From 73d3d3f494f58599d21f5a2cf3bf863d2e4d5894 Mon Sep 17 00:00:00 2001
From: Matthias Seitz <matthias.seitz@outlook.de>
Date: Thu, 30 Jun 2022 00:26:37 +0200
Subject: [PATCH] perf(providers): replace wake_by_ref with loop (#1428)

---
 ethers-providers/src/stream.rs | 71 +++++++++++++++-------------------
 1 file changed, 32 insertions(+), 39 deletions(-)

diff --git a/ethers-providers/src/stream.rs b/ethers-providers/src/stream.rs
index 0a0eb812..6cd73c7d 100644
--- a/ethers-providers/src/stream.rs
+++ b/ethers-providers/src/stream.rs
@@ -1,9 +1,7 @@
 #![allow(clippy::return_self_not_must_use)]
 
 use crate::{JsonRpcClient, Middleware, PinBoxFut, Provider, ProviderError};
-
 use ethers_core::types::{Transaction, TxHash, U256};
-
 use futures_core::{stream::Stream, Future};
 use futures_util::{stream, stream::FuturesUnordered, FutureExt, StreamExt};
 use pin_project::pin_project;
@@ -37,8 +35,8 @@ enum FilterWatcherState<'a, R> {
 }
 
 #[must_use = "filters do nothing unless you stream them"]
-#[pin_project]
 /// Streams data from an installed filter via `eth_getFilterChanges`
+#[pin_project]
 pub struct FilterWatcher<'a, P, R> {
     /// The filter's installed id on the ethereum node
     pub id: U256,
@@ -47,7 +45,7 @@ pub struct FilterWatcher<'a, P, R> {
 
     // The polling interval
     interval: Box<dyn Stream<Item = ()> + Send + Unpin>,
-
+    /// statemachine driven by the Stream impl
     state: FilterWatcherState<'a, R>,
 }
 
@@ -79,8 +77,7 @@ where
     }
 }
 
-// Pattern for flattening the returned Vec of filter changes taken from
-// https://github.com/tomusdrw/rust-web3/blob/f043b222744580bf4be043da757ab0b300c3b2da/src/api/eth_filter.rs#L50-L67
+// Advances the filter's state machine
 impl<'a, P, R> Stream for FilterWatcher<'a, P, R>
 where
     P: JsonRpcClient,
@@ -89,42 +86,38 @@ where
     type Item = R;
 
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
-        let this = self.project();
+        let mut this = self.project();
         let id = *this.id;
 
-        *this.state = match this.state {
-            FilterWatcherState::WaitForInterval => {
-                // Wait the polling period
-                let _ready = futures_util::ready!(this.interval.poll_next_unpin(cx));
-
-                // create a new instance of the future
-                cx.waker().wake_by_ref();
-                let fut = Box::pin(this.provider.get_filter_changes(id));
-                FilterWatcherState::GetFilterChanges(fut)
-            }
-            FilterWatcherState::GetFilterChanges(fut) => {
-                // NOTE: If the provider returns an error, this will return an empty
-                // vector. Should we make this return a Result instead? Ideally if we're
-                // in a streamed loop we wouldn't want the loop to terminate if an error
-                // is encountered (since it might be a temporary error).
-                let items: Vec<R> = futures_util::ready!(fut.as_mut().poll(cx)).unwrap_or_default();
-                cx.waker().wake_by_ref();
-                FilterWatcherState::NextItem(items.into_iter())
-            }
-            // Consume 1 element from the vector. If more elements are in the vector,
-            // the next call will immediately go to this branch instead of trying to get
-            // filter changes again. Once the whole vector is consumed, it will poll again
-            // for new logs
-            FilterWatcherState::NextItem(iter) => {
-                cx.waker().wake_by_ref();
-                match iter.next() {
-                    Some(item) => return Poll::Ready(Some(item)),
-                    None => FilterWatcherState::WaitForInterval,
+        loop {
+            *this.state = match &mut this.state {
+                FilterWatcherState::WaitForInterval => {
+                    // Wait the polling period
+                    let _ready = futures_util::ready!(this.interval.poll_next_unpin(cx));
+                    let fut = Box::pin(this.provider.get_filter_changes(id));
+                    FilterWatcherState::GetFilterChanges(fut)
                 }
-            }
-        };
-
-        Poll::Pending
+                FilterWatcherState::GetFilterChanges(fut) => {
+                    // NOTE: If the provider returns an error, this will return an empty
+                    // vector. Should we make this return a Result instead? Ideally if we're
+                    // in a streamed loop we wouldn't want the loop to terminate if an error
+                    // is encountered (since it might be a temporary error).
+                    let items: Vec<R> =
+                        futures_util::ready!(fut.as_mut().poll(cx)).unwrap_or_default();
+                    FilterWatcherState::NextItem(items.into_iter())
+                }
+                // Consume 1 element from the vector. If more elements are in the vector,
+                // the next call will immediately go to this branch instead of trying to get
+                // filter changes again. Once the whole vector is consumed, it will poll again
+                // for new logs
+                FilterWatcherState::NextItem(iter) => {
+                    if let item @ Some(_) = iter.next() {
+                        return Poll::Ready(item)
+                    }
+                    FilterWatcherState::WaitForInterval
+                }
+            };
+        }
     }
 }