If you perform single threaded async in Rust, you can drop down to the cheap single threaded RefCell rather than the expensive multithreaded Mutex/RwLock
That's one example of a lock you might eliminate, but there are plenty of other cases where it's impossible to eliminate locks even while single threaded.
Consider, for example, something like this (not real rust, I'm rusty there)
lock {
a = foo();
b = io(a).await;
c = bar(b);
}
Eliminating this lock is unsafe because a, b, and c are expected to be updated in tandem. If you remove the lock, then by the time you reach c, a and b may have changed under your feet in an unexpected way because of that await.
Yeah but this problem goes away entirely if you just don’t await within a critical region like that.
I’ve been using nodejs for a decade or so now. Nodejs can also suffer from exactly this problem. In all that time, I think I’ve only reached for a JS locking primitive once.
There is no problem here with the critical region. The problem would be removing the critical region because "there's just one thread".
This is incorrect code
a = foo();
b = io(a).await;
c = bar(b);
Without the lock, `a` can mutate before `b` is done executing which can mess with whether or not `c` is correct. The problem is if you have 2 independent variables that need to be updated in tandem.
Where this might show up. Imagine you have 2 elements on the screen, a span which indicates the contents and a div with the contents.
You now have incorrect code if 2 concurrent loads happen. It could be the original foo, it could be a second foo. There's no way to correctly determine what the content of `myDiv` is from an end user perspective as it depends entirely on what finished last and when. You don't even know if loading is still happening.
I absolutely agree that that code looks buggy. Of course it is - if you just blindly mix view and model logic like that, you’re going to have a bad day. How many different states can the system be in? If multiple concurrent loads can be in progress at the same time, the answer is lots.
But personally I wouldn’t solve it with a lock. I’d solve it by making the state machine more explicit and giving it a little bit of distance from the view logic. If you don’t want multiple loads to happen at once, add an is_loading variable or something to track the loading state. When in the loading state, ignore subsequent load operations.
> add an is_loading variable or something to track the loading state.
Which is definitionally a mutex AKA a lock. However, it's not a lock you are blocking on but rather one that you are trying and leaving.
I know it doesn't look like a traditional lock, but in a language like javascript or python it's a valid locking mechanism. For javascript that's because of the single thread execution model a boolean variable is guaranteed to be consistently set for multiple concurrent actions.
That is to say, you are thinking about concurrency issues, you just aren't thinking about them in concurrency terms.
Yeah I agree. The one time I wrote a lock in javascript it worked like you were hinting at. You could await() the lock's release, and if multiple bits of code were all waiting for the lock, they would acquire it in turn.
But again, I really think in UI code it makes a lot more sense to be clear about what the state is, model it explicitly and make the view a "pure" expression of that state. In the code above:
- The state is 0 or more promises loading data.
- The state is implicit. Ie, the code doesn't list the set of loading promises which are being awaited at any point in time. Its not obvious that there is a collection going on.
- The state is probably wrong. The developer probably wants either 0 or 1 loading states. (Or maybe a queue of them). Because the state hasn't been modelled explicitly, it probably hasn't been considered enough
- The view is updated incorrectly based on the state. If 2 loads happen at the same time, then 1 finishes, the UI removes the "loading..." indicator from the UI. Correct view logic should ensure that the UI is deterministic based on the internal state. 1 in-progress load should result in the UI saying "loading...".
Its a great example. With code like this I think you should always carefully and explicitly consider all of the states of your system, and how the state should change based on user action. Then all UI code can flow naturally from that.
A lock might be a good tool. But without thinking about how you want the program to behave, we have no way to tell. And once you know how you want your program to behave, I find locks to be usually unnecessary.
I think a lot of this type of problem goes away with immutable data and being more careful with side effects (for example, firing them all at once at the end rather than dispersed through the calculation)
