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Brian Anderson f5c3af1c1a Clean up unfinished portions of task tutorial

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% Rust Tasks and Communication Tutorial

Introduction

The Rust language is designed from the ground up to support pervasive and safe concurrency through lightweight, memory-isolated tasks and message passing.

Rust tasks are not the same as traditional threads - they are what are often referred to as green threads, cooperatively scheduled by the Rust runtime onto a small number of operating system threads. Being significantly cheaper to create than traditional threads, Rust can create hundreds of thousands of concurrent tasks on a typical 32-bit system.

Tasks provide failure isolation and recovery. When an exception occurs in rust code (either by calling fail explicitly or by otherwise performing an invalid operation) the entire task is destroyed - there is no way to catch an exception as in other languages. Instead tasks may monitor each other to detect when failure has occurred.

Rust tasks have dynamically sized stacks. When a task is first created it starts off with a small amount of stack (currently in the low thousands of bytes, depending on platform) and more stack is acquired as needed. A Rust task will never run off the end of the stack as is possible in many other languages, but they do have a stack budget, and if a Rust task exceeds its stack budget then it will fail safely.

Tasks make use of Rust's type system to provide strong memory safety guarantees, disallowing shared mutable state. Communication between tasks is facilitated by the transfer of owned data through the global exchange heap.

This tutorial will explain the basics of tasks and communication in Rust, explore some typical patterns in concurrent Rust code, and finally discuss some of the more exotic synchronization types in the standard library.

Warning: This tutorial is incomplete

A note about the libraries

While Rust's type system provides the building blocks needed for safe and efficient tasks, all of the task functionality itself is implemented in the core and standard libraries, which are still under development and do not always present a consistent interface.

In particular, there are currently two independent modules that provide a message passing interface to Rust code: core::comm and core::pipes. core::comm is an older, less efficient system that is being phased out in favor of pipes. At some point the existing core::comm API will be removed and the user-facing portions of core::pipes will be moved to core::comm. In this tutorial we will discuss pipes and ignore the comm API.

For your reference, these are the standard modules involved in Rust concurrency at the moment.

core::task - All code relating to tasks and task scheduling
core::comm - The deprecated message passing API
core::pipes - The new message passing infrastructure and API
std::comm - Higher level messaging types based on core::pipes
std::sync - More exotic synchronization tools, including locks
std::arc - The ARC type, for safely sharing immutable data
std::par - Some basic tools for implementing parallel algorithms

Basics

The programming interface for creating and managing tasks is contained in the task module of the core library, making it available to all Rust code by default. At it's simplest, creating a task is a matter of calling the spawn function, passing a closure to run in the new task.

# use io::println;
use task::spawn;

// Print something profound in a different task using a named function
fn print_message() { println("I am running in a different task!"); }
spawn(print_message);

// Print something more profound in a different task using a lambda expression
spawn( || println("I am also running in a different task!") );

// The canonical way to spawn is using `do` notation
do spawn {
    println("I too am running in a different task!");
}

In Rust, there is nothing special about creating tasks - the language itself doesn't know what a 'task' is. Instead, Rust provides in the type system all the tools necessary to implement safe concurrency, owned types in particular, and leaves the dirty work up to the core library.

The spawn function has a very simple type signature: fn spawn(f: ~fn()). Because it accepts only owned closures, and owned closures contained only owned data, spawn can safely move the entire closure and all its associated state into an entirely different task for execution. Like any closure, the function passed to spawn may capture an environment that it carries across tasks.

# use io::println;
# use task::spawn;
# fn generate_task_number() -> int { 0 }
// Generate some state locally
let child_task_number = generate_task_number();

do spawn {
   // Capture it in the remote task
   println(fmt!("I am child number %d", child_task_number));
}

By default tasks will be multiplexed across the available cores, running in parallel, thus on a multicore machine, running the following code should interleave the output in vaguely random order.

# use io::print;
# use task::spawn;

for int::range(0, 20) |child_task_number| {
    do spawn {
       print(fmt!("I am child number %d\n", child_task_number));
    }
}

Communication

Now that we have spawned a new task, it would be nice if we could communicate with it. Recall that Rust does not have shared mutable state, so one task may not manipulate variables owned by another task. Instead we use pipes.

Pipes are simply a pair of endpoints, with one for sending messages and another for receiving messages. Pipes are low-level communication building-blocks and so come in a variety of forms, appropriate for different use cases, but there are just a few varieties that are most commonly used, which we will cover presently.

The simplest way to create a pipe is to use the pipes::stream function to create a (Chan, Port) pair. In Rust parlance a 'channel' is a sending endpoint of a pipe, and a 'port' is the receiving endpoint. Consider the following example of performing two calculations concurrently.

use task::spawn;
use pipes::{stream, Port, Chan};

let (chan, port): (Chan<int>, Port<int>) = stream();

do spawn {
    let result = some_expensive_computation();
    chan.send(result);
}

some_other_expensive_computation();
let result = port.recv();
# fn some_expensive_computation() -> int { 42 }
# fn some_other_expensive_computation() {}

Let's examine this example in detail. The let statement first creates a stream for sending and receiving integers (recall that let can be used for destructuring patterns, in this case separating a tuple into its component parts).

