fix(rpc): recreate dead and uncleaned subscriptions (backport #22281)

rpc/src/rpc_subscription_tracker.rs

@@ -159,9 +159,10 @@ pub struct SignatureSubscriptionParams {
 
 #[derive(Clone)]
 pub struct SubscriptionControl(Arc<SubscriptionControlInner>);
+pub struct WeakSubscriptionTokenRef(Weak<SubscriptionTokenInner>, SubscriptionId);
 
 struct SubscriptionControlInner {
-    subscriptions: DashMap<SubscriptionParams, Weak<SubscriptionTokenInner>>,
+    subscriptions: DashMap<SubscriptionParams, WeakSubscriptionTokenRef>,
     next_id: AtomicU64,
     max_active_subscriptions: usize,
     sender: crossbeam_channel::Sender<TimestampedNotificationEntry>,
@@ -195,33 +196,44 @@ impl SubscriptionControl {
             self.0.subscriptions.len()
         );
         let count = self.0.subscriptions.len();
-        match self.0.subscriptions.entry(params) {
-            DashEntry::Occupied(entry) => Ok(SubscriptionToken(
-                entry
-                    .get()
-                    .upgrade()
-                    .expect("dead subscription encountered in SubscriptionControl"),
+        let create_token_and_weak_ref = |id, params| {
+            let token = SubscriptionToken(
+                Arc::new(SubscriptionTokenInner {
+                    control: Arc::clone(&self.0),
+                    params,
+                    id,
+                }),
                 self.0.counter.create_token(),
-            )),
+            );
+            let weak_ref = WeakSubscriptionTokenRef(Arc::downgrade(&token.0), token.0.id);
+            (token, weak_ref)
+        };
+
+        match self.0.subscriptions.entry(params) {
+            DashEntry::Occupied(mut entry) => match entry.get().0.upgrade() {
+                Some(token_ref) => Ok(SubscriptionToken(token_ref, self.0.counter.create_token())),
+                // This means the last Arc for this Weak pointer entered the drop just before us,
+                // but could not remove the entry since we are holding the write lock.
+                // See `Drop` implementation for `SubscriptionTokenInner` for further info.
+                None => {
+                    let (token, weak_ref) =
+                        create_token_and_weak_ref(entry.get().1, entry.key().clone());
+                    entry.insert(weak_ref);
+                    Ok(token)
+                }
+            },
             DashEntry::Vacant(entry) => {
                 if count >= self.0.max_active_subscriptions {
                     inc_new_counter_info!("rpc-subscription-refused-limit-reached", 1);
                     return Err(Error::TooManySubscriptions);
                 }
                 let id = SubscriptionId::from(self.0.next_id.fetch_add(1, Ordering::AcqRel));
-                let token = SubscriptionToken(
-                    Arc::new(SubscriptionTokenInner {
-                        control: Arc::clone(&self.0),
-                        params: entry.key().clone(),
-                        id,
-                    }),
-                    self.0.counter.create_token(),
-                );
+                let (token, weak_ref) = create_token_and_weak_ref(id, entry.key().clone());
                 let _ = self
                     .0
                     .sender
                     .send(NotificationEntry::Subscribed(token.0.params.clone(), id).into());
-                entry.insert(Arc::downgrade(&token.0));
+                entry.insert(weak_ref);
                 datapoint_info!(
                     "rpc-subscription",
                     ("total", self.0.subscriptions.len(), i64)
@@ -505,7 +517,9 @@ impl Drop for SubscriptionTokenInner {
             DashEntry::Vacant(_) => {
                 warn!("Subscriptions inconsistency (missing entry in by_params)");
             }
-            DashEntry::Occupied(entry) => {
+            // Check the strong refs count to ensure no other thread recreated this subscription (not token)
+            // while we were acquiring the lock.
+            DashEntry::Occupied(entry) if entry.get().0.strong_count() == 0 => {
                 let _ = self
                     .control
                     .sender
@@ -516,6 +530,9 @@ impl Drop for SubscriptionTokenInner {
                     ("total", self.control.subscriptions.len(), i64)
                 );
             }
+            // This branch handles the case in which this entry got recreated
+            // while we were waiting for the lock (inside the `DashMap::entry` method).
+            DashEntry::Occupied(_entry) /* if _entry.get().0.strong_count() > 0 */ => (),
         }
     }
 }