android: Add UDSChannelBuilder

Allows using Android's LocalSocket via a Socket adapter. Such an adapter
isn't generally 100% safe, since some methods may not have any effect,
but we know what methods are called by gRPC's okhttp transport and can
update the adapter or the transport as appropriate.

android-interop-testing/build.gradle

@@ -52,6 +52,10 @@ android {
         disable 'InvalidPackage', 'HardcodedText', 'UsingOnClickInXml',
                 'MissingClass' // https://github.com/grpc/grpc-java/issues/8799
     }
+    packagingOptions {
+        exclude 'META-INF/INDEX.LIST'
+        exclude 'META-INF/io.netty.versions.properties'
+    }
 }
 
 dependencies {
@@ -60,7 +64,8 @@ dependencies {
     implementation libraries.androidx.annotation
     implementation 'com.google.android.gms:play-services-base:18.0.1'
 
-    implementation project(':grpc-auth'),
+    implementation project(':grpc-android'),
+            project(':grpc-auth'),
             project(':grpc-census'),
             project(':grpc-okhttp'),
             project(':grpc-protobuf-lite'),
@@ -69,6 +74,7 @@ dependencies {
             libraries.hdrhistogram,
             libraries.junit,
             libraries.truth,
+            libraries.androidx.test.rules,
             libraries.opencensus.contrib.grpc.metrics
 
     implementation (libraries.google.auth.oauth2Http) {
@@ -81,7 +87,8 @@ dependencies {
 
     compileOnly libraries.javax.annotation
 
-    androidTestImplementation 'androidx.test.ext:junit:1.1.3',
+    androidTestImplementation project(':grpc-netty'),
+            'androidx.test.ext:junit:1.1.3',
             'androidx.test:runner:1.4.0'
 }
 

android-interop-testing/src/androidTest/java/io/grpc/android/integrationtest/UdsChannelInteropTest.java

@@ -0,0 +1,134 @@
+/*
+ * Copyright 2021 The gRPC Authors
+ *
+ * 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 io.grpc.android.integrationtest;
+
+import static java.util.concurrent.TimeUnit.SECONDS;
+import static org.junit.Assert.assertEquals;
+
+import android.net.LocalSocketAddress.Namespace;
+import androidx.test.InstrumentationRegistry;
+import androidx.test.rule.ActivityTestRule;
+import androidx.test.runner.AndroidJUnit4;
+import com.google.common.util.concurrent.SettableFuture;
+import io.grpc.Server;
+import io.grpc.android.UdsChannelBuilder;
+import io.grpc.android.integrationtest.InteropTask.Listener;
+import io.grpc.netty.NettyServerBuilder;
+import io.grpc.testing.integration.TestServiceImpl;
+import java.io.IOException;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+/**
+ * Tests for channels created with {@link UdsChannelBuilder}. The UDS Channel is only meant to talk
+ * to Unix Domain Socket endpoints on servers that are on-device, so a {@link LocalTestServer} is
+ * set up to expose a UDS endpoint.
+ */
+@RunWith(AndroidJUnit4.class)
+public class UdsChannelInteropTest {
+  private static final int TIMEOUT_SECONDS = 150;
+
+  private static final String UDS_PATH = "udspath";
+  private String testCase;
+
+  private Server server;
+  private UdsTcpEndpointConnector endpointConnector;
+
+  private ScheduledExecutorService executor = Executors.newScheduledThreadPool(2);
+
+  // Ensures Looper is initialized for tests running on API level 15. Otherwise instantiating an
+  // AsyncTask throws an exception.
+  @Rule
+  public ActivityTestRule<TesterActivity> activityRule =
+      new ActivityTestRule<TesterActivity>(TesterActivity.class);
+
+  @Before
+  public void setUp() throws IOException {
+    testCase = InstrumentationRegistry.getArguments().getString("test_case", "all");
+
+    // Start local server.
+    server =
+        NettyServerBuilder.forPort(0)
+            .maxInboundMessageSize(16 * 1024 * 1024)
+            .addService(new TestServiceImpl(executor))
+            .build();
+    server.start();
+
+    // Connect uds endpoint to server's endpoint.
+    endpointConnector = new UdsTcpEndpointConnector(UDS_PATH, "0.0.0.0", server.getPort());
+    endpointConnector.start();
+  }
+
+  @After
+  public void teardown() {
+    server.shutdownNow();
+    endpointConnector.shutDown();
+  }
+
+  @Test
+  public void interopTests() throws Exception {
+    if (testCase.equals("all")) {
+      runTest("empty_unary");
+      runTest("large_unary");
+      runTest("client_streaming");
+      runTest("server_streaming");
+      runTest("ping_pong");
+      runTest("empty_stream");
+      runTest("cancel_after_begin");
+      runTest("cancel_after_first_response");
+      runTest("full_duplex_call_should_succeed");
+      runTest("half_duplex_call_should_succeed");
+      runTest("server_streaming_should_be_flow_controlled");
+      runTest("very_large_request");
+      runTest("very_large_response");
+      runTest("deadline_not_exceeded");
+      runTest("deadline_exceeded");
+      runTest("deadline_exceeded_server_streaming");
+      runTest("unimplemented_method");
+      runTest("timeout_on_sleeping_server");
+      runTest("graceful_shutdown");
+    } else {
+      runTest(testCase);
+    }
+  }
+
+  private void runTest(String testCase) throws Exception {
+    final SettableFuture<String> resultFuture = SettableFuture.create();
+    InteropTask.Listener listener =
+        new Listener() {
+          @Override
+          public void onComplete(String result) {
+            resultFuture.set(result);
+          }
+        };
+
+    new InteropTask(
+            listener,
+            UdsChannelBuilder.forPath(UDS_PATH, Namespace.ABSTRACT)
+                .maxInboundMessageSize(16 * 1024 * 1024)
+                .build(),
+            testCase)
+        .execute();
+    String result = resultFuture.get(TIMEOUT_SECONDS, SECONDS);
+    assertEquals(testCase + " failed", InteropTask.SUCCESS_MESSAGE, result);
+  }
+}

