container/ops: implement overlay op

There are significant limitations to using the overlay mount, and the implementation in the kernel is quite quirky. For now the Op is quite robust, however a higher level interface for it has not been decided yet.

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

container/path_test.go

@@ -0,0 +1,42 @@
+package container
+
+import "testing"
+
+func TestToSysroot(t *testing.T) {
+	testCases := []struct {
+		name string
+		want string
+	}{
+		{"", "/sysroot"},
+		{"/", "/sysroot"},
+		{"//etc///", "/sysroot/etc"},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := toSysroot(tc.name); got != tc.want {
+				t.Errorf("toSysroot: %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestToHost(t *testing.T) {
+	testCases := []struct {
+		name string
+		want string
+	}{
+		{"", "/host"},
+		{"/", "/host"},
+		{"//etc///", "/host/etc"},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := toHost(tc.name); got != tc.want {
+				t.Errorf("toHost: %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+// InternalToHostOvlEscape exports toHost passed to escapeOverlayDataSegment.
+func InternalToHostOvlEscape(s string) string { return escapeOverlayDataSegment(toHost(s)) }