From 24a89e04994f8847272c1771217fc91b3438a853 Mon Sep 17 00:00:00 2001
From: Christian Kaiser <5065635+kaiserc@users.noreply.github.com>
Date: Wed, 2 Sep 2026 01:46:05 +0100
Subject: [PATCH] fix(download): disable uTP to prevent ENOBUFS socket
 exhaustion crashes

WebTorrent's uTP transport uses utp-native, which allocates an independent UDP socket for every outgoing peer connection attempt. Under heavy swarm discovery and peer reconnection churn, Windows rapidly exhausts socket buffer space and dynamic ports (WSAENOBUFS / ENOBUFS: 'no buffer space available').

Furthermore, when binding fails with ENOBUFS, utp-native's UTP.connect emits an 'error' event on an orphaned EventEmitter that has no error listener registered, triggering an unhandled exception that crashes the entire application.

Disabling uTP via { utp: false } instructs WebTorrent to connect to peers exclusively via standard TCP. TCP connections are universally supported by all BitTorrent clients, fully error-handled, pooled, and do not exhaust ephemeral UDP socket buffers.
---
 src/download/engine.test.ts | 15 +++++++++++++++
 src/download/engine.ts      | 16 +++++++++++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/src/download/engine.test.ts b/src/download/engine.test.ts
index 687340f5..b8fdd899 100644
--- a/src/download/engine.test.ts
+++ b/src/download/engine.test.ts
@@ -110,4 +110,19 @@ describe("TorrentEngine macOS port-5350 fix (#22)", () => {
     expect(result?.total).toBe(0);
     engine.destroy();
   });
+
+  it("passes utp:false so utp-native does not exhaust UDP buffers or crash on unhandled bind errors", async () => {
+    const { TorrentEngine } = await import("./engine");
+    const engine = new TorrentEngine();
+    engine.add(
+      "test-id",
+      "magnet:?xt=urn:btih:0000000000000000000000000000000000000000",
+      "/downloads",
+      {},
+    );
+    engine.destroy();
+    expect(constructorCalls).toHaveLength(1);
+    expect(constructorCalls[0]).toMatchObject({ utp: false });
+  });
 });
+
diff --git a/src/download/engine.ts b/src/download/engine.ts
index d3126e54..790df5cf 100644
--- a/src/download/engine.ts
+++ b/src/download/engine.ts
@@ -71,7 +71,21 @@ export class TorrentEngine {
       // the app the moment a download starts. NAT-PMP can never succeed
       // on macOS because the port is permanently taken, so disable it
       // and let UPnP handle NAT traversal instead.
-      const opts = process.platform === "darwin" ? { natPmp: false } : {};
+      //
+      // Disable uTP across all platforms:
+      // WebTorrent's uTP implementation relies on `utp-native`, which allocates
+      // an independent UDP socket for every outgoing connection attempt. Under
+      // peer discovery churn, this rapidly exhausts socket buffer space and
+      // ephemeral ports (leading to WSAENOBUFS / ENOBUFS: "no buffer space available").
+      // Furthermore, when `utp-native` fails to bind, it emits an unhandled 'error'
+      // on an internal EventEmitter with no listeners, crashing the process as an
+      // uncaught exception. Disabling uTP forces WebTorrent to use standard TCP
+      // connections, which are supported by 100% of BitTorrent clients and operate
+      // reliably without socket buffer exhaustion.
+      const opts = {
+        utp: false,
+        ...(process.platform === "darwin" ? { natPmp: false } : {}),
+      };
       this.client = new WebTorrent(opts);
       this.client.on("error", () => {});
       this.applyThrottle();