android-interop-testing/src/main/AndroidManifest.xml

@@ -2,13 +2,16 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android" >
 
     <uses-permission android:name="android.permission.INTERNET" />
+    <!-- For UDS -->
+    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
+    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
 
     <application
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:theme="@style/Base.V7.Theme.AppCompat.Light"
-        android:name="androidx.multidex.MultiDexApplication" >
+        android:name="androidx.multidex.MultiDexApplication">
         <activity
             android:name=".TesterActivity"
             android:exported="true"

android-interop-testing/src/main/java/io/grpc/android/integrationtest/TesterActivity.java

@@ -18,6 +18,7 @@ package io.grpc.android.integrationtest;
 
 import android.content.Context;
 import android.content.Intent;
+import android.net.LocalSocketAddress.Namespace;
 import android.os.Bundle;
 import android.text.TextUtils;
 import android.util.Log;
@@ -30,6 +31,8 @@ import android.widget.TextView;
 import androidx.appcompat.app.AppCompatActivity;
 import com.google.android.gms.security.ProviderInstaller;
 import io.grpc.ManagedChannel;
+import io.grpc.android.UdsChannelBuilder;
+import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.List;
@@ -41,9 +44,13 @@ public class TesterActivity extends AppCompatActivity
   private List<Button> buttons;
   private EditText hostEdit;
   private EditText portEdit;
+  private CheckBox useUdsCheckBox;
+  private EditText udsEdit;
   private TextView resultText;
   private CheckBox testCertCheckBox;
 
+  private UdsTcpEndpointConnector endpointConnector;
+
   @Override
   protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
@@ -57,6 +64,8 @@ public class TesterActivity extends AppCompatActivity
 
     hostEdit = (EditText) findViewById(R.id.host_edit_text);
     portEdit = (EditText) findViewById(R.id.port_edit_text);
+    useUdsCheckBox = (CheckBox) findViewById(R.id.use_uds_checkbox);
+    udsEdit = (EditText) findViewById(R.id.uds_proxy_edit_text);
     resultText = (TextView) findViewById(R.id.grpc_response_text);
     testCertCheckBox = (CheckBox) findViewById(R.id.test_cert_checkbox);
 
@@ -65,6 +74,16 @@ public class TesterActivity extends AppCompatActivity
     enableButtons(false);
   }
 
+  /** Click handler for unix domain socket. */
+  public void enableUds(View view) {
+    boolean enabled = ((CheckBox) view).isChecked();
+    udsEdit.setEnabled(enabled);
+    testCertCheckBox.setEnabled(!enabled);
+    if (enabled) {
+      testCertCheckBox.setChecked(false);
+    }
+  }
+
   public void startEmptyUnary(View view) {
     startTest("empty_unary");
   }
@@ -93,6 +112,10 @@ public class TesterActivity extends AppCompatActivity
 
