use an app-level semaphore to limit tx subscriptions (#238)

* use an app-level semaphore to limit tx subscriptions

* acquire permit in spawned future or it blocks the rest of subscribe

---------

Co-authored-by: Rory Neithinger <rory@llamanodes.com>

web3_proxy/src/app/mod.rs

@@ -127,6 +127,8 @@ pub struct App {
     pub stat_sender: Option<mpsc::UnboundedSender<AppStat>>,
     /// when the app started
     pub start: Instant,
+    /// limit the number of tx subscriptions
+    pub tx_subscriptions: Semaphore,
 
     /// Optional time series database for making pretty graphs that load quickly
     influxdb_client: Option<influxdb2::Client>,
@@ -490,6 +492,8 @@ impl App {
             .name("jsonrpc_response_failed_cache_keys")
             .build();
 
+        let tx_subscriptions = Semaphore::new(1);
+
         let app = Self {
             balanced_rpcs,
             bonus_frontend_public_rate_limiter,
@@ -522,6 +526,7 @@ impl App {
             user_semaphores,
             vredis_pool,
             watch_consensus_head_receiver,
+            tx_subscriptions,
         };
 
         let app = Arc::new(app);

web3_proxy/src/rpcs/one.rs

@@ -5,6 +5,7 @@ use super::request::{OpenRequestHandle, OpenRequestResult};
 use crate::app::Web3ProxyJoinHandle;
 use crate::config::{BlockAndRpc, Web3RpcConfig};
 use crate::errors::{Web3ProxyError, Web3ProxyErrorContext, Web3ProxyResult};
+use crate::globals;
 use crate::jsonrpc::ValidatedRequest;
 use crate::jsonrpc::{self, JsonRpcParams, JsonRpcResultData};
 use crate::rpcs::request::RequestErrorHandler;
@@ -895,7 +896,16 @@ impl Web3Rpc {
         if self.pending_txid_firehose.is_some() && self.ws_provider.load().is_some() {
             let clone = self.clone();
 
-            let f = async move { clone.subscribe_new_transactions().await };
+            let f = async move {
+                let app = globals::APP.get().unwrap();
+                let permit = app.tx_subscriptions.acquire().await?;
+
+                let result = clone.subscribe_new_transactions().await;
+
+                std::mem::drop(permit);
+
+                result
+            };
 
             // TODO: this is waking itself alot
             let h = tokio::spawn(f);