# use pipes::{stream, Chan, Port};
let (chan, port): (Chan<int>, Port<int>) = stream();

The channel will be used by the child task to send data to the parent task, which will wait to receive the data on the port. The next statement spawns the child task.

# use task::{spawn};
# use task::spawn;
# use pipes::{stream, Port, Chan};
# fn some_expensive_computation() -> int { 42 }
# let (chan, port) = stream();
do spawn {
    let result = some_expensive_computation();
    chan.send(result);
}

Notice that chan was transferred to the child task implicitly by capturing it in the task closure. Both Chan and Port are sendable types and may be captured into tasks or otherwise transferred between them. In the example, the child task performs an expensive computation then sends the result over the captured channel.

Finally, the parent continues by performing some other expensive computation and then waiting for the child's result to arrive on the port:

# use pipes::{stream, Port, Chan};
# fn some_other_expensive_computation() {}
# let (chan, port) = stream::<int>();
# chan.send(0);
some_other_expensive_computation();
let result = port.recv();

The Port and Chan pair created by stream enable efficient communication between a single sender and a single receiver, but multiple senders cannot use a single Chan, nor can multiple receivers use a single Port. What if our example needed to perform multiple computations across a number of tasks? The following cannot be written:

# use task::{spawn};
# use pipes::{stream, Port, Chan};
# fn some_expensive_computation() -> int { 42 }
let (chan, port) = stream();

do spawn {
    chan.send(some_expensive_computation());
}

// ERROR! The previous spawn statement already owns the channel,
// so the compiler will not allow it to be captured again
do spawn {
    chan.send(some_expensive_computation());
}

Instead we can use a SharedChan, a type that allows a single Chan to be shared by multiple senders.

# use task::spawn;
use pipes::{stream, SharedChan};

let (chan, port) = stream();
let chan = SharedChan(move chan);

for uint::range(0, 3) |init_val| {
    // Create a new channel handle to distribute to the child task
    let child_chan = chan.clone();
    do spawn {
        child_chan.send(some_expensive_computation(init_val));
    }
}

let result = port.recv() + port.recv() + port.recv();
# fn some_expensive_computation(_i: uint) -> int { 42 }

Here we transfer ownership of the channel into a new SharedChan value. Like Chan, SharedChan is a non-copyable, owned type (sometimes also referred to as an 'affine' or 'linear' type). Unlike Chan though, SharedChan may be duplicated with the clone() method. A cloned SharedChan produces a new handle to the same channel, allowing multiple tasks to send data to a single port. Between spawn, stream and SharedChan we have enough tools to implement many useful concurrency patterns.

Note that the above SharedChan example is somewhat contrived since you could also simply use three stream pairs, but it serves to illustrate the point. For reference, written with multiple streams it might look like the example below.

# use task::spawn;
# use pipes::{stream, Port, Chan};

// Create a vector of ports, one for each child task
let ports = do vec::from_fn(3) |init_val| {
    let (chan, port) = stream();
    do spawn {
        chan.send(some_expensive_computation(init_val));
    }
    port
};

// Wait on each port, accumulating the results
let result = ports.foldl(0, |accum, port| *accum + port.recv() );
# fn some_expensive_computation(_i: uint) -> int { 42 }

Handling task failure

Rust has a built-in mechanism for raising exceptions, written fail (or fail ~"reason", or sometimes assert expr), and it causes the task to unwind its stack, running destructors and freeing memory along the way, and then exit itself. Unlike C++, exceptions in Rust are unrecoverable within a single task - once a task fails there is no way to "catch" the exception.

All tasks are, by default, linked to each other, meaning their fate is intertwined, and if one fails so do all of them.

# use task::spawn;
# fn do_some_work() { loop { task::yield() } }
# do task::try {
// Create a child task that fails
do spawn { fail }

// This will also fail because the task we spawned failed
do_some_work();
# };

While it isn't possible for a task to recover from failure, tasks may be notified when other tasks fail. The simplest way of handling task failure is with the try function, which is similar to spawn, but immediately blocks waiting for the child task to finish.

# fn some_condition() -> bool { false }
# fn calculate_result() -> int { 0 }
let result: Result<int, ()> = do task::try {
    if some_condition() {
        calculate_result()
    } else {
        fail ~"oops!";
    }
};
assert result.is_err();

Unlike spawn, the function spawned using try may return a value, which try will dutifully propagate back to the caller in a Result enum. If the child task terminates successfully, try will return an Ok result; if the child task fails, try will return an Error result.

