usockets: stop the listen addrinfo loop on EADDRINUSE so listen(port, "localhost") reports the conflict

packages/bun-usockets/src/bsd.c

@@ -1147,6 +1147,7 @@ inline __attribute__((always_inline)) LIBUS_SOCKET_DESCRIPTOR bsd_bind_listen_fd
 ) {
 
     if (bsd_set_reuse(listenFd, options) != 0) {
+        *error = LIBUS_ERR;
         return LIBUS_SOCKET_ERROR;
     }
 
@@ -1163,6 +1164,7 @@ inline __attribute__((always_inline)) LIBUS_SOCKET_DESCRIPTOR bsd_bind_listen_fd
     if (listenAddr->ai_family == AF_INET6) {
         int enabled = (options & LIBUS_SOCKET_IPV6_ONLY) != 0;
         if (setsockopt(listenFd, IPPROTO_IPV6, IPV6_V6ONLY, &enabled, sizeof(enabled)) != 0) {
+            *error = LIBUS_ERR;
             return LIBUS_SOCKET_ERROR;
         }
     }
@@ -1193,6 +1195,23 @@ int bsd_set_defer_accept(LIBUS_SOCKET_DESCRIPTOR listenFd) {
 #endif
 }
 
+#ifdef _WIN32
+#define LIBUS_ADDRINUSE WSAEADDRINUSE
+#define LIBUS_ACCES WSAEACCES
+#else
+#define LIBUS_ADDRINUSE EADDRINUSE
+#define LIBUS_ACCES EACCES
+#endif
+
+/* bind() failures that mean "this port is owned by someone else" rather than
+ * "this address family is unusable on this host". The IPv6->IPv4 fallback below
+ * exists for the latter (EAFNOSUPPORT / EADDRNOTAVAIL when IPv6 is disabled),
+ * and must not swallow the former: a second listen(port, "localhost") would
+ * otherwise quietly bind 127.0.0.1 after ::1 returned EADDRINUSE. */
+static int bsd_bind_error_is_fatal(int err) {
+    return err == LIBUS_ADDRINUSE || err == LIBUS_ACCES;
+}
+
 // return LIBUS_SOCKET_ERROR or the fd that represents listen socket
 // listen both on ipv6 and ipv4
 LIBUS_SOCKET_DESCRIPTOR bsd_create_listen_socket(const char *host, int port, int options, int* error) {
@@ -1220,12 +1239,17 @@ LIBUS_SOCKET_DESCRIPTOR bsd_create_listen_socket(const char *host, int port, int
             }
 
             listenAddr = a;
+            *error = 0;
             if (bsd_bind_listen_fd(listenFd, listenAddr, port, options, error) != LIBUS_SOCKET_ERROR) {
                 freeaddrinfo(result);
                 return listenFd;
             }
 
             bsd_close_socket(listenFd);
+            if (bsd_bind_error_is_fatal(*error)) {
+                freeaddrinfo(result);
+                return LIBUS_SOCKET_ERROR;
+            }
         }
     }
 
@@ -1237,12 +1261,17 @@ LIBUS_SOCKET_DESCRIPTOR bsd_create_listen_socket(const char *host, int port, int
             }
 
             listenAddr = a;
+            *error = 0;
             if (bsd_bind_listen_fd(listenFd, listenAddr, port, options, error) != LIBUS_SOCKET_ERROR) {
                 freeaddrinfo(result);
                 return listenFd;
             }
 
             bsd_close_socket(listenFd);
+            if (bsd_bind_error_is_fatal(*error)) {
+                freeaddrinfo(result);
+                return LIBUS_SOCKET_ERROR;
+            }
         }
     }
 

src/js/internal/cluster/RoundRobinHandle.ts

@@ -37,6 +37,8 @@
         // Currently, net module only supports `ipv6Only` option in `flags`.
         ipv6Only: !!(flags & UV_TCP_IPV6ONLY),
         backlog,
+        // Workers bind this port directly; open the reuse group for them.
+        reusePort: true,
       });
     } else
       this.server.listen({

src/js/node/net.ts

@@ -4032,7 +4032,8 @@ function listenInCluster(
       hostname,
       exclusive,
       ipv6Only,
-      reusePort,
+      // Workers share the port via SO_REUSEPORT; matches _http_server.ts.
+      true /* reusePort */,
       readableAll,
       writableAll,
       tls,

test/js/node/http/node-http.test.ts

@@ -1295,6 +1295,28 @@ describe("node:http", () => {
     const err = await promise;
     expect(err.code).toBe("EADDRINUSE");
   });
+
+  // https://github.com/oven-sh/bun/issues/15560
+  test("listen(port, 'localhost') reports EADDRINUSE instead of silently binding the other loopback family", async () => {
+    const a = createServer();
+    const b = createServer();
+    try {
+      await new Promise<void>((resolve, reject) => {
+        a.on("error", reject);
+        a.listen(0, "localhost", () => resolve());
+      });
+      const port = a.address().port;
+
+      const err = await new Promise<NodeJS.ErrnoException>((resolve, reject) => {
+        b.on("error", resolve);
+        b.listen(port, "localhost", () => reject(new Error(`second listen bound ${JSON.stringify(b.address())}`)));
+      });
+      expect(err.code).toBe("EADDRINUSE");
+    } finally {
+      b.close();
+      a.close();
+    }
+  });
 });
 
 describe("node https server", async () => {

test/js/node/net/node-net.test.ts

@@ -1870,3 +1870,25 @@ it.skipIf(isWindows)("connect({ localPort }) succeeds when the local port has TI
     target.close();
   }
 });
+
+// https://github.com/oven-sh/bun/issues/15560
+it('listen(port, "localhost") reports EADDRINUSE when the port is already held on localhost', async () => {
+  const a = createServer();
+  const b = createServer();
+  try {
+    await new Promise<void>((resolve, reject) => {
+      a.on("error", reject);
+      a.listen(0, "localhost", resolve);
+    });
+    const port = (a.address() as import("node:net").AddressInfo).port;
+
+    const err = await new Promise<NodeJS.ErrnoException>((resolve, reject) => {
+      b.on("error", resolve);
+      b.listen(port, "localhost", () => reject(new Error(`second listen bound ${JSON.stringify(b.address())}`)));
+    });
+    expect(err.code).toBe("EADDRINUSE");
+  } finally {
+    b.close();
+    a.close();
+  }
+});