8330940: Impossible to create a socket backlog greater than 200 on Windows 8+

Reviewed-by: michaelm, dfuchs, alanb

Author: jaikiran

src/java.base/windows/native/libnio/ch/Net.c

@@ -214,6 +214,13 @@ Java_sun_nio_ch_Net_bind0(JNIEnv *env, jclass clazz, jobject fdo, jboolean prefe
 JNIEXPORT void JNICALL
 Java_sun_nio_ch_Net_listen(JNIEnv *env, jclass cl, jobject fdo, jint backlog)
 {
+    /*
+     * Use SOMAXCONN_HINT when backlog larger than 200. It will adjust the value
+     * to be within the range (200, 65535).
+     */
+    if (backlog > 200) {
+        backlog = SOMAXCONN_HINT(backlog);
+    }
     if (listen(fdval(env,fdo), backlog) == SOCKET_ERROR) {
         NET_ThrowNew(env, WSAGetLastError(), "listen");
     }

test/jdk/java/net/ServerSocket/LargeBacklogTest.java

@@ -0,0 +1,110 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import java.io.IOException;
+import java.net.InetAddress;
+import java.net.InetSocketAddress;
+import java.net.ServerSocket;
+import java.net.Socket;
+import java.nio.channels.AsynchronousServerSocketChannel;
+import java.nio.channels.ServerSocketChannel;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.fail;
+
+/*
+ * @test
+ * @bug 8330940
+ * @summary verify that java.net.ServerSocket and the server socket channels in java.nio.channels
+ *          when configured with a backlog of >=200 on Windows, will allow for those many
+ *          backlogged Socket connections
+ * @requires os.family == "windows"
+ * @run junit LargeBacklogTest
+ */
+class LargeBacklogTest {
+
+    @Test
+    void testServerSocket() throws Exception {
+        final int backlog = 242;
+        // Create a ServerSocket configured with the given backlog.
+        // The ServerSocket never accept()s a connection so each connect() attempt
+        // will be backlogged.
+        try (var server = new ServerSocket(0, backlog, InetAddress.getLoopbackAddress())) {
+            final int serverPort = server.getLocalPort();
+            testBackloggedConnects(backlog, serverPort);
+        }
+    }
+
+    @Test
+    void testServerSocketChannel() throws Exception {
+        final int backlog = 213;
+        // Create a ServerSocketChannel configured with the given backlog.
+        // The channel never accept()s a connection so each connect() attempt
+        // will be backlogged.
+        try (var serverChannel = ServerSocketChannel.open()) {
+            serverChannel.bind(new InetSocketAddress(InetAddress.getLoopbackAddress(), 0), backlog);
+            final int serverPort = ((InetSocketAddress) serverChannel.getLocalAddress()).getPort();
+            testBackloggedConnects(backlog, serverPort);
+        }
+    }
+
+    @Test
+    void testAsynchronousServerSocketChannel() throws Exception {
+        final int backlog = 209;
+        // Create a AsynchronousServerSocketChannel configured with the given backlog.
+        // The channel never accept()s a connection so each connect() attempt
+        // will be backlogged.
+        try (var serverChannel = AsynchronousServerSocketChannel.open()) {
+            serverChannel.bind(new InetSocketAddress(InetAddress.getLoopbackAddress(), 0), backlog);
+            final int serverPort = ((InetSocketAddress) serverChannel.getLocalAddress()).getPort();
+            testBackloggedConnects(backlog, serverPort);
+        }
+    }
+
+    private static void testBackloggedConnects(final int backlog, final int serverPort) {
+        int numSuccessfulConnects = 0;
+        System.err.println("attempting " + backlog + " connections to port " + serverPort);
+        // attempt the Socket connections
+        for (int i = 1; i <= backlog; i++) {
+            try (final Socket sock = new Socket(InetAddress.getLoopbackAddress(), serverPort)) {
+                numSuccessfulConnects++;
+                System.err.println("connection " + i + " established " + sock);
+            } catch (IOException ioe) {
+                System.err.println("connection attempt " + i + " failed: " + ioe);
+                // do not attempt any more connections
+                break;
+            }
+        }
+        System.err.println(numSuccessfulConnects + " connections successfully established");
+        // ideally we expect the number of successful connections to be equal to the backlog value.
+        // however in certain environments, it's possible that some other process attempts a
+        // connection to the server's port. so we allow for a small number of connection attempts
+        // to fail (due to exceeding the backlog)
+        final int minimumExpectedSuccessfulConns = backlog - 5;
+        if (numSuccessfulConnects < minimumExpectedSuccessfulConns) {
+            fail("expected at least " + minimumExpectedSuccessfulConns
+                    + " successful connections for a backlog of " + backlog + ", but only "
+                    + numSuccessfulConnects + " were successful");
+        }
+    }
+}