On Sun, Sep 15, 2024 at 3:49 PM Gary Guo gary@garyguo.net wrote:
On Fri, 13 Sep 2024 23:28:37 -0700 Boqun Feng boqun.feng@gmail.com wrote:
Hmm.. I think it makes more sense to make `access()` requires `where T: Sync` instead of the current fix? I.e. I propose we do:
impl<T, U> LockedBy<T, U> { pub fn access<'a>(&'a self, owner: &'a U) -> &'a T where T: Sync { ... } }The current fix in this patch disallows the case where a user has a `Foo: !Sync`, but want to have multiple `&LockedBy<Foo, X>` in different threads (they would use `access_mut()` to gain unique accesses), which seems to me is a valid use case.
The where-clause fix disallows the case where a user has a `Foo: !Sync`, a `&LockedBy<Foo, X>` and a `&X`, and is trying to get a `&Foo` with `access()`, this doesn't seems to be a common usage, but maybe I'm missing something?
+1 on this. Our `LockedBy` type only works with `Lock` -- which provides mutual exclusion rather than `RwLock`-like semantics, so I think it should be perfectly valid for people to want to use `LockedBy` for `Send + !Sync` types and only use `access_mut`. So placing `Sync` bound on `access` sounds better.
I will add the `where` bound to `access`.
There's even a way to not requiring `Sync` bound at all, which is to ensure that the owner itself is a `!Sync` type:
impl<T, U> LockedBy<T, U> { pub fn access<'a, B: Backend>(&'a self, owner: &'a Guard<U, B>) -> &'a T { ... } }Because there's no way for `Guard<U, B>` to be sent across threads, we can also deduce that all caller of `access` must be from a single thread and thus the `Sync` bound is unnecessary.
Isn't Guard Sync? Either way, it's inconvenient to make Guard part of the interface. That prevents you from using it from within `&self`/`&mut self` methods on the owner.
Alice