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#![recursion_limit = "256"] #![deny(missing_docs)] //! # A leaky-bucket rate limiter //! //! This implements a leaky bucket from which you can acquire tokens. //! //! If the tokens are already available, the acquisition will be instant (fast path) and the //! acquired number of tokens will be added to the bucket. //! //! If the bucket overflows (i.e. goes over max), the task that tried to acquire the tokens will //! be suspended until the required number of tokens has been added. //! //! ## Example //! //! If the project is built with the `static` feature (default), you can use //! `LeakyBucket` directly as long as you are inside a tokio runtime, like so: //! //! ```no_run //! use leaky_bucket::LeakyBucket; //! use std::{error::Error, time::Duration}; //! //! #[tokio::main] //! async fn main() -> Result<(), Box<dyn Error>> { //! let rate_limiter = LeakyBucket::builder() //! .max(5) //! .tokens(5) //! .build()?; //! //! println!("Waiting for permit..."); //! // should take about 5 seconds to acquire. //! rate_limiter.acquire(10).await?; //! println!("I made it!"); //! Ok(()) //! } //! ``` //! //! ## Example using explicit coordinator //! //! ```no_run //! use leaky_bucket::LeakyBuckets; //! use std::{error::Error, time::Duration}; //! //! #[tokio::main] //! async fn main() -> Result<(), Box<dyn Error>> { //! let mut buckets = LeakyBuckets::new(); //! let coordinator = buckets.coordinate()?; //! // spawn the coordinate thread to refill the rate limiter. //! tokio::spawn(async move { coordinator.await.expect("coordinate thread errored") }); //! //! let rate_limiter = buckets //! .rate_limiter() //! .max(5) //! .tokens(5) //! .build()?; //! //! println!("Waiting for permit..."); //! // should take about 5 seconds to acquire. //! rate_limiter.acquire(10).await?; //! println!("I made it!"); //! Ok(()) //! } //! ``` use futures_channel::mpsc; use futures_util::{ ready, select, stream::{FuturesUnordered, StreamExt as _}, }; use std::{ collections::VecDeque, future::Future, pin::Pin, sync::{ atomic::{AtomicBool, AtomicUsize, Ordering}, Arc, }, task::{Context, Poll, Waker}, time::Duration, }; use thiserror::Error; #[cfg(feature = "static")] lazy_static::lazy_static! { static ref LEAKY_BUCKETS: LeakyBuckets = { let mut buckets = LeakyBuckets::new(); let coordinator = buckets.coordinate().expect("no other running coordinator"); tokio::spawn(async move { coordinator.await.expect("coordinate thread errored") }); buckets }; } /// Error type for the rate limiter. #[derive(Debug, Error)] #[non_exhaustive] pub enum Error { /// The bucket has already been started. #[error("Coordinator already started")] AlreadyStarted, /// There was an issue enqueueing a task. #[error("Failed to send task to coordinator: {0}")] TaskSendError(#[source] mpsc::SendError), /// Failed to queue up new task. #[error("Failed to queue up new task coordinator")] NewTaskError, /// Tried to acquire more tokens than what is possible. #[error("Acquiring tokens would cause an overflow")] TokenOverflow, } struct NewTask { inner: Arc<Inner>, task_rx: mpsc::Receiver<Task>, } struct LeakyBucketsInner { new_task_tx: mpsc::UnboundedSender<NewTask>, } /// Coordinator for rate limiters. Is used to create new rate limiters as needed. pub struct LeakyBuckets { inner: Arc<LeakyBucketsInner>, new_task_rx: Option<mpsc::UnboundedReceiver<NewTask>>, } impl Default for LeakyBuckets { fn default() -> Self { Self::new() } } impl LeakyBuckets { /// Construct a new coordinator for rate limiters. pub fn new() -> Self { let (new_task_tx, new_task_rx) = mpsc::unbounded(); let inner = Arc::new(LeakyBucketsInner { new_task_tx }); LeakyBuckets { inner, new_task_rx: Some(new_task_rx), } } /// Run the coordinator. pub fn coordinate( &mut self, ) -> Result<impl Future<Output = Result<(), Error>> + 'static, Error> { let mut new_task_rx = match self.new_task_rx.take() { Some(new_task_rx) => new_task_rx, None => return Err(Error::AlreadyStarted), }; Ok(async move { let mut futures = FuturesUnordered::new(); loop { while futures.is_empty() { select! { NewTask { inner, task_rx } = new_task_rx.select_next_some() => { futures.push(inner.coordinate(task_rx)); } } } select! { _ = futures.next() => { panic!