artifact as input.

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

internal/pkg/pkg_test.go

@@ -117,6 +117,59 @@ func newStubFile(
 	}
 }
 
+// destroyArtifact removes all traces of an [Artifact] from the on-disk cache.
+// Do not use this in a test case without a very good reason to do so.
+func destroyArtifact(
+	t *testing.T,
+	base *check.Absolute,
+	c *pkg.Cache,
+	a pkg.Artifact,
+) {
+	if pathname, checksum, err := c.Cure(a); err != nil {
+		t.Fatalf("Cure: error = %v", err)
+	} else if err = os.Remove(pathname.String()); err != nil {
+		t.Fatal(err)
+	} else {
+		p := base.Append(
+			"checksum",
+			pkg.Encode(checksum),
+		)
+		if err = filepath.WalkDir(p.String(), func(
+			path string,
+			d fs.DirEntry,
+			err error,
+		) error {
+			if err != nil {
+				return err
+			}
+			if d.IsDir() {
+				return os.Chmod(path, 0700)
+			}
+			return nil
+		}); err != nil && !errors.Is(err, os.ErrNotExist) {
+			t.Fatal(err)
+		}
+		if err = os.RemoveAll(p.String()); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+// newDestroyArtifactFunc returns a function that calls destroyArtifact.
+func newDestroyArtifactFunc(a pkg.Artifact) func(
+	t *testing.T,
+	base *check.Absolute,
+	c *pkg.Cache,
+) {
+	return func(
+		t *testing.T,
+		base *check.Absolute,
+		c *pkg.Cache,
+	) {
+		destroyArtifact(t, base, c, a)
+	}
+}
+
 func TestIdent(t *testing.T) {
 	t.Parallel()