Keep track of name and number of results in prepared_lua_function_t

Keep track of function name in prepared_lua_function_t to be able to
output a better error message if something goes wrong in this
function.

Also stores the number of results we are expecting from this Lua
function in prepared_lua_function_t. We'll need this later.

Author: joto

src/output-flex.cpp

@@ -69,14 +69,17 @@ static char const osm2pgsql_table_name[] = "osm2pgsql.table";
 static char const osm2pgsql_object_metatable[] = "osm2pgsql.object_metatable";
 
 prepared_lua_function_t::prepared_lua_function_t(lua_State *lua_state,
-                                                 char const *name)
+                                                 char const *name,
+                                                 int nresults)
 {
     int const index = lua_gettop(lua_state);
 
     lua_getfield(lua_state, 1, name);
 
     if (lua_type(lua_state, -1) == LUA_TFUNCTION) {
         m_index = index;
+        m_name = name;
+        m_nresults = nresults;
         return;
     }
 
@@ -1012,9 +1015,10 @@ void output_flex_t::call_lua_function(prepared_lua_function_t func,
         get_options()->extra_attributes); // the single argument
 
     luaX_set_context(lua_state(), this);
-    if (luaX_pcall(lua_state(), 1, 0)) {
-        throw std::runtime_error{"Failed to execute lua processing function:"
-                                 " {}"_format(lua_tostring(lua_state(), -1))};
+    if (luaX_pcall(lua_state(), 1, func.nresults())) {
+        throw std::runtime_error{
+            "Failed to execute Lua function 'osm2pgsql.{}':"
+            " {}"_format(func.name(), lua_tostring(lua_state(), -1))};
     }
 }
 

src/output-flex.hpp

@@ -43,17 +43,30 @@ class prepared_lua_function_t
     /**
      * Get function with the name "osm2pgsql.name" from Lua and put pointer
      * to it on the Lua stack.
+     *
+     * \param lua_state Current Lua state.
+     * \param name Name of the function.
+     * \param nresults The number of results this function is supposed to have.
      */
-    prepared_lua_function_t(lua_State *lua_state, const char *name);
+    prepared_lua_function_t(lua_State *lua_state, const char *name,
+                            int nresults = 0);
 
     /// Return the index of the function on the Lua stack.
     int index() const noexcept { return m_index; }
 
+    /// The name of the function.
+    char const* name() const noexcept { return m_name; }
+
+    /// The number of results this function is expected to have.
+    int nresults() const noexcept { return m_nresults; }
+
     /// Is this function defined in the users Lua code?
     operator bool() const noexcept { return m_index != 0; }
 
 private:
+    char const *m_name = nullptr;
     int m_index = 0;
+    int m_nresults = 0;
 };
 
 class output_flex_t : public output_t