Refactor SocketNetlinkMonitor to make it buildable with system sdk

NetlinkMonitor cannot be used when building with system sdk. This CL
refactored the SocketNetlinkMonitor into a separate class and use
factory method and droidstub to avoid referring NetlinkMonitor when
building with system sdk.

Test: m service-connectivity-mdns-standalone-build-test
Bug: 272392042
Change-Id: I2d97f6fe8a17febd155c81303d86cd558654ddf5
diff --git a/service-t/Android.bp b/service-t/Android.bp
index 5bf2973..7de749c 100644
--- a/service-t/Android.bp
+++ b/service-t/Android.bp
@@ -75,3 +75,47 @@
         "//packages/modules/IPsec/tests/iketests",
     ],
 }
+
+// Test building mDNS as a standalone, so that it can be imported into other repositories as-is.
+// The mDNS code is platform code so it should use framework-annotations-lib, contrary to apps that
+// should use sdk_version: "system_current" and only androidx.annotation_annotation. But this build
+// rule verifies that the mDNS code can be built into apps, if code transformations are applied to
+// the annotations.
+// When using "system_current", framework annotations are not available; they would appear as
+// package-private as they are marked as such in the system_current stubs. So build against
+// core_platform and add the stubs manually in "libs". See http://b/147773144#comment7.
+java_library {
+    name: "service-connectivity-mdns-standalone-build-test",
+    sdk_version: "core_platform",
+    srcs: [
+        ":service-mdns-droidstubs",
+        "src/com/android/server/connectivity/mdns/**/*.java",
+    ],
+    exclude_srcs: [
+        "src/com/android/server/connectivity/mdns/internal/SocketNetlinkMonitor.java",
+        "src/com/android/server/connectivity/mdns/SocketNetLinkMonitorFactory.java"
+    ],
+    static_libs: [
+        "net-utils-device-common-mdns-standalone-build-test",
+    ],
+    libs: [
+        "framework-annotations-lib",
+        "android_system_stubs_current",
+        "androidx.annotation_annotation",
+    ],
+    visibility: [
+        "//visibility:private",
+    ],
+}
+
+droidstubs {
+    name: "service-mdns-droidstubs",
+    srcs: ["src/com/android/server/connectivity/mdns/SocketNetLinkMonitorFactory.java"],
+    libs: [
+        "net-utils-device-common-mdns-standalone-build-test",
+        "service-connectivity-tiramisu-pre-jarjar"
+    ],
+    visibility: [
+        "//visibility:private",
+    ],
+}
\ No newline at end of file
diff --git a/service-t/src/com/android/server/connectivity/mdns/ISocketNetLinkMonitor.java b/service-t/src/com/android/server/connectivity/mdns/ISocketNetLinkMonitor.java
new file mode 100644
index 0000000..ef3928c
--- /dev/null
+++ b/service-t/src/com/android/server/connectivity/mdns/ISocketNetLinkMonitor.java
@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.connectivity.mdns;
+
+/**
+ * The interface for netlink monitor.
+ */
+public interface ISocketNetLinkMonitor {
+
+    /**
+     * Returns if the netlink monitor is supported or not. By default, it is not supported.
+     */
+    default boolean isSupported() {
+        return false;
+    }
+
+    /**
+     * Starts the monitor.
+     */
+    default void startMonitoring() {
+    }
+
+    /**
+     * Stops the monitor.
+     */
+    default void stopMonitoring() {
+    }
+}
diff --git a/service-t/src/com/android/server/connectivity/mdns/MdnsSocketProvider.java b/service-t/src/com/android/server/connectivity/mdns/MdnsSocketProvider.java
index 0c709ea..2823f92 100644
--- a/service-t/src/com/android/server/connectivity/mdns/MdnsSocketProvider.java
+++ b/service-t/src/com/android/server/connectivity/mdns/MdnsSocketProvider.java
@@ -29,16 +29,12 @@
 import android.net.TetheringManager.TetheringEventCallback;
 import android.os.Handler;
 import android.os.Looper;
-import android.system.OsConstants;
 import android.util.ArrayMap;
 import android.util.Log;
 
 import com.android.internal.annotations.VisibleForTesting;
 import com.android.net.module.util.CollectionUtils;
 import com.android.net.module.util.LinkPropertiesUtils.CompareResult;
-import com.android.net.module.util.ip.NetlinkMonitor;
-import com.android.net.module.util.netlink.NetlinkConstants;
-import com.android.net.module.util.netlink.NetlinkMessage;
 import com.android.server.connectivity.mdns.util.MdnsLogger;
 
 import java.io.IOException;
@@ -70,7 +66,7 @@
     @NonNull private final Dependencies mDependencies;
     @NonNull private final NetworkCallback mNetworkCallback;
     @NonNull private final TetheringEventCallback mTetheringEventCallback;
-    @NonNull private final NetlinkMonitor mNetlinkMonitor;
+    @NonNull private final ISocketNetLinkMonitor mSocketNetlinkMonitor;
     private final ArrayMap<Network, SocketInfo> mNetworkSockets = new ArrayMap<>();
     private final ArrayMap<String, SocketInfo> mTetherInterfaceSockets = new ArrayMap<>();
     private final ArrayMap<Network, LinkProperties> mActiveNetworksLinkProperties =
@@ -117,7 +113,8 @@
             }
         };
 
