Special case Lua copy-construction of trackable instances

This fixes an crashing issue with ArdourUI.SelectionList a bug
introduced in 6dc3bdf252c and 35dcd46d7d.

Since removal of the special cases in 35dcd46d7d, when using
a C-pointer in a std::list<>,
  std::list<class*>::push_back(TypeListValue)
TypeListValues<>'s Head was expanded to  "class*& const"
implied by void ::push_back(const T& value);

This resulted in lifetime issues with a classes that derive
from sigc::trackable (e.g. Ardour's Selection).
The reference leaves scope and isn't duplicated when it is pushed back
to the std::list<>.

The script scripts/select_every_2nd_region.lua crashed because entries
in the SelectionList were no longer valid.

Previously (before 6dc3bdf252c) TypeListValues explicitly
copy-constructed the value to work around the lifetime issue.
This new solution bypasses the issue by directly using the c-pointer
without dereferencing it.

2 files changed, 15 insertions, 1 deletions

diff --git a/libs/lua/LuaBridge/detail/CFunctions.h b/libs/lua/LuaBridge/detail/CFunctions.h
index 7300e9cb70..22dc115a6a 100644
--- a/libs/lua/LuaBridge/detail/CFunctions.h
+++ b/libs/lua/LuaBridge/detail/CFunctions.h
@@ -1234,6 +1234,20 @@ struct CFunc
   }
 
   //--------------------------------------------------------------------------
+  // push back a C-pointer to a std::list<T*>
+
+  template <class T, class C>
+  static int pushbackptr (lua_State *L)
+  {
+    C * const c = Userdata::get <C> (L, 1, false);
+    if (!c) { return luaL_error (L, "invalid pointer to std::list<>"); }
+    T * const v = Userdata::get <T> (L, 2, true);
+    if (!v) { return luaL_error (L, "invalid pointer to std::list<>::value_type"); }
+    c->push_back (v);
+    return 0;
+  }
+
+  //--------------------------------------------------------------------------
   // generate std::map from table
 
   template <class K, class V>
diff --git a/libs/lua/LuaBridge/detail/Namespace.h b/libs/lua/LuaBridge/detail/Namespace.h
index 5ea139ee22..76474aa703 100644
--- a/libs/lua/LuaBridge/detail/Namespace.h
+++ b/libs/lua/LuaBridge/detail/Namespace.h
@@ -1911,7 +1911,7 @@ public:
     typedef std::list<TP> LT;
     return beginConstStdCPtrList<T> (name)
       .addFunction ("unique", (void (LT::*)())&LT::unique)
-      .addFunction ("push_back", (void (LT::*)(const TP&))&LT::push_back);
+      .addExtCFunction ("push_back", &CFunc::pushbackptr<T, LT>);
   }