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//! A cache of services.
use super::error;
use futures_core::Stream;
use futures_util::{stream::FuturesUnordered, task::AtomicWaker};
pub use indexmap::Equivalent;
use indexmap::IndexMap;
use std::fmt;
use std::future::Future;
use std::hash::Hash;
use std::pin::Pin;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use std::task::{Context, Poll};
use tower_service::Service;
use tracing::{debug, trace};
/// Drives readiness over a set of services.
///
/// The cache maintains two internal data structures:
///
/// * a set of _pending_ services that have not yet become ready; and
/// * a set of _ready_ services that have previously polled ready.
///
/// As each `S` typed [`Service`] is added to the cache via [`ReadyCache::push`], it
/// is added to the _pending set_. As [`ReadyCache::poll_pending`] is invoked,
/// pending services are polled and added to the _ready set_.
///
/// [`ReadyCache::call_ready`] (or [`ReadyCache::call_ready_index`]) dispatches a
/// request to the specified service, but panics if the specified service is not
/// in the ready set. The `ReadyCache::check_*` functions can be used to ensure
/// that a service is ready before dispatching a request.
///
/// The ready set can hold services for an abitrarily long time. During this
/// time, the runtime may process events that invalidate that ready state (for
/// instance, if a keepalive detects a lost connection). In such cases, callers
/// should use [`ReadyCache::check_ready`] (or [`ReadyCache::check_ready_index`])
/// immediately before dispatching a request to ensure that the service has not
/// become unavailable.
///
/// Once `ReadyCache::call_ready*` is invoked, the service is placed back into
/// the _pending_ set to be driven to readiness again.
///
/// When `ReadyCache::check_ready*` returns `false`, it indicates that the
/// specified service is _not_ ready. If an error is returned, this indicats that
/// the server failed and has been removed from the cache entirely.
///
/// [`ReadyCache::evict`] can be used to remove a service from the cache (by key),
/// though the service may not be dropped (if it is currently pending) until
/// [`ReadyCache::poll_pending`] is invoked.
///
/// Note that the by-index accessors are provided to support use cases (like
/// power-of-two-choices load balancing) where the caller does not care to keep
/// track of each service's key. Instead, it needs only to access _some_ ready
/// service. In such a case, it should be noted that calls to
/// [`ReadyCache::poll_pending`] and [`ReadyCache::evict`] may perturb the order of
/// the ready set, so any cached indexes should be discarded after such a call.
pub struct ReadyCache<K, S, Req>
where
K: Eq + Hash,
{
/// A stream of services that are not yet ready.
pending: FuturesUnordered<Pending<K, S, Req>>,
/// An index of cancelation handles for pending streams.
pending_cancel_txs: IndexMap<K, CancelTx>,
/// Services that have previously become ready. Readiness can become stale,
/// so a given service should be polled immediately before use.
///
/// The cancelation oneshot is preserved (though unused) while the service is
/// ready so that it need not be reallocated each time a request is
/// dispatched.
ready: IndexMap<K, (S, CancelPair)>,
}
// Safety: This is safe because we do not use `Pin::new_unchecked`.
impl<S, K: Eq + Hash, Req> Unpin for ReadyCache<K, S, Req> {}
#[derive(Debug)]
struct Cancel {
waker: AtomicWaker,
canceled: AtomicBool,
}
#[derive(Debug)]
struct CancelRx(Arc<Cancel>);
#[derive(Debug)]
struct CancelTx(Arc<Cancel>);
type CancelPair = (CancelTx, CancelRx);
#[derive(Debug)]
enum PendingError<K, E> {
Canceled(K),
Inner(K, E),
}
pin_project_lite::pin_project! {
/// A [`Future`] that becomes satisfied when an `S`-typed service is ready.
