Free a preview port slot when a forwarded connection errors

backend/src/runners/loopback.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, test } from "bun:test";
+import { describe, expect, spyOn, test } from "bun:test";
 import type { MuxStream } from "@useagent/runner-protocol";
 import { FORWARDER_IDLE_MS, LoopbackForwarders } from "./loopback";
 
@@ -31,6 +31,29 @@ describe("loopback forwarders", () => {
     forwarders.closeAll();
   });
 
+  test("a connection that errors and then closes frees its listener for the sweep", () => {
+    let now = 1_000_000;
+    // Capture the socket handlers so the error-then-close order can be driven directly.
+    let handlers: Bun.SocketHandler<{ closed: boolean }> | undefined;
+    const listen = spyOn(Bun, "listen").mockImplementation(((options: { socket: typeof handlers }) => {
+      handlers = options.socket;
+      return { port: 40_000, stop() {} };
+    }) as unknown as typeof Bun.listen);
+    try {
+      const forwarders = new LoopbackForwarders({ now: () => now });
+      forwarders.address("c1", 80, never);
+      const socket = { data: undefined, end() {} } as unknown as Bun.Socket<{ closed: boolean }>;
+      handlers!.open!(socket);
+      handlers!.error!(socket, new Error("ECONNRESET"));
+      handlers!.close!(socket, undefined);
+      now += FORWARDER_IDLE_MS;
+      expect(forwarders.sweep()).toBe(1);
+      expect(forwarders.size).toBe(0);
+    } finally {
+      listen.mockRestore();
+    }
+  });
+
   test("a stream that cannot be opened ends the browser connection", async () => {
     const forwarders = new LoopbackForwarders();
     const address = forwarders.address("c1", 80, () => {

backend/src/runners/loopback.ts

@@ -55,6 +55,14 @@ export class LoopbackForwarders {
     }
     const entry: Entry = { forwarder: null as unknown as Forwarder, connections: 0, lastActivity: this.now() };
     const forwarders = this;
+    // A socket can report error() and then close(); count it out of the listener once.
+    const releaseConnection = (socket: Socket): boolean => {
+      if (socket.data.closed) return false;
+      socket.data.closed = true;
+      entry.connections = Math.max(0, entry.connections - 1);
+      entry.lastActivity = forwarders.now();
+      return true;
+    };
     const listener = Bun.listen<Socket["data"]>({
       hostname: "127.0.0.1",
       port: 0,
@@ -100,15 +108,11 @@ export class LoopbackForwarders {
           void drain(socket);
         },
         close(socket) {
-          const data = socket.data;
-          if (data.closed) return;
-          data.closed = true;
-          entry.connections = Math.max(0, entry.connections - 1);
-          entry.lastActivity = forwarders.now();
-          data.stream?.end();
+          if (!releaseConnection(socket)) return;
+          socket.data.stream?.end();
         },
         error(socket) {
-          socket.data.closed = true;
+          releaseConnection(socket);
           socket.data.stream?.reset("browser side errored");
         },
         drain() {