   @Override
   public void onComplete(String result) {
+    if (endpointConnector != null) {
+      endpointConnector.shutDown();
+      endpointConnector = null;
+    }
     resultText.setText(result);
     enableButtons(true);
   }
@@ -106,6 +129,9 @@ public class TesterActivity extends AppCompatActivity
     String host = hostEdit.getText().toString();
     String portStr = portEdit.getText().toString();
     int port = TextUtils.isEmpty(portStr) ? 8080 : Integer.valueOf(portStr);
+    boolean udsEnabled = useUdsCheckBox.isChecked();
+    String udsPath =
+        TextUtils.isEmpty(udsEdit.getText()) ? "default" : udsEdit.getText().toString();
 
     String serverHostOverride;
     InputStream testCert;
@@ -116,10 +142,27 @@ public class TesterActivity extends AppCompatActivity
       serverHostOverride = null;
       testCert = null;
     }
-    ManagedChannel channel =
-        TesterOkHttpChannelBuilder.build(host, port, serverHostOverride, true, testCert);
 
-    new InteropTask(this, channel, testCase).execute();
+    // Create Channel
+    ManagedChannel channel;
+    if (udsEnabled) {
+      channel = UdsChannelBuilder.forPath(udsPath, Namespace.ABSTRACT).build();
+    } else {
+      channel = TesterOkHttpChannelBuilder.build(host, port, serverHostOverride, true, testCert);
+    }
+
+    // Port-forward uds local port to server exposing tcp endpoint.
+    if (udsEnabled) {
+      endpointConnector = new UdsTcpEndpointConnector(udsPath, host, port);
+      try {
+        endpointConnector.start();
+      } catch (IOException e) {
+        Log.e(LOG_TAG, "Failed to start UDS-TCP Endpoint Connector.");
+      }
+    }
+
+    // Start Test.
+    new InteropTask(TesterActivity.this, channel, testCase).execute();
   }
 
   @Override

android-interop-testing/src/main/java/io/grpc/android/integrationtest/UdsTcpEndpointConnector.java