///
/// May fail due to cancelation, i.e. if the service is evicted from the balancer.
struct Pending<K, S, Req> {
key: Option<K>,
cancel: Option<CancelRx>,
ready: Option<S>,
_pd: std::marker::PhantomData<Req>,
}
}
// === ReadyCache ===
impl<K, S, Req> Default for ReadyCache<K, S, Req>
where
K: Eq + Hash,
S: Service<Req>,
{
fn default() -> Self {
Self {
ready: IndexMap::default(),
pending: FuturesUnordered::new(),
pending_cancel_txs: IndexMap::default(),
}
}
}
impl<K, S, Req> fmt::Debug for ReadyCache<K, S, Req>
where
K: fmt::Debug + Eq + Hash,
S: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let Self {
pending,
pending_cancel_txs,
ready,
} = self;
f.debug_struct("ReadyCache")
.field("pending", pending)
.field("pending_cancel_txs", pending_cancel_txs)
.field("ready", ready)
.finish()
}
}
impl<K, S, Req> ReadyCache<K, S, Req>
where
K: Eq + Hash,
{
/// Returns the total number of services in the cache.
pub fn len(&self) -> usize {
self.ready_len() + self.pending_len()
}
/// Returns whether or not there are any services in the cache.
pub fn is_empty(&self) -> bool {
self.ready.is_empty() && self.pending.is_empty()
}
/// Returns the number of services in the ready set.
pub fn ready_len(&self) -> usize {
self.ready.len()
}
/// Returns the number of services in the unready set.
pub fn pending_len(&self) -> usize {
self.pending.len()
}
/// Returns true iff the given key is in the unready set.
pub fn pending_contains<Q: Hash + Equivalent<K>>(&self, key: &Q) -> bool {
self.pending_cancel_txs.contains_key(key)
}
/// Obtains a reference to a service in the ready set by key.
pub fn get_ready<Q: Hash + Equivalent<K>>(&self, key: &Q) -> Option<(usize, &K, &S)> {
self.ready.get_full(key).map(|(i, k, v)| (i, k, &v.0))
}
/// Obtains a mutable reference to a service in the ready set by key.
pub fn get_ready_mut<Q: Hash + Equivalent<K>>(
&mut self,
key: &Q,
) -> Option<(usize, &K, &mut S)> {
self.ready
.get_full_mut(key)
.map(|(i, k, v)| (i, k, &mut v.0))
}
/// Obtains a reference to a service in the ready set by index.
pub fn get_ready_index(&self, idx: usize) -> Option<(&K, &S)> {
self.ready.get_index(idx).map(|(k, v)| (k, &v.0))
}
/// Obtains a mutable reference to a service in the ready set by index.
pub fn get_ready_index_mut(&mut self, idx: usize) -> Option<(&mut K, &mut S)> {
self.ready.get_index_mut(idx).map(|(k, v)| (k, &mut v.0))
}
/// Evicts an item from the cache.
///
/// Returns true if a service was marked for eviction.
///
/// Services are dropped from the ready set immediately. Services in the
/// pending set are marked for cancellation, but [`ReadyCache::poll_pending`]
/// must be called to cause the service to be dropped.
pub fn evict<Q: Hash + Equivalent<K>>(&mut self, key: &Q) -> bool {
let canceled = if let Some(c) = self.pending_cancel_txs.swap_remove(key) {
c.cancel();
true
} else {
false
};
self.ready
.swap_remove_full(key)
.map(|_| true)
.unwrap_or(canceled)
}
}
impl<K, S, Req> ReadyCache<K, S, Req>
where
K: Clone + Eq + Hash,
S: Service<Req>,
<S as Service<Req>>::Error: Into<crate::BoxError>,
S::Error: Into<crate::BoxError>,
{
/// Pushes a new service onto the pending set.
///
/// The service will be promoted to the ready set as [`poll_pending`] is invoked.
///
/// Note that this does **not** remove services from the ready set. Once the
/// old service is used, it will be dropped instead of being added back to
/// the pending set; OR, when the new service becomes ready, it will replace
/// the prior service in the ready set.
