diff --git a/postinstall_runner_action.h b/postinstall_runner_action.h
index 2308a0f..0136ee2 100644
--- a/postinstall_runner_action.h
+++ b/postinstall_runner_action.h
@@ -17,7 +17,9 @@
 
 class PostinstallRunnerAction : public InstallPlanAction {
  public:
-  PostinstallRunnerAction(): powerwash_marker_created_(false) {}
+  PostinstallRunnerAction()
+      : powerwash_marker_created_(false),
+        powerwash_marker_file_(NULL) {}
 
   void PerformAction();
 
@@ -42,6 +44,17 @@
   // False otherwise. Used for cleaning up if post-install fails.
   bool powerwash_marker_created_;
 
+  // Non-NULL value will cause post-install to override the default marker file
+  // name; used for testing.
+  const char* powerwash_marker_file_;
+
+  // Special ctor + friend declaration for testing purposes.
+  PostinstallRunnerAction(const char* powerwash_marker_file)
+      : powerwash_marker_created_(false),
+        powerwash_marker_file_(powerwash_marker_file) {}
+
+  friend class PostinstallRunnerActionTest;
+
   DISALLOW_COPY_AND_ASSIGN(PostinstallRunnerAction);
 };