Structured Concurrency

   Provides various primitives useful for structured concurrency and async tasks.

Senders/Receivers

A Sender is a lazy Task (in the general sense of the word). It needs to be connected to a Receiver and then started before it will (eventually) call one of the three receiver methods exactly once: setValue, setDone, setError.

It can be used to model many asynchronous operations: Futures, Fiber, Coroutines, Threads, etc. It enforces structured concurrency because a Sender cannot start without it being awaited on.

setValue is the only one allowed to throw exceptions, and if it does, setError is called with the Throwable. setDone is called when the operation has been cancelled.

See https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2300r5.html for the C++ proposal for introducing Senders/Receivers.

Currently we have the following Senders:

• ValueSender. Just produces a plain value. just is a convenient construction function for it.
• ThreadSender. Calls the setValue function in the context of a new thread.
• Nursery. A place to await multiple Senders.
• ForkSender. Forks the program and executes the supplied function.
• ThrowingSender. Always throws.
• DoneSender. Always cancels.
• VoidSender. Always calls setValue with no arguments.
• ErrorSender. Always calls setError with supplied exception.
• PromiseSender. Creates a promise-like object that can be fulfilled, canceled or errored manually. Useful for when the Sender/Receiver connection isn't statically known or very dynamic.
• DeferSender. Creates a sender that calls a supplied callable and runs the returned Sender. defer is a convenient constructor function for it.

Writing your own Sender

Most of the asynchronous tasks you will do involve writing your own Sender.

Here is the implementation of the ValueSender.

/// A Sender that sends a single value of type T
struct ValueSender(T) {
  alias Value = T;
  T value;
  static struct Op(Receiver) {
    Receiver receiver;
    T value;
    @disable this(ref return scope typeof(this) rhs);
    @disable this(this);
    void start() {
      receiver.setValue(value);
    }
  }
  Op!Receiver connect(Receiver)(Receiver receiver) {
    // ensure NVRO
    auto op = Op!(Receiver)(receiver, value);
    return op;
  }
}

A ValueSender!int is nothing more than a int wrapped in a struct with a connect method. It can be constructed and passed around, but it won't produce a value until it is connected and started. The Op object (operational-state) returned by connect represents the state of a connected Sender/Receiver pair, which in case of the ValueSender includes the value to be send. After connecting the operational-state still need its start method called, before it actually produces a value.

A Receiver needs to implement the setValue, setError and setDone. A Sender is required to call exactly one of the three functions once. Both setError and setdone are required to be nothrow. If setValue is not nothrow then the Sender must call setError if setValue throws.

Most Senders should call receiver.getStopToken to retrieve a stoptoken by which they can be notified (or polled) whether they are cancelled. See the section of stoptokens how this works.

Operations

Senders enjoy the following operations.

• syncWait. It takes a Sender and blocks the current execution context until the Sender is completed. It then returns or throws anything the Sender has send, if any. (note: attributes are inferred when possible, so that e.g. if the Sender doesn't call setError, syncWait itself is nothrow).

• then. Chains a callable to be invoked when the Sender is completed with a value.

• via. Start one Sender in the setValue of another. Useful for when you want to change the execution context. ValueSender!int(4).via(ThreadSender()) produces an int in the context of a new thread.

• withStopToken. Like then but injects a StopToken as well.

• withStopSource. When applied after a Sender you can stop the Sender manually with the stopsource. It will still stop when the downstream receiver's StopToken is triggered.

• race. Runs multiple Senders and completes with the value produced by the first to complete, after first cancelling and awaiting the others. When all Senders complete with an error, the first error is propagated. When all Senders complete with cancellation, race completes with cancellation as well. Unlike raceAll it allows Senders to error or complete with cancellation as long as one is still running.

• raceAll. Runs multiple Senders and completes with the value or error produced by the first to complete, after first cancelling and awaiting the others. Unlike race, the only way raceAll can complete succesfully is when all Senders are still running when one of them completes. So not only does it forward the first value, also the first error.

• ignoreError. Redirects the setError to setDone, so as not to trigger the downstream error path.

• whenAll. Produces a tuple of values after all Senders produced their values. If one or more Senders complete with an error, whenAll will complete with the first error, after stopping and awaiting the remaining Senders. Likewise, if one Sender completes with cancellation, whenAll completes with cancellation as well, after stopping and awaiting the remaining Senders.

