RwLockReadGuard and RwLockWriteGuard should only be Sync when RawRwLock is

[ammaraskar]

Currently RwLockReadGuards are marked as Sync so long as the contained type is, without having the same bound on the RawRwLock type:

parking_lot/lock_api/src/rwlock.rs

Line 878 in ecfe20b

unsafe impl<'a, R: RawRwLock + 'a, T: ?Sized + Sync + 'a> Sync for RwLockReadGuard<'a, R, T> {}

This means that even if an implementer marks their guard as GuardNoSend, it's actually possible to send guard objects across threads using references to them (references are marked as Send so long as the underlying type is Sync).

Much like MappedMutexGuard this should probably only be Sync when the raw lock is.

An example of sending parking_lot's non sendable guard objects across threads:

use parking_lot::RwLock;
use crossbeam_utils::thread;

fn is_send<T: Send>(_: &T) {}

fn main() {
    let lock = RwLock::new(42);
    let read_guard = lock.read();
    // RwLockReadGuard itself is not Send-able, uncommenting
    // the line below causes a compilation failure.
    //is_send(&read_guard);
    
    thread::scope(|s| {
        let guard_ref = &read_guard;
        
        s.spawn(move |_| {
            // But they can be sent as a reference.
            let sent_lock_guard = guard_ref;
            println!("In thread: {:p}", sent_lock_guard);
        });
        println!("In main:   {:p}", &read_guard);
    });
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=625dd71fdcd34658f0c2d2752a86ca0e

(Issue found by @sslab-gatech's Rust group)

[Amanieu]

I believe this is not a problem since the only thing you can do with a &RwLockReadGuard is dereference it to access the underlying data. There is no interaction with the RawRwLock itself so it should still be sound.

[ammaraskar]

Oh right, the problem is the:

pub fn rwlock(s: &Self) -> &'a RwLock<R, T>

method which would allow you to get a reference to the underlying RwLock object, which might not itself be safe across threads.

[Amanieu]

Ah good point, I forgot about that one. I will adjust the trait bounds.