Maximum Size of Object Maps

Rhai by default does not limit how large (i.e. the number of properties) an object map can be.

This can be changed via Engine::set_max_map_size, with zero being unlimited (the default).

A script attempting to create an object map literal with more properties than the maximum will terminate with a parse error.

Any script operation that produces an object map with more properties than the maximum also terminates the script with an error.

This check can be disabled via the unchecked feature for higher performance (but higher risks as well).

let mut engine = Engine::new();

engine.set_max_map_size(500);       // allow object maps with only up to 500 properties

engine.set_max_map_size(0);         // allow unlimited object maps

Maximum size

Be conservative when setting a maximum limit and always consider the fact that a registered function may grow an object map’s size without Rhai noticing until the very end.

For instance, the built-in + operator for object maps concatenates two object maps together to form one larger object map; if both object maps are slightly below the maximum size limit, the resultant object map may be almost twice the maximum size.

As a malicious script may create a deeply-nested object map which consumes huge amounts of memory while each individual object map still stays under the maximum size limit, Rhai also recursively adds up the sizes of all strings, arrays and object maps contained within each object map to make sure that the aggregate sizes of none of these data structures exceed their respective maximum size limits (if any).

// Small, innocent object map...
let small_map: #{ x: 42 };          // 1-deep... 1 item, 1 object map

// ... becomes huge when multiplied!
small_map.y = small_map;            // 2-deep... 2 items, 2 object maps
small_map.y = small_map;            // 3-deep... 4 items, 4 object maps
small_map.y = small_map;            // 4-deep... 8 items, 8 object maps
small_map.y = small_map;            // 5-deep... 16 items, 16 object maps
small_map.y = small_map;            // <- Rhai raises an error somewhere here
small_map.y = small_map;            //    when the TOTAL number of items in
small_map.y = small_map;            //    the entire array tree exceeds limit

// Or this abomination...
let map = #{ x: 42 };

loop {
    map.x = map;    // <- only 1 item, but infinite number of object maps