• retry. It retries the underlying Sender until success or cancellation. The retry logic is customizable. Included is a Times, that will retry n times and then propagate the latest failure.

• retryWhen. It retries the underlying Sender until success or cancellation. The retry logic is customizable. Unlike retry, retryWhen logic returns an additional sender on failure to be run before the next retry, allowing deeper customisability such as delaying retries.

• completeWithCancellation. Wraps the Sender and redirects the setValue termination to complete with cancellation. The Sender is not allowed to produce a Value.

• completeWithError. Wraps the Sender and redirects the all terminations to complete with error. If an Throwable is forwarded in the Sender it takes precedence over the error value if the latter is only an Exception.

• toShared. Wraps a Sender in a SharedSender that forwards the same termination call to each connected Receiver.

• forwardOn. Run the completion of a Sender on a specific Scheduler.

• toSingleton. Only allows one instantiation of the underlying Sender, regardless of how many Receivers are connected. In contrast with toShared this starts the underlying Sender each time the receiver count goes from 0 to 1, whereas toShared keeps the last termination cached.

• stopOn. Allows to explicitely set which StopToken to use. Normally StopToken's are chained so that triggering stop will propagate through the whole task chain. stopOn allows you to have Senders that only listen to a specific StopToken.

• withChild. Creates an ordering of stop triggers between the parent and the child. When the resulting Sender's StopToken is triggered, the parent's is only triggered after the child has completed. This creates certainty of child operations having ran cleanup code before the parent is triggered.

• onTermination. Invokes the supplied callable on any termination of the underlying Sender.

• onError. Invokes the supplied callable whenever the underlying Sender completes with an error.

• onCompletion. Invokes the supplied callable whenever the underlying Sender completes with either an value or cancellation.

• stopWhen. Cancels the source when the trigger completes normally. If the either source or trigger completes with cancellation or with an error, the first one is propagates after both are completed.

• onResult. Invokes the supplied callable on termination and passes the resulting value, similar to the return value of .syncWait.

• repeat. Repeats the supplied sender until it is cancelled or produces an error.

Streams

A Stream has a .collect function that accepts a shared callable and returns a Sender. Once the Sender is connected and started the Stream will call the callable zero or more times before one of the three terminal functions of the Receiver is called.

An exception throw in the callable will cancel the stream and complete the Sender with that exception.

Streams can be cancelled by triggering the StopToken supplied via the Receiver.

The callable supplied to the Stream has to annotated with shared because the execution context where the callable is called from is undefined.

Currently there are the following Streams:

• infiniteStream. Continously emits the same value.
• iotaStream. Emits the values that span the given starting and stopping values.
• arrayStream. Emits every value from the array.
• intervalStream. Emits every interval.
• doneStream. Upon start immediately emits cancellation.
• errorStream. Upon start immediately emits an error.
• sharedStream. Is used for broadcasting values to zero or more receivers. Receivers can be added and removed at any time.
• cycleStream. Cycles through a ranges until cancelled.
• deferStream. Creates a stream of the values produced by the Senders returned by the supplied callable.
• cronStream. Emits at the schedule specified. Currently only supports minutes and hours.

With the following operations:

• take. Emits at most the first n values.
• transform. Applies a tranformation function to each value.
• filter. Filters out all values where predicate is false.
• scan. Applies an accumulator function with seed to each value.
• sample. Forwards the latest value of the base Stream every time the trigger Stream emits a value. If the base stream hasn't produced a (new) value the trigger is ignored.
• via. Starts the Stream on the context of another Sender.
• throttleFirst. Limits a Stream by starting a cooldown period after each value during which no newer values are emitted.
• throttleLast. Like throttleFirst but only emits the latest value after the cooldown.
• debounce. Limits a Stream by only emitting the last value after the Stream has not emitted for a duration.
• slide. Slides a window over the stream and emits each full window as an array.
• toList. Converts the Stream into a Sender that completes with an array that contains all the items emitted. Be careful to use this on finite streams only.
• flatMapConcat. For each value runs the supplied function and starts the returned Sender. The inner Sender must be completed before another is started.
• flatMapLatest. For each value runs the supplied function and starts the returned Sender. A Sender that is still running when the next value arrives is cancelled.

