2 stable releases
1.0.1 | Jun 4, 2022 |
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#738 in Concurrency
8KB
65 lines
One At A Time Please
Mark functions as being #[one_at_a_time]
, and instantly only one
thread can call them at a time. It's quick and dirty synchronisation of
calls to functions.
What is for?
Lets say you have a number of functions. They will get called from multiple threads. You only want one of them ever called at a time. When that happens you want the rest to all be blocked.
For example; you have a load of tests. You want to run them in parallel, except for four of them. Which should run in serial.
(Single-threaded recursion is still allowed.)
How does it work?
Internally there is a big lock. Functions marked with one_at_a_time
will aquire that big lock. When they have it, then everyone else is blocked.
API
#[one_at_a_time]
-- An annotation to add to functions. Functions marked with this can only be called on one thread at a time. Otherwise they (politely) wait until it's their turn.one_at_a_time()
-- A helper function to do this yourself in code. You call it, pass in a lambda, and it will (politely) wait until the lock is free. Then run.OneAtATime
- All of the other APIs share one giant lock. This is a struct that allows you to make your own lock, and use that instead.
Examples
#[one_at_a_time]
use ::one_at_a_time_please::one_at_a_time;
/// This __cannot__ be called from different threads at the same time.
#[one_at_a_time]
fn add(a: u32, b: u32) -> u32 {
a + b
}
one_at_a_time<F>(f: FnOnce() -> R) -> F
use ::one_at_a_time_please::one_at_a_time;
/// This __can__ be called from different threads at the same time ...
fn some_function(a: u32, b: u32) -> u32 {
/// but this __cannot__ be called from different threads.
one_at_a_time(move || {
a + b
})
}
struct OneAtATime
use ::one_at_a_time_please::one_at_a_time;
use ::std::sync::Arc;
/// This example is for showing that you can make your own lock,
/// and block on that.
///
/// This is a different lock to what `#[one_at_a_time]` uses.
/// It's behaviour will not overlap with them.
#[one_at_a_time]
fn some_function(a: u32, b: u32) -> u32 {
let oaat = Arc::new(OneAtATime::new());
for i in 0 .. 10 {
let thread_local_oaat = oaat.clone();
let thread = spawn(move || {
for _ in 0..10 {
thread_local_oaat.call(|| {
// do stuff here
});
}
});
}
}
Advanced Example
An example of a (terrible) unsafe counter is provided in tests.
Where it's made safe to call using #[one_at_a_time]
.
You can find it at tests/unsafe_counter.rs.
Dependencies
~0.4–5.5MB
~13K SLoC