("coordinator task exited unexpectedly"); } NewTask { inner, task_rx } = new_task_rx.select_next_some() => { futures.push(inner.coordinate(task_rx)); } } } }) } /// Construct a new rate limiter. pub fn rate_limiter(&self) -> Builder<'_> { Builder { new_task_tx: &self.inner.new_task_tx, tokens: None, max: None, refill_interval: None, refill_amount: None, } } } /// Builder for a leaky bucket. pub struct Builder<'a> { new_task_tx: &'a mpsc::UnboundedSender<NewTask>, tokens: Option<usize>, max: Option<usize>, refill_interval: Option<Duration>, refill_amount: Option<usize>, } impl Builder<'_> { /// Set the max value for the builder. #[inline(always)] pub fn max(mut self, max: usize) -> Self { self.max = Some(max); self } /// The number of tokens that the bucket should start with. /// /// If set to larger than `max` at build time, will only saturate to max. #[inline(always)] pub fn tokens(mut self, tokens: usize) -> Self { self.tokens = Some(tokens); self } /// Set the max value for the builder. #[inline(always)] pub fn refill_interval(mut self, refill_interval: Duration) -> Self { self.refill_interval = Some(refill_interval); self } /// Set the refill amount to use. #[inline(always)] pub fn refill_amount(mut self, refill_amount: usize) -> Self { self.refill_amount = Some(refill_amount); self } /// Construct a new leaky bucket. pub fn build(self) -> Result<LeakyBucket, Error> { const DEFAULT_MAX: usize = 120; const DEFAULT_TOKENS: usize = 0; const DEFAULT_REFILL_INTERVAL: Duration = Duration::from_secs(1); const DEFAULT_REFILL_AMOUNT: usize = 1; let max = self.max.unwrap_or(DEFAULT_MAX); let tokens = max.saturating_sub(self.tokens.unwrap_or(DEFAULT_TOKENS)); let refill_interval = self.refill_interval.unwrap_or(DEFAULT_REFILL_INTERVAL); let refill_amount = self.refill_amount.unwrap_or(DEFAULT_REFILL_AMOUNT); let tokens = AtomicUsize::new(tokens); let (task_tx, task_rx) = mpsc::channel(1); let inner = Arc::new(Inner { tokens, max, refill_interval, refill_amount, task_tx, }); self.new_task_tx .clone() .unbounded_send(NewTask { inner: inner.clone(), task_rx, }) .map_err(|_| Error::NewTaskError)?; Ok(LeakyBucket { inner }) } } /// A single queued task waiting to be woken up. struct Task { /// Amount required to wake up the given task. required: usize, /// Waker to call. waker: Waker, /// Indicates if the task is completed. complete: Arc<AtomicBool>, } struct Inner { /// Current number of tokens that have been acquired. /// /// This has an inverted relationship to `max`. /// The current number of active tokens can be found be subtracting the /// the number of acquire tokens (this), from max. /// /// It is structure like this since adding to an atomic growing number to /// acquire tokens is much easier than trying to subtract and truncate it /// towards zero. /// /// This means that there is a risk the value will overflow. tokens: AtomicUsize, /// Max number of tokens. max: usize, /// Period to use when refilling. refill_interval: Duration, /// Amount to add when refilling. refill_amount: usize, /// Sender for emitting queued tasks. task_tx: mpsc::Sender<Task>, } impl std::fmt::Debug for Inner { fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { fmt.debug_struct("LeakyBucket") .field("tokens", &self.tokens.load(Ordering::SeqCst)) .field("max", &self.max) .field("refill_interval", &self.refill_interval) .field("refill_amount", &self.refill_amount) .finish() } } impl Inner { /// Coordinate tasks. async fn coordinate(self: Arc<Inner>, mut task_rx: mpsc::Receiver<Task>) -> Result<(), Error> { // The queue of tasks to process. let mut tasks = VecDeque::new(); let first = std::time::Instant::now() .checked_add(self.refill_interval) .unwrap_or_else(std::time::Instant::now); // The interval at which we refill tokens. let mut interval = tokio::time::interval_at(first.into(), self.refill_interval).fuse(); // The current number of tokens accumulated locally. // This will increase until we have enough to satisfy the next waking task. let mut amount = 0; let mut current = None; 'outer: loop { select! { waker = task_rx.select_next_some() => { tasks.push_back(waker); }, _ = interval.select_next_some() => { amount += self.refill_amount; let mut task = match current.take().or_else(|| tasks.pop_front()) { Some(task) => task, None => { // Nothing to wake up, subtract the number of // tokens immediately allowing future acquires to // enter the fast path. self.balance_tokens(amount); amount = 0; continue; } }; while amount >= task.required { // We have enough tokens to wake up the next task. // Subtract it from the