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Document the correctness of Poll::Pending wakeups

This commit is contained in:
teor 2021-03-27 13:50:46 +10:00 committed by Deirdre Connolly
parent 829a6f11c5
commit 306fa88214
9 changed files with 100 additions and 8 deletions
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7
tower-batch/src/future.rs
View File

@ -49,6 +49,13 @@ where
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut this = self.project();
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`.
//
// This loop ensures that the task is scheduled as required, because it
// only returns Pending when another future returns Pending.
loop {
match this.state.as_mut().project() {
ResponseStateProj::Failed(e) => {

19
tower-batch/src/service.rs
View File

@ -134,12 +134,19 @@ where
return Poll::Ready(Err(self.get_worker_error()));
}
// Then, poll to acquire a semaphore permit. If we acquire a permit,
// then there's enough buffer capacity to send a new request. Otherwise,
// we need to wait for capacity.
// In tokio 0.3.7, `acquire_owned` panics if its semaphore returns an error,
// so we don't need to handle errors until we upgrade to tokio 1.0.
// CORRECTNESS
//
// Poll to acquire a semaphore permit. If we acquire a permit, then
// there's enough buffer capacity to send a new request. Otherwise, we
// need to wait for capacity.
//
// In tokio 0.3.7, `acquire_owned` panics if its semaphore returns an
// error, so we don't need to handle errors until we upgrade to
// tokio 1.0.
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`. If it returns Pending, the semaphore also schedules
// the task for wakeup when the next permit is available.
ready!(self.semaphore.poll_acquire(cx));
Poll::Ready(Ok(()))

7
tower-fallback/src/future.rs
View File

@ -74,6 +74,13 @@ where
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut this = self.project();
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`.
//
// This loop ensures that the task is scheduled as required, because it
// only returns Pending when a future or service returns Pending.
loop {
match this.state.as_mut().project() {
ResponseStateProj::PollResponse1 { fut, .. } => match ready!(fut.poll(cx)) {

8
zebra-consensus/src/chain.rs
View File

@ -71,7 +71,13 @@ where
Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>> + Send + 'static>>;
fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
// Correctness:
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`.
//
// If either verifier is unready, this task is scheduled for wakeup when it becomes
// ready.
//
// We acquire checkpoint readiness before block readiness, to avoid an unlikely
// hang during the checkpoint to block verifier transition. If the checkpoint and

21
zebra-network/src/peer/client.rs
View File

@ -113,7 +113,13 @@ impl Stream for ClientRequestReceiver {
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
match self.inner.poll_next_unpin(cx) {
Poll::Ready(client_request) => Poll::Ready(client_request.map(Into::into)),
// `inner.poll_next_unpin` parks the task for this future
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we
// return `Poll::Pending`.
//
// inner.poll_next_unpin` schedules this task for wakeup when
// there are new items available in the inner stream.
Poll::Pending => Poll::Pending,
}
}
@ -198,6 +204,19 @@ impl Service<Request> for Client {
Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>> + Send + 'static>>;
fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`.
//
//`ready!` returns `Poll::Pending` when `server_tx` is unready, and
// schedules this task for wakeup.
//
// Since `shutdown_tx` is used for oneshot communication to the heartbeat
// task, it will never be `Pending`.
//
// TODO: should the Client exit if the heartbeat task exits and drops
// `shutdown_tx`?
if ready!(self.server_tx.poll_ready(cx)).is_err() {
Poll::Ready(Err(self
.error_slot

19
zebra-network/src/peer_set/set.rs
View File

@ -464,6 +464,25 @@ where
if self.preselected_p2c_index.is_none() {
trace!("no ready services, sending demand signal");
let _ = self.demand_signal.try_send(());
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we
// return `Poll::Pending`.
//
// As long as there are unready or new peers, this task will run,
// because:
// - `poll_discover` schedules this task for wakeup when new
// peers arrive.
// - if there are unready peers, `poll_unready` schedules this
// task for wakeup when peer services become ready.
// - if the preselected peer is not ready, `service.poll_ready`
// schedules this task for wakeup when that service becomes
// ready.
//
// To avoid peers blocking on a full background error channel:
// - if no background tasks have exited since the last poll,
// `poll_background_errors` schedules this task for wakeup when
// the next task exits.
return Poll::Pending;
}
}

9
zebra-network/src/peer_set/unready_service.rs
View File

@ -40,6 +40,15 @@ impl<K, S: Service<Req>, Req> Future for UnreadyService<K, S, Req> {
return Poll::Ready(Err((key, Error::Canceled)));
}
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`.
//
//`ready!` returns `Poll::Pending` when the service is unready, and
// schedules this task for wakeup.
//
// `cancel.poll` also schedules this task for wakeup if it is canceled.
let res = ready!(this
.service
.as_mut()

9
zebrad/src/components/inbound/downloads.rs
View File

@ -108,6 +108,15 @@ where
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
let this = self.project();
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`.
//
// If no download and verify tasks have exited since the last poll, this
// task is scheduled for wakeup when the next task becomes ready.
//
// TODO:
// This would be cleaner with poll_map #63514, but that's nightly only.
if let Some(join_result) = ready!(this.pending.poll_next(cx)) {
match join_result.expect("block download and verify tasks must not panic") {

9
zebrad/src/components/sync/downloads.rs
View File

@ -72,6 +72,15 @@ where
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
let this = self.project();
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`.
//
// If no download and verify tasks have exited since the last poll, this
// task is scheduled for wakeup when the next task becomes ready.
//
// TODO:
// This would be cleaner with poll_map #63514, but that's nightly only.
if let Some(join_result) = ready!(this.pending.poll_next(cx)) {
match join_result.expect("block download and verify tasks must not panic") {