runner: initial automatic update (#2141)

src/runner/update_state.h

@@ -2,11 +2,16 @@
 
 #include <ctime>
 #include <optional>
+#include <functional>
 
+// All fields must be default-initialized
 struct UpdateState
 {
     std::optional<std::time_t> github_update_last_checked_date;
+    bool pending_update = false;
 
-    static UpdateState load();
-    void save();
+    // To prevent concurrent modification of the file, we enforce this interface, which locks the file while
+    // the state_modifier is active.
+    static void store(std::function<void(UpdateState&)> state_modifier);
+    static UpdateState read();
 };