Implement Send and Sync for PreparedQuery

[da-shalev]

PreparedQuery does not automatically implement Send and Sync because it retains pointer-capable fetch caches. Its prepared iterators and views already implement both traits when every Q::Item<'a> is Send; this applies the same bound to PreparedQuery itself.

ChangeTracker contains prepared queries, so it inherits Send + Sync transitively. This lets multithreaded schedulers store a ChangeTracker without introducing a local unsafe newtype.

The compile-time test covers prepared shared and mutable queries and confirms that ChangeTracker inherits both traits.

[Ralith]

Thanks, this is an oversight. In fact, I think these impls should probably be on PreparedQuery... do you see any scenario where that would be a problem?

While fixes to ChangeTracker and PreparedQuery are absolutely welcome, be aware that you don't need either of them; the former is implemented in terms of the public API, and the latter should almost never be faster than the now built-in query caching.

[da-shalev]

Thanks, this is an oversight. In fact, I think these impls should probably be on PreparedQuery... do you see any scenario where that would be a problem?

While fixes to ChangeTracker and PreparedQuery are absolutely welcome, be aware that you don't need either of them; the former is implemented in terms of the public API, and the latter should almost never be faster than the now built-in query caching.

You're right, I'm updating the PR to have the Send + Sync impl's directly on PreparedQuery. Having it on the ChangeTracker was the only thing I was concerned about because as you said PreparedQuery already caches itself.

Yes, thanks! I've looked into a event based impl of ChangeTracker.

The only issue I've ran into with event-based tracking is live change detection, so I explored using the provided ChangeTracker as a whole before over-optimizing with my own approach.

I'm exploring multiple forms of change detection before committing to one specific approach. And that's how I realized the oversight.

[Ralith]

Are you sure the bound on Sync is correct? Bear in mind that all public methods are &mut. Also please include a safety comment.

[Ralith]

as you said PreparedQuery already caches itself.

To be clear: all queries are automatically cached. PreparedQuery is mostly redundant now.