///
/// [`poll_pending`]: crate::ready_cache::cache::ReadyCache::poll_pending
pub fn push(&mut self, key: K, svc: S) {
let cancel = cancelable();
self.push_pending(key, svc, cancel);
}
fn push_pending(&mut self, key: K, svc: S, (cancel_tx, cancel_rx): CancelPair) {
if let Some(c) = self.pending_cancel_txs.insert(key.clone(), cancel_tx) {
// If there is already a service for this key, cancel it.
c.cancel();
}
self.pending.push(Pending {
key: Some(key),
cancel: Some(cancel_rx),
ready: Some(svc),
_pd: std::marker::PhantomData,
});
}
/// Polls services pending readiness, adding ready services to the ready set.
///
/// Returns [`Poll::Ready`] when there are no remaining unready services.
/// [`poll_pending`] should be called again after [`push`] or
/// [`call_ready_index`] are invoked.
///
/// Failures indicate that an individual pending service failed to become
/// ready (and has been removed from the cache). In such a case,
/// [`poll_pending`] should typically be called again to continue driving
/// pending services to readiness.
///
/// [`poll_pending`]: crate::ready_cache::cache::ReadyCache::poll_pending
/// [`push`]: crate::ready_cache::cache::ReadyCache::push
/// [`call_ready_index`]: crate::ready_cache::cache::ReadyCache::call_ready_index
pub fn poll_pending(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), error::Failed<K>>> {
loop {
match Pin::new(&mut self.pending).poll_next(cx) {
Poll::Pending => return Poll::Pending,
Poll::Ready(None) => return Poll::Ready(Ok(())),
Poll::Ready(Some(Ok((key, svc, cancel_rx)))) => {
trace!("endpoint ready");
let cancel_tx = self.pending_cancel_txs.swap_remove(&key);
if let Some(cancel_tx) = cancel_tx {
// Keep track of the cancelation so that it need not be
// recreated after the service is used.
self.ready.insert(key, (svc, (cancel_tx, cancel_rx)));
} else {
assert!(
cancel_tx.is_some(),
"services that become ready must have a pending cancelation"
);
}
}
Poll::Ready(Some(Err(PendingError::Canceled(_)))) => {
debug!("endpoint canceled");
// The cancellation for this service was removed in order to
// cause this cancellation.
}
Poll::Ready(Some(Err(PendingError::Inner(key, e)))) => {
let cancel_tx = self.pending_cancel_txs.swap_remove(&key);
assert!(
cancel_tx.is_some(),
"services that return an error must have a pending cancelation"
);
return Err(error::Failed(key, e.into())).into();
}
}
}
}
/// Checks whether the referenced endpoint is ready.
///
/// Returns true if the endpoint is ready and false if it is not. An error is
/// returned if the endpoint fails.
pub fn check_ready<Q: Hash + Equivalent<K>>(
&mut self,
cx: &mut Context<'_>,
key: &Q,
) -> Result<bool, error::Failed<K>> {
match self.ready.get_full_mut(key) {
Some((index, _, _)) => self.check_ready_index(cx, index),
None => Ok(false),
}
}
/// Checks whether the referenced endpoint is ready.
///
/// If the service is no longer ready, it is moved back into the pending set
/// and `false` is returned.
///
/// If the service errors, it is removed and dropped and the error is returned.
pub fn check_ready_index(
&mut self,
cx: &mut Context<'_>,
index: usize,
) -> Result<bool, error::Failed<K>> {
let svc = match self.ready.get_index_mut(index) {
None => return Ok(false),
Some((_, (svc, _))) => svc,
};
match svc.poll_ready(cx) {
Poll::Ready(Ok(())) => Ok(true),
Poll::Pending => {
// became unready; so move it back there.
let (key, (svc, cancel)) = self
.ready
.swap_remove_index(index)
.expect("invalid ready index");
// If a new version of this service has been added to the
// unready set, don't overwrite it.
if !self.pending_contains(&key) {
self.push_pending(key, svc, cancel);
}
Ok(false)
}
Poll::Ready(Err(e)) => {
// failed, so drop it.
let (key, _) = self
.ready
.swap_remove_index(index)
.expect("invalid ready index");
Err(error::Failed(key, e.into()))
}
}
}
/// Calls a ready service by key.
