Make Dictionary Send

[RicardRC]

I don't even know if it possible, but it would be awesome.

Sync would be a blast, too.
I need it to be able to use crossbeam with gdext.

[Bromeon]

I don't think it's possible. Reading Godot's thread-safety guidelines:

GDScript arrays, dictionaries

In GDScript, reading and writing elements from multiple threads is OK, but anything that changes the container size (resizing, adding or removing elements) requires locking a mutex.

Since the Dictionary is reference-counted, it would be possible to mutate state from an other thread, which is unsound without mutex. We also don't have access to the reference count itself.

See also #18 and #212.

[lilizoey]

This will be quite an involved addition. It's easy to make a newtype for the dictionary that ensures exclusive access to the dictionary for the right methods. However since a dictionary can store arbitrary values, that means this dictionary could be used to smuggle a non-send value between threads, or to smuggle a reference to a non-sync value between threads.

It would be nice to have such a newtype wrapper, but it'll require a bit of care to make it safe. Perhaps even forbidding the use of Gd<T> values in the dictionary.

How we enforce that is a big harder. We might need some recursive constructor that check every key/value for a non-send/sync value and reject those keys/values. This should probably also come with an unsafe constructor that doesn't check for this.

And of course we'd need to deep duplicate any dictionary used in this newtype.

[Bromeon]

Closing as there is currently no way to safely implement this. For more general discussion about threading design, we can use #18.

If you need this in an unsafe way, you can write a wrapper UnsafeDict(Dictionary) which implements Send -- but then it's your responsibility to keep up the guarantees.