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substream: Decrement the bytes counter to avoid excessive flushing #511

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Merged
lexnv merged 3 commits into from
Jan 20, 2026
Merged

substream: Decrement the bytes counter to avoid excessive flushing #511

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8f976bd
substream/fix: Decrement pending bytes to avoid unnecessary flush sys
lexnv Jan 13, 2026
7daebc5
substream/fix: Flush pending frames with the socket flush
lexnv Jan 14, 2026
5f2f4bf
Merge branch 'master' into lexnv/opt-len-enc
lexnv Jan 20, 2026
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14 changes: 13 additions & 1 deletion src/substream/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -677,7 +677,15 @@ impl Sink<Bytes> for Substream {
delegate_poll_ready!(&mut self.substream, cx);

if self.pending_out_bytes >= BACKPRESSURE_BOUNDARY {
return poll_flush!(&mut self.substream, cx).map_err(From::from);
// This attempts to empty 'pending_out_frames' into the socket.
match futures::Sink::poll_flush(self.as_mut(), cx) {
Poll::Ready(Ok(())) => {}
Poll::Ready(Err(e)) => return Poll::Ready(Err(e)),
Poll::Pending => {
// Still flushing. We cannot accept new data yet.
return Poll::Pending;
}
}
Comment on lines +680 to +688
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@dmitry-markin dmitry-markin Jan 15, 2026

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How is this different from poll_flush!?

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@lexnv lexnv Jan 20, 2026

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The poll_flush! should call into the socket / TCP under the hood via AsyncWrite::poll_flush.

By calling the futures::Sink::poll_flush we ensure our impl is always called and try to write any pending frames before calling poll_flush

}

Poll::Ready(Ok(()))
Expand Down Expand Up @@ -745,6 +753,10 @@ impl Sink<Bytes> for Substream {
Poll::Ready(Ok(nwritten)) => {
pending_frame.advance(nwritten);

// The number of pending bytes is reduced by the number of bytes written
// to ensure that backpressure is properly handled.
self.pending_out_bytes = self.pending_out_bytes.saturating_sub(nwritten);

if !pending_frame.is_empty() {
self.pending_out_frame = Some(pending_frame);
}
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