Replace usage of atomics with `tokio::sync::watch` (#2272)

Rust atomics have an API that's very easy to use incorrectly, leading to
hard to find bugs. For that reason, it's best to avoid it unless there's
a good reason not to.

zebra-network/src/peer_set/candidate_set/tests/vectors.rs

@@ -3,10 +3,7 @@ use std::{
     convert::TryInto,
     iter,
     net::{IpAddr, SocketAddr},
-    sync::{
-        atomic::{AtomicUsize, Ordering},
-        Arc, Mutex,
-    },
+    sync::{Arc, Mutex},
     time::Duration as StdDuration,
 };
 
@@ -14,6 +11,7 @@ use chrono::{DateTime, Duration, Utc};
 use futures::future;
 use tokio::{
     runtime::Runtime,
+    sync::watch,
     time::{self, Instant},
 };
 use tower::Service;
@@ -165,7 +163,7 @@ fn candidate_set_updates_are_rate_limited() {
         }
 
         assert_eq!(
-            call_count.load(Ordering::SeqCst),
+            *call_count.borrow(),
             INTERVALS_TO_RUN as usize * GET_ADDR_FANOUT
         );
     });
@@ -191,7 +189,7 @@ fn candidate_set_update_after_update_initial_is_rate_limited() {
             .await
             .expect("Call to CandidateSet::update should not fail");
 
-        assert_eq!(call_count.load(Ordering::SeqCst), GET_ADDR_FANOUT);
+        assert_eq!(*call_count.borrow(), GET_ADDR_FANOUT);
 
         // The following two calls to `update` should be skipped
         candidate_set
@@ -204,7 +202,7 @@ fn candidate_set_update_after_update_initial_is_rate_limited() {
             .await
             .expect("Call to CandidateSet::update should not fail");
 
-        assert_eq!(call_count.load(Ordering::SeqCst), GET_ADDR_FANOUT);
+        assert_eq!(*call_count.borrow(), GET_ADDR_FANOUT);
 
         // After waiting for at least the minimum interval the call to `update` should succeed
         time::advance(MIN_PEER_GET_ADDR_INTERVAL).await;
@@ -213,7 +211,7 @@ fn candidate_set_update_after_update_initial_is_rate_limited() {
             .await
             .expect("Call to CandidateSet::update should not fail");
 
-        assert_eq!(call_count.load(Ordering::SeqCst), 2 * GET_ADDR_FANOUT);
+        assert_eq!(*call_count.borrow(), 2 * GET_ADDR_FANOUT);
     });
 }
 
@@ -242,7 +240,7 @@ fn mock_gossiped_peers(last_seen_times: impl IntoIterator<Item = DateTime<Utc>>)
 
 /// Create a mock `PeerSet` service that checks that requests to it are rate limited.
 ///
-/// The function also returns an atomic counter, that can be used for checking how many times the
+/// The function also returns a call count watcher, that can be used for checking how many times the
 /// service was called.
 fn mock_peer_service<E>() -> (
     impl Service<
@@ -251,12 +249,12 @@ fn mock_peer_service<E>() -> (
             Future = future::Ready<Result<Response, E>>,
             Error = E,
         > + 'static,
-    Arc<AtomicUsize>,
+    watch::Receiver<usize>,
 ) {
     let rate_limit_interval = MIN_PEER_GET_ADDR_INTERVAL;
 
-    let call_counter = Arc::new(AtomicUsize::new(0));
-    let call_counter_to_return = call_counter.clone();
+    let mut call_counter = 0;
+    let (call_count_sender, call_count_receiver) = watch::channel(call_counter);
 
     let mut peer_request_tracker: VecDeque<_> =
         iter::repeat(Instant::now()).take(GET_ADDR_FANOUT).collect();
@@ -274,7 +272,8 @@ fn mock_peer_service<E>() -> (
                 peer_request_tracker.push_back(Instant::now() + rate_limit_interval);
 
                 // Increment count of calls
-                call_counter.fetch_add(1, Ordering::SeqCst);
+                call_counter += 1;
+                let _ = call_count_sender.send(call_counter);
 
                 // Return an empty list of peer addresses
                 future::ok(Response::Peers(vec![]))
@@ -283,5 +282,5 @@ fn mock_peer_service<E>() -> (
         }
     });
 
-    (service, call_counter_to_return)
+    (service, call_count_receiver)
 }