Add createGetLinkMessage() at RtNetlinkLinkMessage

This CL adds the createGetLinkMessage() method at the
RtNetlinkLinkMessage class, that creates a link message to get the
information of a interface. The functionality is the same as
"ip link show $interface".

Bug: 364501824
Test: atest NetworkStaticLibTests

Change-Id: I87a120f488e5b6f7f24c663c68ba26835fa2157a
diff --git a/staticlibs/device/com/android/net/module/util/netlink/RtNetlinkLinkMessage.java b/staticlibs/device/com/android/net/module/util/netlink/RtNetlinkLinkMessage.java
index 27869ef..037d95f 100644
--- a/staticlibs/device/com/android/net/module/util/netlink/RtNetlinkLinkMessage.java
+++ b/staticlibs/device/com/android/net/module/util/netlink/RtNetlinkLinkMessage.java
@@ -20,7 +20,9 @@
 
 import static com.android.net.module.util.NetworkStackConstants.ETHER_ADDR_LEN;
 import static com.android.net.module.util.netlink.NetlinkConstants.IFF_UP;
+import static com.android.net.module.util.netlink.NetlinkConstants.RTM_GETLINK;
 import static com.android.net.module.util.netlink.NetlinkConstants.RTM_NEWLINK;
+import static com.android.net.module.util.netlink.StructNlMsgHdr.NLM_F_REQUEST;
 import static com.android.net.module.util.netlink.StructNlMsgHdr.NLM_F_REQUEST_ACK;
 
 import android.net.MacAddress;
@@ -281,6 +283,35 @@
                 DEFAULT_MTU, null, newName);
     }
 
+    /**
+     * Creates an {@link RtNetlinkLinkMessage} instance that can be used to get the link information
+     * of a network interface.
+     *
+     * @param interfaceName The name of the network interface to query.
+     * @param sequenceNumber The sequence number for the Netlink message.
+     * @return An `RtNetlinkLinkMessage` instance representing the request to query the interface.
+     */
+    @Nullable
+    public static RtNetlinkLinkMessage createGetLinkMessage(@NonNull String interfaceName,
+            int sequenceNumber) {
+        return createGetLinkMessage(interfaceName, sequenceNumber, new OsAccess());
+    }
+
+    @VisibleForTesting
+    @Nullable
+    protected static RtNetlinkLinkMessage createGetLinkMessage(@NonNull String interfaceName,
+            int sequenceNumber, @NonNull OsAccess osAccess) {
+        final int interfaceIndex = osAccess.if_nametoindex(interfaceName);
+        if (interfaceIndex == OsAccess.INVALID_INTERFACE_INDEX) {
+            return null;
+        }
+
+        return RtNetlinkLinkMessage.build(
+                new StructNlMsgHdr(0, RTM_GETLINK, NLM_F_REQUEST, sequenceNumber),
+                new StructIfinfoMsg((short) AF_UNSPEC, (short) 0, interfaceIndex, 0, 0),
+                DEFAULT_MTU, null, null);
+    }
+
     @Override
     public String toString() {
         return "RtNetlinkLinkMessage{ "
diff --git a/staticlibs/tests/unit/src/com/android/net/module/util/netlink/RtNetlinkLinkMessageTest.java b/staticlibs/tests/unit/src/com/android/net/module/util/netlink/RtNetlinkLinkMessageTest.java
index ee74468..bd0e31d 100644
--- a/staticlibs/tests/unit/src/com/android/net/module/util/netlink/RtNetlinkLinkMessageTest.java
+++ b/staticlibs/tests/unit/src/com/android/net/module/util/netlink/RtNetlinkLinkMessageTest.java
@@ -26,7 +26,6 @@
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.when;
 
-import android.annotation.SuppressLint;
 import android.net.MacAddress;
 import android.system.OsConstants;
 
@@ -289,6 +288,24 @@
     }
 
     @Test
+    public void testCreateGetLinkMessage() {
+        final String expectedHexBytes =
+                "20000000120001006824000000000000"    // struct nlmsghdr
+                + "00000000080000000000000000000000"; // struct ifinfomsg
+        final String interfaceName = "wlan0";
+        final int interfaceIndex = 8;
+        final int sequenceNumber = 0x2468;
+
+        when(mOsAccess.if_nametoindex(interfaceName)).thenReturn(interfaceIndex);
+
+        final RtNetlinkLinkMessage msg = RtNetlinkLinkMessage.createGetLinkMessage(
+                interfaceName, sequenceNumber, mOsAccess);
+        assertNotNull(msg);
+        final byte[] bytes = msg.pack(ByteOrder.LITTLE_ENDIAN);  // For testing.
+        assertEquals(expectedHexBytes, HexDump.toHexString(bytes));
+    }
+
+    @Test
     public void testToString() {
         final ByteBuffer byteBuffer = toByteBuffer(RTM_NEWLINK_HEX);
         byteBuffer.order(ByteOrder.LITTLE_ENDIAN);  // For testing.