///
/// # Panics
///
/// If the specified key does not exist in the ready
pub fn call_ready<Q: Hash + Equivalent<K>>(&mut self, key: &Q, req: Req) -> S::Future {
let (index, _, _) = self
.ready
.get_full_mut(key)
.expect("check_ready was not called");
self.call_ready_index(index, req)
}
/// Calls a ready service by index.
///
/// # Panics
///
/// If the specified index is out of range.
pub fn call_ready_index(&mut self, index: usize, req: Req) -> S::Future {
let (key, (mut svc, cancel)) = self
.ready
.swap_remove_index(index)
.expect("check_ready_index was not called");
let fut = svc.call(req);
// If a new version of this service has been added to the
// unready set, don't overwrite it.
if !self.pending_contains(&key) {
self.push_pending(key, svc, cancel);
}
fut
}
}
// === impl Cancel ===
/// Creates a cancelation sender and receiver.
///
/// A `tokio::sync::oneshot` is NOT used, as a `Receiver` is not guaranteed to
/// observe results as soon as a `Sender` fires. Using an `AtomicBool` allows
/// the state to be observed as soon as the cancelation is triggered.
fn cancelable() -> CancelPair {
let cx = Arc::new(Cancel {
waker: AtomicWaker::new(),
canceled: AtomicBool::new(false),
});
(CancelTx(cx.clone()), CancelRx(cx))
}
impl CancelTx {
fn cancel(self) {
self.0.canceled.store(true, Ordering::SeqCst);
self.0.waker.wake();
}
}
// === Pending ===
impl<K, S, Req> Future for Pending<K, S, Req>
where
S: Service<Req>,
{
type Output = Result<(K, S, CancelRx), PendingError<K, S::Error>>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.project();
// Before checking whether the service is ready, check to see whether
// readiness has been canceled.
let CancelRx(cancel) = this.cancel.as_mut().expect("polled after complete");
if cancel.canceled.load(Ordering::SeqCst) {
let key = this.key.take().expect("polled after complete");
return Err(PendingError::Canceled(key)).into();
}
match this
.ready
.as_mut()
.expect("polled after ready")
.poll_ready(cx)
{
Poll::Pending => {
// Before returning Pending, register interest in cancelation so
// that this future is polled again if the state changes.
let CancelRx(cancel) = this.cancel.as_mut().expect("polled after complete");
cancel.waker.register(cx.waker());
// Because both the cancel receiver and cancel sender are held
// by the `ReadyCache` (i.e., on a single task), then it must
// not be possible for the cancelation state to change while
// polling a `Pending` service.
assert!(
!cancel.canceled.load(Ordering::SeqCst),
"cancelation cannot be notified while polling a pending service"
);
Poll::Pending
}
Poll::Ready(Ok(())) => {
let key = this.key.take().expect("polled after complete");
let cancel = this.cancel.take().expect("polled after complete");
Ok((key, this.ready.take().expect("polled after ready"), cancel)).into()
}
Poll::Ready(Err(e)) => {
let key = this.key.take().expect("polled after compete");
Err(PendingError::Inner(key, e)).into()
}
}
}
}
impl<K, S, Req> fmt::Debug for Pending<K, S, Req>
where
K: fmt::Debug,
S: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let Self {
key,
cancel,
ready,
_pd,
} = self;
f.debug_struct("Pending")
.field("key", key)
.field("cancel", cancel)
.field("ready", ready)
.finish()
}
}