-        mNetlinkMonitor = new SocketNetlinkMonitor(mHandler);
+        mSocketNetlinkMonitor = SocketNetLinkMonitorFactory.createNetLinkMonitor(mHandler,
+                LOGGER.mLog);
     }
 
     /**
@@ -156,18 +153,6 @@
         }
     }
 
-    private static class SocketNetlinkMonitor extends NetlinkMonitor {
-        SocketNetlinkMonitor(Handler handler) {
-            super(handler, LOGGER.mLog, TAG, OsConstants.NETLINK_ROUTE,
-                    NetlinkConstants.RTMGRP_IPV4_IFADDR | NetlinkConstants.RTMGRP_IPV6_IFADDR);
-        }
-
-        @Override
-        public void processNetlinkMessage(NetlinkMessage nlMsg, long whenMs) {
-            // TODO: Handle netlink message.
-        }
-    }
-
     /*** Ensure that current running thread is same as given handler thread */
     public static void ensureRunningOnHandlerThread(Handler handler) {
         if (handler.getLooper().getThread() != Thread.currentThread()) {
@@ -192,7 +177,9 @@
         final TetheringManager tetheringManager = mContext.getSystemService(TetheringManager.class);
         tetheringManager.registerTetheringEventCallback(mHandler::post, mTetheringEventCallback);
 
-        mHandler.post(mNetlinkMonitor::start);
+        if (mSocketNetlinkMonitor.isSupported()) {
+            mHandler.post(mSocketNetlinkMonitor::startMonitoring);
+        }
         mMonitoringSockets = true;
     }
 
@@ -209,7 +196,9 @@
                     TetheringManager.class);
             tetheringManager.unregisterTetheringEventCallback(mTetheringEventCallback);
 
-            mHandler.post(mNetlinkMonitor::stop);
+            if (mSocketNetlinkMonitor.isSupported()) {
+                mHandler.post(mSocketNetlinkMonitor::stopMonitoring);
+            }
             // Clear all saved status.
             mActiveNetworksLinkProperties.clear();
             mNetworkSockets.clear();
diff --git a/service-t/src/com/android/server/connectivity/mdns/SocketNetLinkMonitorFactory.java b/service-t/src/com/android/server/connectivity/mdns/SocketNetLinkMonitorFactory.java
new file mode 100644
index 0000000..8f6aecc
--- /dev/null
+++ b/service-t/src/com/android/server/connectivity/mdns/SocketNetLinkMonitorFactory.java
@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.connectivity.mdns;
+
+import android.annotation.NonNull;
+import android.os.Handler;
+
+import com.android.net.module.util.SharedLog;
+import com.android.server.connectivity.mdns.internal.SocketNetlinkMonitor;
+
+/**
+ * The factory class for creating the netlink monitor.
+ */
+public class SocketNetLinkMonitorFactory {
+
+    /**
+     * Creates a new netlink monitor.
+     */
+    public static ISocketNetLinkMonitor createNetLinkMonitor(@NonNull final Handler handler,
+            @NonNull SharedLog log) {
+        return new SocketNetlinkMonitor(handler, log);
+    }
+
+    private SocketNetLinkMonitorFactory() {
+    }
+
+}
diff --git a/service-t/src/com/android/server/connectivity/mdns/internal/SocketNetlinkMonitor.java b/service-t/src/com/android/server/connectivity/mdns/internal/SocketNetlinkMonitor.java
new file mode 100644
index 0000000..e053413
--- /dev/null
+++ b/service-t/src/com/android/server/connectivity/mdns/internal/SocketNetlinkMonitor.java
@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.connectivity.mdns.internal;
+
+import android.annotation.NonNull;
+import android.os.Handler;
+import android.system.OsConstants;
+
+import com.android.net.module.util.SharedLog;
+import com.android.net.module.util.ip.NetlinkMonitor;
+import com.android.net.module.util.netlink.NetlinkConstants;
+import com.android.net.module.util.netlink.NetlinkMessage;
+import com.android.server.connectivity.mdns.ISocketNetLinkMonitor;
+
+/**
+ * The netlink monitor for MdnsSocketProvider.
+ */
+public class SocketNetlinkMonitor extends NetlinkMonitor implements ISocketNetLinkMonitor {
+
+    public SocketNetlinkMonitor(@NonNull final Handler handler, @NonNull SharedLog log) {
+        super(handler, log, SocketNetlinkMonitor.class.getSimpleName(), OsConstants.NETLINK_ROUTE,
+                NetlinkConstants.RTMGRP_IPV4_IFADDR | NetlinkConstants.RTMGRP_IPV6_IFADDR);
+    }
+
+    @Override
+    public void processNetlinkMessage(NetlinkMessage nlMsg, long whenMs) {
+
+    }
+
+    @Override
+    public boolean isSupported() {
+        return true;
+    }
+
+    @Override
+    public void startMonitoring() {
+        this.start();
+    }
+
+    @Override
+    public void stopMonitoring() {
+        this.stop();
+    }
+}