current amount and notify the task to wake up. amount -= task.required; task.complete.store(true, Ordering::Release); task.waker.wake(); task = match tasks.pop_front() { Some(task) => task, None => { self.balance_tokens(amount); amount = 0; continue 'outer; }, }; } current = Some(task); }, } } } /// Subtract the given amount of tokens, allowing them to be used by the fast path acquire. fn balance_tokens(&self, amount: usize) { if amount == 0 { return; } let mut current = self.tokens.load(Ordering::Acquire); while current > 0 { let new = current.saturating_sub(amount); match self.tokens.compare_exchange_weak( current, new, Ordering::SeqCst, Ordering::Acquire, ) { Ok(_) => break, Err(x) => current = x, } } } } /// The leaky bucket. #[derive(Clone)] pub struct LeakyBucket { inner: Arc<Inner>, } impl std::fmt::Debug for LeakyBucket { fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { self.inner.fmt(f) } } impl LeakyBucket { /// Construct a new leaky bucket through a builder. #[cfg(feature = "static")] pub fn builder() -> Builder<'static> { LEAKY_BUCKETS.rate_limiter() } /// Query how many tokens are available. /// /// This is just a best-effort estimate, calling this to ensure that there /// are enough tokens available to avoid blocking does not guarantee that /// the acquire operation won't block. /// /// Tokens is always reported as less than or equal to `max`. /// /// # Example /// /// ```rust /// use leaky_bucket::LeakyBucket; /// use std::{error::Error, time::Duration}; /// /// #[tokio::main] /// async fn main() -> Result<(), Box<dyn Error>> { /// let rate_limiter = LeakyBucket::builder() /// .max(5) /// .tokens(0) /// .build()?; /// /// assert_eq!(0, rate_limiter.tokens()); /// /// println!("Waiting for permit..."); /// // should take about 5 seconds to acquire. /// rate_limiter.acquire_one().await?; /// println!("I made it!"); /// /// assert!(rate_limiter.tokens() >= 0 && rate_limiter.tokens() <= rate_limiter.max()); /// Ok(()) /// } /// ``` pub fn tokens(&self) -> usize { let tokens = self.inner.tokens.load(Ordering::Acquire); self.inner.max.saturating_sub(tokens) } /// Get the max number of tokens this rate limiter is configured for. #[inline] pub fn max(&self) -> usize { self.inner.max } /// Acquire a single token. /// /// This is identical to [`acquire`] with an argument of `1`. /// /// [`acquire`]: LeakyBucket::acquire /// /// # Example /// /// ```rust /// use leaky_bucket::LeakyBucket; /// use std::{error::Error, time::Duration}; /// /// #[tokio::main] /// async fn main() -> Result<(), Box<dyn Error>> { /// let rate_limiter = LeakyBucket::builder() /// .max(5) /// .tokens(0) /// .build()?; /// /// assert_eq!(0, rate_limiter.tokens()); /// /// println!("Waiting for permit..."); /// // should take about 5 seconds to acquire. /// rate_limiter.acquire_one().await?; /// println!("I made it!"); /// /// assert_eq!(0, rate_limiter.tokens()); /// Ok(()) /// } /// ``` #[inline] pub fn acquire_one(&self) -> Acquire<'_> { self.acquire(1) } /// Acquire the given `amount` of tokens. /// /// Note that you _are_ allowed to acquire more tokens than the current /// `max`, but the acquire will have to suspend the task until enough /// tokens has built up to satisfy the acquire request. /// /// # Example /// /// ```rust /// use leaky_bucket::LeakyBucket; /// use std::{error::Error, time::Duration}; /// /// #[tokio::main] /// async fn main() -> Result<(), Box<dyn Error>> { /// let rate_limiter = LeakyBucket::builder() /// .max(5) /// .tokens(5) /// .build()?; /// /// assert_eq!(5, rate_limiter.tokens()); /// /// println!("Waiting for permit..."); /// // should take about 5 seconds to acquire. /// rate_limiter.acquire(10).await?; /// println!("I made it!"); /// /// assert_eq!(0, rate_limiter.tokens()); /// Ok(()) /// } /// ``` /// /// # Errors /// /// The returned future will fail with an [`Error::TokenOverflow`] if the /// tracked number of tokens attempts to overflow the internal token /// counter, which is an `usize`. /// /// For this reason you should prefer to use smaller values in the amount /// acquired. /// /// [`Error::TokenOverflow`]: self::Error::TokenOverflow pub fn acquire(&self, amount: usize) -> Acquire<'_> { Acquire { inner: &self.inner, amount, in_progress: None, queued: None, } } } /// Future associated with acquiring a single token. pub struct Acquire<'a> { /// Reference to leaky bucket internals. inner: &'a Inner, /// The