Limit concurrency of torrent initialization

crates/librqbit/src/session.rs

@@ -112,6 +112,8 @@ pub struct Session {
     reqwest_client: reqwest::Client,
     connector: Arc<StreamConnector>,
 
+    concurrent_initialize_semaphore: Arc<tokio::sync::Semaphore>,
+
     // This is stored for all tasks to stop when session is dropped.
     _cancellation_token_drop_guard: DropGuard,
 }
@@ -375,6 +377,9 @@ pub struct SessionOptions {
 
     // socks5://[username:password@]host:port
     pub socks_proxy_url: Option<String>,
+
+    // how many concurrent torrent initializations can happen
+    pub concurrent_init_limit: Option<usize>,
 }
 
 async fn create_tcp_listener(
@@ -562,6 +567,7 @@ impl Session {
                 default_storage_factory: opts.default_storage_factory,
                 reqwest_client,
                 connector: stream_connector,
+                concurrent_initialize_semaphore: Arc::new(tokio::sync::Semaphore::new(opts.concurrent_init_limit.unwrap_or(3)))
             });
 
             if let Some(mut disk_write_rx) = disk_write_rx {
@@ -1085,7 +1091,12 @@ impl Session {
             let _ = span.enter();
 
             managed_torrent
-                .start(peer_rx, opts.paused, self.cancellation_token.child_token())
+                .start(
+                    peer_rx,
+                    opts.paused,
+                    self.cancellation_token.child_token(),
+                    self.concurrent_initialize_semaphore.clone(),
+                )
                 .context("error starting torrent")?;
         }
 
@@ -1233,7 +1244,12 @@ impl Session {
             self.tcp_listen_port,
             handle.info().options.force_tracker_interval,
         )?;
-        handle.start(peer_rx, false, self.cancellation_token.child_token())?;
+        handle.start(
+            peer_rx,
+            false,
+            self.cancellation_token.child_token(),
+            self.concurrent_initialize_semaphore.clone(),
+        )?;
         self.try_update_persistence_metadata(handle).await;
         Ok(())
     }

crates/librqbit/src/torrent_state/mod.rs

@@ -208,6 +208,7 @@ impl ManagedTorrent {
         peer_rx: Option<PeerStream>,
         start_paused: bool,
         live_cancellation_token: CancellationToken,
+        init_semaphore: Arc<tokio::sync::Semaphore>,
     ) -> anyhow::Result<()> {
         let mut g = self.locked.write();
 
@@ -283,10 +284,16 @@ impl ManagedTorrent {
                 let t = self.clone();
                 let span = self.info().span.clone();
                 let token = live_cancellation_token.clone();
+
                 spawn_with_cancel(
                     error_span!(parent: span.clone(), "initialize_and_start"),
                     token.clone(),
                     async move {
+                        let _permit = init_semaphore
+                            .acquire()
+                            .await
+                            .context("bug: concurrent init semaphore was closed")?;
+
                         match init.check().await {
                             Ok(paused) => {
                                 let mut g = t.locked.write();
@@ -344,7 +351,12 @@ impl ManagedTorrent {
                 drop(g);
 
                 // Recurse.
-                self.start(peer_rx, start_paused, live_cancellation_token)
+                self.start(
+                    peer_rx,
+                    start_paused,
+                    live_cancellation_token,
+                    init_semaphore,
+                )
             }
             ManagedTorrentState::None => bail!("bug: torrent is in empty state"),
         }

crates/rqbit/src/main.rs

@@ -122,6 +122,10 @@ struct Opts {
     /// Alternatively, set this as an environment variable RQBIT_SOCKS_PROXY_URL
     #[arg(long)]
     socks_url: Option<String>,
+
+    /// How many torrents can be initializing (rehashing) at the same time
+    #[arg(long, default_value = "5")]
+    concurrent_init_limit: usize,
 }
 
 #[derive(Parser)]
@@ -335,6 +339,7 @@ async fn async_main(opts: Opts) -> anyhow::Result<()> {
             }
         }),
         socks_proxy_url: socks_url,
+        concurrent_init_limit: Some(opts.concurrent_init_limit),
     };
 
     let stats_printer = |session: Arc<Session>| async move {