pd: fix a deadlock

[overvenus]

This PR fix deadlock pitfall. Reading then writing a RwLock in a match block will cause a deadlock.

Demo:

use std::sync::RwLock;

fn main() {
    let mu = RwLock::new(Some(1));
    {
        let clone = foo(&mu.read().unwrap());
        let mut data = mu.write().unwrap();
        *data = Some(2);
        println!("none match: {:?}", clone);
    }

    {
        match foo(&mu.read().unwrap()) {
            Some(_) | None => {
                let mut data = mu.write().unwrap();
                *data = Some(2);
                println!("in match");
            }
        }
    }

    println!("done");
}

fn foo(a: &Option<usize>) -> Option<usize> {
    a.clone()
}

Playground

[siddontang]

can we add a test for it?

For the next write, we use try_write to check the lock is still locked.

[hhkbp2]

LGTM
This issue looks like a compiler impl bug to me.

[BusyJay]

I think compiler will translate it to

let read_lock = lock.read().unwrap();
match func(read_lock) {
    ...
}
...
drop(read_lock)

.

[siddontang]

test_rwlock_deadlock

[siddontang]

LGTM

[siddontang]

PTAL @BusyJay

[BusyJay]

LGTM

[BusyJay]

Though I don't think the test case is related to the bug.

[overvenus]

FYI, may be solved in RFC#2094(non-lexical lifetimes)