Concurrency model for rust analyzer

Zulip discussion thread: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/Concurrency.20in.20RA

Rust analyzer is a long-lived process which oversees many concurrent activities.
The code for this is mainly in the main_loop.rs. Luckely, the amount of
concurrency we need is small, so the thing is still manageable. Nonetheless, it
is hard to understand, and, being the heart of rust-analyzer, it would be cool
to simplify it.

The list of concurrent activities we need to handle:

• LSP requests and notifications
• background request processing
• cargo check/cargo metadata project loading
• cargo check diagnostics
• vfs

Cross-cutting concerns:

• cancellation
  • during shutdown
  • when switching Cargo.toml, it makes sense to kill cargo check.
    • this is the bit which doesn't map nicely to trio's scopes.
• eventually consistent non-concurrent updates
  • for cargo check / cargo metadata we want to run at most one process at a time.
    If there's a current process, we should wait for it to finish and schedule a new one.
• selection: in vfs, main loop and flycheck we need an ability to select among n concurrent activities.

Possible approaches:

1 Actorish model

This is mostly what we do today. Main thread owns all the state and has a
mailbox. Concurrent activities happen on background threads and communicate with
the main thread asynchronously and unidirectionaly, by posting the message

Project loading looks like this:

fn load_project():
    spawn(|| {
        send(Begin)
        for {
            send(Progress)
        }
        send(End(loaded_project))
    })


fn main_loop():
    let mut state = ...;
    loop {
        match next_event() {
            Begin => start_progress(&mut state),
            Progress => report_progress(&mut state)
            End(project) => {
                end_progress(&mut state)
                switch_to(&mut state, project)
            }
        }
    }

The benefit is that it's crystal clear what actually happens, as one can
dbg!(next_event) and get a linear history of all events as data.

The drawback is the tearing of control flow -- to understand a project reload
process (which conceptually is a single continous activity), one need to jump between load_project and main_loop.

2 High order actorish model

Rather than sending n different message types, we can have only one message type which packs a closure to be applied to the state

fn load_project():
    spawn(|| {
        send(|state| start_process(state))
        for {
            send(|state| report_progress(state))
        }
        send(|state| {
            end_progress(state)
            switch_to(state, loaded_project)
        })
    })


fn main_loop():
    let mut state = ...;
    loop {
        next_event()(&mut state);
    }

The benefit here is somewhat more linear control flow (though, everything still happens asynchronously) and extensibility with new "message types"

Also, because the set of functions is open, the set of actions supported by an Agent is also open, a sharp contrast to pattern matching message handling loops provided by some other languages.

https://clojure.org/reference/agents

The drawback is that the main loop is no-longer as obvious -- instead of a stream of events as data, you get a stream of closures.
Because there's no single place where state modification happens, it's harder to spot interferences between distinct concurrent activities

3 Mutexes

The third approach is to wrap the state into a mutex. The code looks exactly as
in the last example, with the difference that it actually is synchronous
(background activity resumes after critical section is finished, and not merely
scheduled).

fn load_project():
    spawn(|| {
        with_lock(|state| start_process(state))
        for {
            with_lock(|state| report_progress(state))
        }
        with_lock(|state| {
            end_progress(state)
            switch_to(state, loaded_project)
        })
    })

The main benefit here is simplicity -- each activity is a fully straight-line code.

The main drawback is that we don't have a central place where all mutation happens.

Misc

It seems the following abstraction of a concurrent process might be helpful:

let task: Task<R> = Task::spawn(|stop_token| {
    ...
    if stop_token.stop_requested() {
        return -1;
    }
    ...
    92
});

// Two-phase cancellation: we first request a stop, then we wait for the task to
// actually finish.
let task: StoppedTask = task.request_stop();
drop(task);

Alas, to be useful we need a way to plug this (a vector of these actually) into
select, and its unclear how to do that.

Start by reading main_loop.rs and the linked Zulip discussion, then map the listed activities and cross-cutting concerns: cancellation, scheduling, and selection. The issue does not define a chosen concurrency model or specific files beyond main_loop.rs, so done would require an agreed design and a concrete simplification plan.