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Wake the task up when inserting a new response into PendingResponses #1735

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Wake the task up when inserting a new response into PendingResponses #1735

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2a97b8a
Wake the task up when pushing a new response into `PendingResponses`
dmitry-markin Sep 28, 2023
6b9eca8
minor: add missing import
dmitry-markin Sep 28, 2023
a61f47d
minor: add missing init
dmitry-markin Sep 28, 2023
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21 changes: 15 additions & 6 deletions substrate/client/network/sync/src/pending_responses.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,7 @@
// along with this program. If not, see <https://www.gnu.org/licenses/>.

//! [`PendingResponses`] is responsible for keeping track of pending responses and
//! polling them.
//! polling them. The [`Stream`] implemented by [`PendingResponses`] never terminates.

use futures::{
channel::oneshot,
Expand All @@ -30,7 +30,7 @@ use log::error;
use sc_network::request_responses::RequestFailure;
use sc_network_common::sync::PeerRequest;
use sp_runtime::traits::Block as BlockT;
use std::task::{Context, Poll};
use std::task::{Context, Poll, Waker};
use tokio_stream::StreamMap;

/// Response result.
Expand All @@ -50,11 +50,12 @@ pub(crate) struct ResponseEvent<B: BlockT> {
pub(crate) struct PendingResponses<B: BlockT> {
/// Pending responses
pending_responses: StreamMap<PeerId, BoxStream<'static, (PeerRequest<B>, ResponseResult)>>,
waker: Option<Waker>,
}

impl<B: BlockT> PendingResponses<B> {
pub fn new() -> Self {
Self { pending_responses: StreamMap::new() }
Self { pending_responses: StreamMap::new(), waker: None }
}

pub fn insert(
Expand All @@ -79,6 +80,10 @@ impl<B: BlockT> PendingResponses<B> {
);
debug_assert!(false);
}

if let Some(waker) = self.waker.take() {
waker.wake();
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This requires that we called poll before. Not sure that this is removing any footgun.

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Looking at the code we are already polling pending_responses after we inserted any kind of element, so why is the task not waking up?

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This requires that we called poll before. Not sure that this is removing any footgun.

Yeah, but we definitely called it at least once.

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Looking at the code we are already polling pending_responses after we inserted any kind of element, so why is the task not waking up?

There is no problem with waking up right now, but it can shoot us once the polling in SyncingEngine is converted into select! and the order of inserting / polling pending_responses is not strictly defined.

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But for a select!. Once you insert data to the pending_responses your match arm will be finished and you start again from the top of the loop, aka calling select! again. This will poll all futures, including your pending_responses futures. So, not sure why we could miss there to poll it.

}
}

pub fn remove(&mut self, peer_id: &PeerId) -> bool {
Expand All @@ -99,16 +104,20 @@ impl<B: BlockT> Stream for PendingResponses<B> {
mut self: std::pin::Pin<&mut Self>,
cx: &mut Context<'_>,
) -> Poll<Option<Self::Item>> {
match futures::ready!(self.pending_responses.poll_next_unpin(cx)) {
Some((peer_id, (request, response))) => {
match self.pending_responses.poll_next_unpin(cx) {
Poll::Ready(Some((peer_id, (request, response)))) => {
// We need to manually remove the stream, because `StreamMap` doesn't know yet that
// it's going to yield `None`, so may not remove it before the next request is made
// to the same peer.
self.pending_responses.remove(&peer_id);

Poll::Ready(Some(ResponseEvent { peer_id, request, response }))
},
None => Poll::Ready(None),
Poll::Ready(None) | Poll::Pending => {
self.waker = Some(cx.waker().clone());

Poll::Pending
},
}
}
}
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