5.5 KiB
Tasks
Now that we know what Futures are, we now want to run them!
In async-std, the tasks (TODO: link) module is responsible for this. The simplest way is using the block_on function:
use async_std::fs::File;
use async_std::task;
async fn read_file(path: &str) -> Result<String, io::Error> {
let mut file = File.open(path).await?;
let mut contents = String::new();
file.read_to_string(&mut contents).await?;
contents
}
fn main() {
let task = task::spawn(async {
let result = read_file("data.csv");
match result {
Ok(s) => println!("{}", s),
Err(e) => println!("Error reading file: {:?}", e)
}
});
println!("Started task!");
task::block_on(task);
println!("Stopped task!");
}
This asks the runtime baked into async_std to execute the code that reads a file. Let’s go one by one, though, inside to outside.
async {
let result = read_file("data.csv");
match result {
Ok(s) => println!("{}", s),
Err(e) => println!("Error reading file: {:?}", e)
}
}
This is an async block. Async blocks are necessary to call async functions, and will instruct the compiler to include all the relevant instructions to do so. In Rust, all blocks return a value and async blocks happen to return a value of the kind Future.
But let’s get to the interesting part:
task::spawn(async { })
spawn takes a Future and starts running it on a Task. It returns a JoinHandle. Futures in Rust are sometimes called cold Futures. You need something that starts running them. To run a Future, there may be some additional bookkeeping required, e.g. if it’s running or finished, where it is being placed in memory and what the current state is. This bookkeeping part is abstracted away in a Task. A Task is similar to a Thread, with some minor differences: it will be scheduled by the program instead of the operating system kernel and if it encounters a point where it needs to wait, the program itself responsible for waking it up again. We’ll talk a little bit about that later. An async_std task can also has a name and an ID, just like a thread.
For now, it is enough to know that once you spawned a task, it will continue running in the background. The JoinHandle in itself is a future that will finish once the Task ran to conclusion. Much like with threads and the join function, we can now call block_on on the handle to block the program (or the calling thread, to be specific) to wait for it to finish.
Tasks in async_std
Tasks in async_std are one of the core abstractions. Much like Rust’s threads, they provide some practical functionality over the raw concept. Tasks have a relationship to the runtime, but they are in themselves separate. async_std tasks have a number of desirable properties:
- They are allocated in one single allocation
- All tasks have a backchannel, which allows them to propagate results and errors to the spawning task through the
JoinHandle - The carry desirable metadata for debugging
- They support task local storage
async_stds task api handles setup and teardown of a backing runtime for you and doesn’t rely on a runtime being started.
Blocking
Tasks are assumed to run concurrently, potentially by sharing a thread of execution. This means that operations blocking an operating system thread, such as std::thread::sleep or io function from Rusts stdlib will stop execution of all tasks sharing this thread. Other libraries (such as database drivers) have similar behaviour. Note that blocking the current thread is not in and by itself bad behaviour, just something that does not mix well with they concurrent execution model of async-std. Essentially, never do this:
fn main() {
task::block_on(async {
// this is std::fs, which blocks
std::fs::read_to_string("test_file");
})
}
If you want to mix operation kinds, consider putting such operations on a thread.
Errors and panics
Tasks report errors through normal channels: If they are fallible, their Output should be of kind Result<T,E>.
In case of panic, behaviour differs depending on if there's a reasonable part that addresses the panic. If not, the program aborts.
In practice, that means that block_on propagates panics to the blocking component:
fn main() {
task::block_on(async {
panic!("test");
});
}
thread 'async-task-driver' panicked at 'test', examples/panic.rs:8:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
While panicing a spawned tasks will abort:
task::spawn(async {
panic!("test");
});
task::block_on(async {
task::sleep(Duration::from_millis(10000)).await;
})
thread 'async-task-driver' panicked at 'test', examples/panic.rs:8:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
Aborted (core dumped)
That might seem odd at first, but the other option would be to silently ignore panics in spawned tasks. The current behaviour can be changed by catching panics in the spawned task and reacting with custom behaviour. This gives users the choice of panic handling strategy.
Conclusion
async_std comes with a useful Task type that works with an API similar to std::thread. It covers error and panic behaviour in a structured and defined way.
Tasks are separate concurrent units and sometimes they need to communicate. That’s where Streams come in.