This patch introduces native-level support for creating MPTCP sockets by:

1. Extending the socket0 native method in both Unix and Windows
   implementations to accept an mptcp parameter
2. Adding new overloaded socket() and serverSocket() methods in
   Net.java that support explicit MPTCP enabling
3. Introducing convenience methods mptcpSocket() and mptcpServerSocket()
   for easier MPTCP socket creation

When MPTCP is enabled and supported by the OS, the socket is created with
the IPPROTO_MPTCP protocol for stream sockets. All existing call sites
are updated to pass 'false' for the mptcp parameter to maintain backward
compatibility.

Co-Developed-by: Gang Yan <yangang@kylinos.cn>
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Co-Developed-by: Xiang Gao <gaoxiang@kylinos.cn>
Signed-off-by: Xiang Gao <gaoxiang@kylinos.cn>
Signed-off-by: Geliang Tang <geliang@kernel.org>
---
 .../share/classes/sun/nio/ch/Net.java         | 19 ++++++++++++++-----
 src/java.base/unix/native/libnio/ch/Net.c     | 14 ++++++++++++--
 src/java.base/windows/native/libnio/ch/Net.c  |  4 +++-
 3 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/src/java.base/share/classes/sun/nio/ch/Net.java b/src/java.base/share/classes/sun/nio/ch/Net.java
index 9ec7975a35c..20caa62859f 100644
--- a/src/java.base/share/classes/sun/nio/ch/Net.java
+++ b/src/java.base/share/classes/sun/nio/ch/Net.java
@@ -485,25 +485,34 @@ static FileDescriptor socket() throws IOException {
         return socket(UNSPEC, true);
     }
 
-    static FileDescriptor socket(ProtocolFamily family, boolean stream) throws IOException {
+    static FileDescriptor socket(ProtocolFamily family, boolean stream, boolean mptcp)
+	throws IOException {
         boolean preferIPv6 = isIPv6Available() &&
             (family != StandardProtocolFamily.INET);
-        return IOUtil.newFD(socket0(preferIPv6, stream, false, FAST_LOOPBACK));
+        return IOUtil.newFD(socket0(preferIPv6, stream, false, FAST_LOOPBACK, mptcp));
+    }
+
+    static FileDescriptor socket(ProtocolFamily family, boolean stream) throws IOException {
+        return socket(family, stream, false);
     }
 
     static FileDescriptor serverSocket() {
         return serverSocket(UNSPEC);
     }
 
-    static FileDescriptor serverSocket(ProtocolFamily family) {
+    static FileDescriptor serverSocket(ProtocolFamily family, boolean mptcp) {
         boolean preferIPv6 = isIPv6Available() &&
             (family != StandardProtocolFamily.INET);
-        return IOUtil.newFD(socket0(preferIPv6, true, true, FAST_LOOPBACK));
+        return IOUtil.newFD(socket0(preferIPv6, true, true, FAST_LOOPBACK, mptcp));
+    }
+
+    static FileDescriptor serverSocket(ProtocolFamily family) {
+        return serverSocket(family, false);
     }
 
     // Due to oddities SO_REUSEADDR on Windows reuse is ignored
     private static native int socket0(boolean preferIPv6, boolean stream, boolean reuse,
-                                      boolean fastLoopback);
+                                      boolean fastLoopback, boolean mptcp);
 
     public static void bind(FileDescriptor fd, InetAddress addr, int port)
         throws IOException
diff --git a/src/java.base/unix/native/libnio/ch/Net.c b/src/java.base/unix/native/libnio/ch/Net.c
index 28c1814f422..1c53c197fd8 100644
--- a/src/java.base/unix/native/libnio/ch/Net.c
+++ b/src/java.base/unix/native/libnio/ch/Net.c
@@ -255,13 +255,23 @@ Java_sun_nio_ch_Net_canUseIPv6OptionsWithIPv4LocalAddress0(JNIEnv* env, jclass c
 
 JNIEXPORT jint JNICALL
 Java_sun_nio_ch_Net_socket0(JNIEnv *env, jclass cl, jboolean preferIPv6,
-                            jboolean stream, jboolean reuse, jboolean ignored)
+                            jboolean stream, jboolean reuse, jboolean ignored,
+                            jboolean mptcp)
 {
     int fd;
     int type = (stream ? SOCK_STREAM : SOCK_DGRAM);
     int domain = (ipv6_available() && preferIPv6) ? AF_INET6 : AF_INET;
+    int protocol = 0;
 
-    fd = socket(domain, type, 0);
+#if defined(__linux__) && defined(IPPROTO_MPTCP)
+    if (stream == JNI_TRUE && mptcp == JNI_TRUE) {
+        protocol = IPPROTO_MPTCP;
+    }
+#else
+    (void)mptcp; /* Avoid compile warning when MPTCP is not supported */
+#endif
+
+    fd = socket(domain, type, protocol);
     if (fd < 0) {
         return handleSocketError(env, errno);
     }
diff --git a/src/java.base/windows/native/libnio/ch/Net.c b/src/java.base/windows/native/libnio/ch/Net.c
index 814f502c48a..5b9ecdd2c77 100644
--- a/src/java.base/windows/native/libnio/ch/Net.c
+++ b/src/java.base/windows/native/libnio/ch/Net.c
@@ -152,11 +152,13 @@ Java_sun_nio_ch_Net_canUseIPv6OptionsWithIPv4LocalAddress0(JNIEnv* env, jclass c
 
 JNIEXPORT jint JNICALL
 Java_sun_nio_ch_Net_socket0(JNIEnv *env, jclass cl, jboolean preferIPv6,
-                            jboolean stream, jboolean reuse, jboolean fastLoopback)
+                            jboolean stream, jboolean reuse, jboolean fastLoopback,
+                            jboolean mptcp)
 {
     SOCKET s;
     int domain = (preferIPv6) ? AF_INET6 : AF_INET;
 
+    (void)mptcp; /* not support MPTCP yet */
     s = socket(domain, (stream ? SOCK_STREAM : SOCK_DGRAM), 0);
     if (s != INVALID_SOCKET) {
         SetHandleInformation((HANDLE)s, HANDLE_FLAG_INHERIT, 0);
-- 
2.48.1