Most of the time you will need to write your own Stream however. The following helpers can speed that up:

• loopStream. Takes a struct with a loop function and calls that with an emit and stopToken while ensuring the struct is alive during that.
• fromStreamOp. Constructs a full Stream given only a templated OperationalState. Allows passing in custom values into the OperationalState's constructor. Since Streams build on Senders they require a bit of boilerplate to setup, this helper eliminates that.

Scheduler

Schedulers create Senders that run on specific execution contexts. A Sender can query a Receiver with .getScheduler() to get a Scheduler and from there can schedule additional tasks to be ran immediately or after a certain Duration.

syncWait automatically inserts a LocalThreadScheduler with a timingwheels implementation to fulfull the Scheduler contract. This means that by default any Sender can schedule timers that run on the thread that awaits the whole chain.

For testing purposes there is a ManualTimeScheduler which can be used to advance the timingwheels manually.

ThreadPool

stdTaskPool creates a RAII thread pool where Senders can be scheduled on using the .on scheduling operator. Both the sender scheduled will run in the thread pool as well any additional scheduled Senders using getScheduler. It uses the std.parallelism's TaskPool implementation underneath.

Nursery

A place where Senders can be awaited in. Senders placed in the Nursery are started only when the Nursery is started.

In many ways it is like the whenAll, except as an object. That allows it to be passed around and for work to be registered into it dynamically.

Cancellation

Cancellation is a very important aspect of asynchronous work. So much so that it is baked-in into the Receiver's API. It is a cooperative mechanism however, and it requires each Sender to respond to requests. In response to a stop request a Sender should call the setDone function, so that any downstream work is cancelled as well. However, in case of a race, it is perfectly fine to terminate with any of the other termination functions.

See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2175r0.html for a thorough explanation for why we need stop tokens in particular and cancellation in general.

Cancellation requests happen through the use of a StopSource and a StopToken. Each Sender should request a StopToken from its Receiver by calling getStopToken. Senders should respond to cancellation through polling the StopToken's isStopRequested method or calling the onStop method to attach a callback. Note that the callback might be called immediately in case a stop has already been requested.

By default the syncWait call will create a StopSource and a StopToken. The StopSource will be connected to any enclosing syncWait operation or otherwise to the globalStopSource. You can supply a StopSource to the syncWait function explicitely, but note that it won't be connected to any enclosing StopSource, for which you are responsible yourself.

Signals and termination

By default the library sets up a signal handler on the first use of syncWait. It spins up a dedicated thread to listen for both SIGINT and SIGTERM. Either signal causes the globalStopSource's stop method to be called on that dedicated thread.

All calls to syncWait that do not supply a StopSource explicitely will create a StopSource which will be connected to the globalStopSource, or, in the case of nested syncWaits, to the parent StopSource.

This ensures that by default both SIGINT and SIGTERM will cancel all work.

This behavior can be overridden by calling setGlobalStopSource before any call to syncWait. If you do so you are responsible for setting up signal handlers yourself. See the functions in signal.d, specifically setupCtrlCHandler.

Additional methods of termination

In certain scenarios you want to have additional ways to terminate outstanding work.

Simply calling globalStopSource().stop() will cause any work to be cancelled. It will use the current thread to run all stop callbacks.

If you want the termination to happen asynchronously, for instance because the current thread is not async-safe, you can call SignalHandler.notify(SIGINT). Note that this does rely on SignalHandler.launchHandlerThread to be called at one point. This happens by default unless you call setGlobalStopSource and it returns true. In that case you need to call SignalHandler.launchHandlerThread yourself too. See the functions in signal.d.

Dynamic libraries

The concurrency library is designed to work with dynamic libraries. It exports 2 functions that it uses to load important globals and thread-locals from the host process.

On Posix, the only requirement is that the linker you are using supports --export-dynamic-symbol (at least gold, lld do).

On Windows, you need to export the 2 symbols explicitly from the executable to make the whole process (i.e., DLLs with statically linked concurrency) use these. This can be achieved by adding according linker flags for the executable, e.g., in dub.sdl:

lflags "/EXPORT:concurrency_getLocalThreadExecutor" "/EXPORT:concurrency_globalStopSourcePointer" platform="windows"

DSemver

This package uses dsemver to calculate the next semantic version.

run dub run dsemver@1.1.0 -- -p $(pwd) -c to calcuate the next version.