amount of tokens to acquire. amount: usize, // An owned Sender, used for sending the `Task` struct over. // Only created lazily when necessary. in_progress: Option<(mpsc::Sender<Task>, usize)>, // State of a queued task, or if it was woken up. // If defined, this needs to be checked when the task is woken to test if // the current acquire is awake or not. queued: Option<Arc<AtomicBool>>, } impl Future for Acquire<'_> { type Output = Result<(), Error>; fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { let Self { ref inner, ref mut in_progress, ref mut queued, amount, .. } = *self; // Test if it has been woken up by the coordinator. // If that is the case, complete should be `true`. // // Otherwise we are still pending. if let Some(complete) = queued { return if complete.load(Ordering::Acquire) { Poll::Ready(Ok(())) } else { Poll::Pending }; } loop { // handle send-in-progress. if let Some((ref mut tx, required)) = *in_progress { // queue up thread to be released once more tokens are available. if let Err(e) = ready!(tx.poll_ready(cx)) { return Poll::Ready(Err(Error::TaskSendError(e))); } let complete = Arc::new(AtomicBool::new(false)); *queued = Some(complete.clone()); let task = Task { required, waker: cx.waker().clone(), complete, }; if let Err(e) = tx.start_send(task) { return Poll::Ready(Err(Error::TaskSendError(e))); } return Poll::Pending; } let mut required = amount; let mut current = inner.tokens.load(Ordering::Acquire); let mut new; loop { new = match current.checked_add(required) { Some(new) => new, None => { return Poll::Ready(Err(Error::TokenOverflow)); } }; match inner.tokens.compare_exchange_weak( current, new, Ordering::SeqCst, Ordering::Acquire, ) { Ok(_) => break, Err(x) => { current = x; } } } // fast path, we successfully acquired the number of tokens needed to proceed. if new < inner.max { return Poll::Ready(Ok(())); } // Slow path: // // Subtract the number of tokens already consumed from required and // enqueue a task to be awoken when the given number of tokens is // available. if current < inner.max { required -= inner.max - current; } // hand back the sender for the next time. *in_progress = Some((inner.task_tx.clone(), required)); } } } #[cfg(test)] mod tests { use super::{Error, LeakyBuckets}; use futures::prelude::*; use std::time::{Duration, Instant}; use tokio::time; #[tokio::test] async fn test_debug() { let expected_debug = "LeakyBucket { tokens: 16, max: 20, refill_interval: 2s, refill_amount: 10 }"; let buckets = LeakyBuckets::new(); let leaky = buckets .rate_limiter() .tokens(5) .max(20) .refill_amount(10) .refill_interval(Duration::from_millis(2000)) .build() .expect("build rate limiter"); leaky.acquire_one().await.unwrap(); let actual_debug = format!("{:?}", leaky); assert_eq!(expected_debug, &actual_debug); } #[tokio::test] async fn test_leaky_bucket() { let interval = Duration::from_millis(20); let mut buckets = LeakyBuckets::new(); let leaky = buckets .rate_limiter() .tokens(0) .max(10) .refill_amount(10) .refill_interval(interval) .build() .expect("build rate limiter"); let mut wakeups = 0u32; let mut duration = None; let test = async { let start = Instant::now(); leaky.acquire(10).await?; wakeups += 1; leaky.acquire(10).await?; wakeups += 1; leaky.acquire(10).await?; wakeups += 1; duration = Some(Instant::now().duration_since(start)); Ok::<_, Error>(()) }; futures::future::select(test.boxed(), buckets.coordinate().unwrap().boxed()).await; assert_eq!(3, wakeups); assert!(duration.expect("expected measured duration") > interval * 2); } #[tokio::test] async fn test_concurrent_rate_limited() { let interval = Duration::from_millis(20); let mut buckets = LeakyBuckets::new(); let leaky = buckets .rate_limiter() .tokens(0) .max(10) .refill_amount(1) .refill_interval(interval) .build() .expect("build rate limiter"); let mut one_wakeups = 0; let one = async { loop { leaky.acquire(1).await?; one_wakeups += 1; } #[allow(unreachable_code)] Ok::<_, Error>(()) }; let mut two_wakeups = 0u32; let two = async { loop { leaky.acquire(1).await?; two_wakeups += 1; } #[allow(unreachable_code)] Ok::<_, Error>(()) }; let delay = time::delay_for(Duration::from_millis(200)); let task = future::select(one.boxed(), two.boxed()); let task = future::select(task, delay); future::select(task, buckets.coordinate().unwrap().boxed()).await; let total = one_wakeups + two_wakeups; assert!(total > 5 && total < 15); } }