Note: A failed task does not currently produce a useful error value (all error results from try are equal to Err(())). In the future it may be possible for tasks to intercept the value passed to fail.

TODO: Need discussion of future_result in order to make failure modes useful.

But not all failure is created equal. In some cases you might need to abort the entire program (perhaps you're writing an assert which, if it trips, indicates an unrecoverable logic error); in other cases you might want to contain the failure at a certain boundary (perhaps a small piece of input from the outside world, which you happen to be processing in parallel, is malformed and its processing task can't proceed). Hence the need for different linked failure modes.

Failure modes

By default, task failure is bidirectionally linked, which means if either task dies, it kills the other one.

# fn sleep_forever() { loop { task::yield() } }
# do task::try {
do task::spawn {
    do task::spawn {
        fail;  // All three tasks will die.
    }
    sleep_forever();  // Will get woken up by force, then fail
}
sleep_forever();  // Will get woken up by force, then fail
# };

If you want parent tasks to kill their children, but not for a child task's failure to kill the parent, you can call task::spawn_supervised for unidirectionally linked failure. The function task::try, which we saw previously, uses spawn_supervised internally, with additional logic to wait for the child task to finish before returning. Hence:

# use pipes::{stream, Chan, Port};
# use task::{spawn, try};
# fn sleep_forever() { loop { task::yield() } }
# do task::try {
let (sender, receiver): (Chan<int>, Port<int>) = stream();
do spawn {  // Bidirectionally linked
    // Wait for the supervised child task to exist.
    let message = receiver.recv();
    // Kill both it and the parent task.
    assert message != 42;
}
do try {  // Unidirectionally linked
    sender.send(42);
    sleep_forever();  // Will get woken up by force
}
// Flow never reaches here -- parent task was killed too.
# };

Supervised failure is useful in any situation where one task manages multiple fallible child tasks, and the parent task can recover if any child files. On the other hand, if the parent (supervisor) fails then there is nothing the children can do to recover, so they should also fail.

Supervised task failure propagates across multiple generations even if an intermediate generation has already exited:

# fn sleep_forever() { loop { task::yield() } }
# fn wait_for_a_while() { for 1000.times { task::yield() } }
# do task::try::<int> {
do task::spawn_supervised {
    do task::spawn_supervised {
        sleep_forever();  // Will get woken up by force, then fail
    }
    // Intermediate task immediately exits
}
wait_for_a_while();
fail;  // Will kill grandchild even if child has already exited
# };

Finally, tasks can be configured to not propagate failure to each other at all, using task::spawn_unlinked for isolated failure.

# fn random() -> int { 100 }
# fn sleep_for(i: int) { for i.times { task::yield() } }
# do task::try::<()> {
let (time1, time2) = (random(), random());
do task::spawn_unlinked {
    sleep_for(time2);  // Won't get forced awake
    fail;
}
sleep_for(time1);  // Won't get forced awake
fail;
// It will take MAX(time1,time2) for the program to finish.
# };

Creating a task with a bi-directional communication path

A very common thing to do is to spawn a child task where the parent and child both need to exchange messages with each other. The function std::comm::DuplexStream() supports this pattern. We'll look briefly at how it is used.

To see how spawn_conversation() works, we will create a child task that receives uint messages, converts them to a string, and sends the string in response. The child terminates when 0 is received. Here is the function that implements the child task:

# use std::comm::DuplexStream;
# use pipes::{Port, Chan};
fn stringifier(channel: &DuplexStream<~str, uint>) {
    let mut value: uint;
    loop {
        value = channel.recv();
        channel.send(uint::to_str(value, 10u));
        if value == 0u { break; }
    }
}

The implementation of DuplexStream supports both sending and receiving. The stringifier function takes a DuplexStream that can send strings (the first type parameter) and receive uint messages (the second type parameter). The body itself simply loops, reading from the channel and then sending its response back. The actual response itself is simply the strified version of the received value, uint::to_str(value).

Here is the code for the parent task:

# use std::comm::DuplexStream;
# use pipes::{Port, Chan};
# use task::spawn;
# fn stringifier(channel: &DuplexStream<~str, uint>) {
#     let mut value: uint;
#     loop {
#         value = channel.recv();
#         channel.send(uint::to_str(value, 10u));
#         if value == 0u { break; }
#     }
# }
# fn main() {

let (from_child, to_child) = DuplexStream();

do spawn || {
    stringifier(&to_child);
};

from_child.send(22u);
assert from_child.recv() == ~"22";

from_child.send(23u);
from_child.send(0u);

assert from_child.recv() == ~"23";
assert from_child.recv() == ~"0";

# }

The parent task first calls DuplexStream to create a pair of bidirectional endpoints. It then uses task::spawn to create the child task, which captures one end of the communication channel. As a result, both parent and child can send and receive data to and from the other.

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