Check that Lua functions on OSM object are called correctly

The code that deals with functions called on the Lua OSM object given as
parameter to the process callbacks is somewhat strange. The result is
that calling "object.as_point()" etc. will work, but the correct Lua way
to do this is "object:as_point()". This adds a check to all the
functions that detects this case and prints a warning. It will also
check that those functions are not called with any additional arguments.

Checking for this now, users will hopefully fix this. This gives us the
opportunity later to change or code to be more in line with usual Lua
syntax.

This will only print a warning once per function.

Author: joto

src/output-flex.cpp

@@ -45,6 +45,7 @@
 #include <limits>
 #include <memory>
 #include <mutex>
+#include <set>
 #include <stdexcept>
 #include <string>
 #include <string_view>
@@ -295,6 +296,40 @@ void create_expire_tables(std::vector<expire_output_t> const &expire_outputs,
     }
 }
 
+void check_for_object(lua_State *lua_state, char const *const function_name)
+{
+    // This is used to make sure we are printing warnings only once per
+    // function name.
+    static std::set<std::string> message_shown;
+    if (message_shown.count(function_name)) {
+        return;
+    }
+
+    int const num_params = lua_gettop(lua_state);
+    if (num_params == 0) {
+        log_warn("You should use the syntax 'object:{}()' (with the colon, not "
+                 "a point) to call functions on the OSM object.",
+                 function_name);
+
+        message_shown.emplace(function_name);
+        return;
+    }
+
+    if (lua_getmetatable(lua_state, 1)) {
+        luaL_getmetatable(lua_state, osm2pgsql_object_metatable.data());
+        if (lua_rawequal(lua_state, -1, -2)) {
+            lua_pop(lua_state, 2); // remove the two metatables
+            return;
+        }
+        lua_pop(lua_state, 2); // remove the two metatables
+    }
+
+    message_shown.emplace(function_name);
+    log_warn("First and only parameter for {0}() must be the OSM object. Call "
+             "it like this: 'object:{0}()'.",
+             function_name);
+}
+
 } // anonymous namespace
 
 /**
@@ -304,6 +339,8 @@ void create_expire_tables(std::vector<expire_output_t> const &expire_outputs,
 void output_flex_t::check_context_and_state(char const *name,
                                             char const *context, bool condition)
 {
+    check_for_object(lua_state(), name);
+
     if (condition) {
         throw fmt_error(
             "The function {}() can only be called (directly or indirectly) "