feature request: get_or_insert method

[laralove143]

a utility method to do this, possibly more optimized, would be nice since i think it's used very often

let key = key();
let value = match dashmap.get(key) {
    Some(value) => value,
    None => {
        let default = default();
        dashmap.insert(key, default);
        default
    }
}

this way we could just do let value = dashmap.get_or_insert(key(), default());

[wangnengjie]

If dashmap.get returns None, then there is no lock guard for target shard and dashmap.insert may replace some value.
The dashmap.entry api can do this by

let dashmap = DashMap::<String, i32>::new();
dashmap.insert("a".to_string(), 1);
{
    let entry = dashmap.entry("a".to_string());
    match entry {
        Entry::Occupied(ref o) => println!("{}", o.get()),
        Entry::Vacant(_) => panic!("should not go here"),
    }
}
{
    let entry = dashmap.entry("b".to_string());
    match entry {
        Entry::Occupied(_) => panic!("should not go here"),
        Entry::Vacant(vacant) => {
            vacant.insert(2);
        }
    }
}
println!("{}", dashmap.get("b").unwrap().value());

Both OccupiedEntry and VacantEntry has a write guard to target shard so it's safe. (I don't know whether this api is stable?)
Problem is that OccupiedEntry will keep a write guard and for get_or_insert situation maybe you need a read guard? It seems that parking_lot provide some method to upgrade or downgrade the guard but I'm not so familiar with them.
I'm new to rust language. Please tell me if I have something wrong in the example code.

[laralove143]

do you mean this is how it could be implemented? i don’t know how dashmap’s private code works but one of the contributors could probably come up with the best solution using methods that aren’t exposed publicly

[wangnengjie]

not exactly. I just show that your example code is not concurrent safe and if you had used it in your project to temporarily fill the lack of get_or_insert, you may need to have a look on your code. Just a kindly reminder...
my example is only a compromise for the lack of api at this moment. It's sure that contributors can implemented it in a nice and effective way.

[xacrimon]

The entry API is stable and this is one of it's intended use cases. If this doesn't work for you and you have a proposition for a better API, please reopen. Closing as I don't want two API's that does the same thing. This is still a fairly niche use case which is why it's under the entry API.

[AshleySchaeffer]

For those stumbling across this in future. See a possible solution here:

#292 (comment)

[geekbeast]

So I took a crack at implementing this here (#293) for reason mentioned in #78 #292. The work around in #292 works great, but ends up computing the usize hash and shard twice.