fix(lua/splay_tree): for_each method was crashing if the map was updated during for_each

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/bindings/lua/splay_map.cpp

@@ -168,7 +168,11 @@ static int splay_map_pred(lua_State * L) {
 }
 
 static int splay_map_for_each(lua_State * L) {
-    lua_splay_map & m = to_splay_map(L, 1);    // map
+    // Remark: we take a copy of the map to make sure
+    // for_each will not crash if the map is updated while being
+    // traversed.
+    // The copy operation is very cheap O(1).
+    lua_splay_map m(to_splay_map(L, 1)); // map
     luaL_checktype(L, 2, LUA_TFUNCTION); // user-fun
     m.for_each([&](lua_ref const & k, lua_ref const & v) {
         lua_pushvalue(L, 2); // push user-fun

tests/lua/map2.lua

@@ -0,0 +1,33 @@
+local m = splay_map()
+for i = 1, 100 do
+   m:insert(i, i * 3)
+end
+
+local prev_k = nil
+-- It is safe to add/erase elements from m while we are traversing.
+-- The for_each method will traverse the elements that are in m
+-- when the for_each is invoked
+m:for_each(
+   function(k, v)
+      if prev_k then
+         assert(prev_k < k)
+      end
+      print(k .. " -> " .. v)
+      prev_k = k
+      m:insert(-k, v)
+   end
+)
+
+assert(m:size() == 200)
+m2 = m:copy()
+m:for_each(
+   function(k, v)
+      assert(m2:contains(-k))
+      assert(m2:find(-k) == v)
+      if k > 0 then
+         m:erase(k)
+      end
+   end
+)
+assert(m:size() == 100)
+m:for_each(function(k, v) assert(k < 0) end)