@@ -0,0 +1,198 @@
+/*
+ * Copyright 2021 The gRPC Authors
+ *
+ * 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 io.grpc.android.integrationtest;
+
+import static java.util.concurrent.TimeUnit.SECONDS;
+
+import android.net.LocalServerSocket;
+import android.net.LocalSocket;
+import android.util.Log;
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.net.Socket;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.atomic.AtomicInteger;
+
+/**
+ * Funnels traffic between a given UDS endpoint and a local TCP endpoint. A client binds to the UDS
+ * endpoint, but effectively communicates with the TCP endpoint.
+ */
+public class UdsTcpEndpointConnector {
+
+  private static final String LOG_TAG = "EndpointConnector";
+
+  // Discard-policy, to allow dropping tasks that were received immediately after shutDown()
+  private final ThreadPoolExecutor executor =
+      new ThreadPoolExecutor(
+          5,
+          10,
+          1,
+          SECONDS,
+          new LinkedBlockingQueue<>(),
+          new ThreadPoolExecutor.DiscardOldestPolicy());
+
+  private final String udsPath;
+
+  private final String host;
+  private final int port;
+  private InetSocketAddress socketAddress;
+
+  private LocalServerSocket clientAcceptor;
+
+  private volatile boolean shutDownRequested = false;
+  private volatile boolean shutDownComplete = false;
+
+  /** Listen on udsPath and forward connections to host:port. */
+  public UdsTcpEndpointConnector(String udsPath, String host, int port) {
+    this.udsPath = udsPath;
+    this.host = host;
+    this.port = port;
+  }
+
+  /** Start listening and accept connections. */
+  public void start() throws IOException {
+    clientAcceptor = new LocalServerSocket(udsPath);
+    executor.execute(
+        new Runnable() {
+          @Override
+          public void run() {
+            Log.i(LOG_TAG, "Starting connection from " + udsPath + " to " + socketAddress);
+            socketAddress = new InetSocketAddress(host, port);
+            while (!shutDownRequested) {
+              try {
+                LocalSocket clientSocket = clientAcceptor.accept();
+                if (shutDownRequested) {
+                  // Check if shut down during blocking accept().
+                  clientSocket.close();
+                  shutDownComplete = true;
+                  break;
+                }
+                Socket serverSocket = new Socket();
+                serverSocket.connect(socketAddress);
+                startWorkers(clientSocket, serverSocket);
+              } catch (IOException e) {
+                throw new RuntimeException(e);
+              }
+            }
+          }
+        });
+  }
+
+  /** Stop listening and release resources. */
+  public void shutDown() {
+    Log.i(LOG_TAG, "Shutting down connection from " + udsPath + " to " + socketAddress);
+    shutDownRequested = true;
+
+    try {
+      // Upon clientAcceptor.close(), clientAcceptor.accept() continues to block.
+      // Thus, once shutDownRequested=true, must send a connection request to unblock accept().
+      LocalSocket localSocket = new LocalSocket();
+      localSocket.connect(clientAcceptor.getLocalSocketAddress());
+      localSocket.close();
+      clientAcceptor.close();
+    } catch (IOException e) {
+      Log.w(LOG_TAG, "Failed to close LocalServerSocket", e);
+    }
+    executor.shutdownNow();
+  }
+
+  public boolean isShutdown() {
+    return shutDownComplete && executor.isShutdown();
+  }
+
+  private void startWorkers(LocalSocket clientSocket, Socket serverSocket) throws IOException {
+    DataInputStream clientIn = new DataInputStream(clientSocket.getInputStream());
+    DataOutputStream clientOut = new DataOutputStream(serverSocket.getOutputStream());
+    DataInputStream serverIn = new DataInputStream(serverSocket.getInputStream());
+    DataOutputStream serverOut = new DataOutputStream(clientSocket.getOutputStream());
+
+    AtomicInteger completionCount = new AtomicInteger(0);
+    StreamConnector.Listener cleanupListener =
+        new StreamConnector.Listener() {
+          @Override
+          public void onFinished() {
+            if (completionCount.incrementAndGet() == 2) {
+              try {
+                serverSocket.close();
+                clientSocket.close();
+              } catch (IOException e) {
+                Log.e(LOG_TAG, "Failed to clean up connected sockets.", e);
+              }
+            }
+          }
+        };
+    executor.execute(new StreamConnector(clientIn, clientOut).addListener(cleanupListener));
+    executor.execute(new StreamConnector(serverIn, serverOut).addListener(cleanupListener));
+  }
+
+  /**
+   * Funnels everything that comes in to a DataInputStream into an DataOutputStream, until the
+   * DataInputStream is closed. (detected by IOException).
+   */
+  private static final class StreamConnector implements Runnable {
+
+    interface Listener {
+      void onFinished();
+    }
+
+    private static final int BUFFER_SIZE = 1000;
+
+    private final DataInputStream in;
+    private final DataOutputStream out;
+    private final byte[] buffer = new byte[BUFFER_SIZE];
+
+    private boolean finished = false;
+
+    private final Collection<Listener> listeners = new ArrayList<>();
+
+    StreamConnector(DataInputStream in, DataOutputStream out) {
+      this.in = in;
+      this.out = out;
+    }
+
+    StreamConnector addListener(Listener listener) {
+      listeners.add(listener);
+      return this;
+    }
+
+    @Override
+    public void run() {
+      while (!finished) {
+        int bytesRead;
+        try {
+          bytesRead = in.read(buffer);
+          if (bytesRead == -1) {
+            finished = true;
+            out.close();
+            continue;
+          }
+          out.write(buffer, 0, bytesRead);
+        } catch (IOException e) {
+          finished = true;
+        }
+      }
+      for (StreamConnector.Listener listener : listeners) {
+        listener.onFinished();
+      }
+    }
+  }
+}

android-interop-testing/src/main/res/color/focus.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<selector xmlns:android="http://schemas.android.com/apk/res/android">
+  <item android:color="#AAA" android:state_focused="false"/>
+  <item android:color="#000" android:state_focused="true"/>
+</selector>

android-interop-testing/src/main/res/layout/activity_tester.xml

@@ -28,6 +28,25 @@
         />
   </LinearLayout>
 
+  <LinearLayout
+      android:layout_width="match_parent"
+      android:layout_height="wrap_content"
+    android:orientation="horizontal">
+    <CheckBox android:id="@+id/use_uds_checkbox"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:onClick="enableUds"
+        android:textColor="@color/focus"
+        android:text="Use UDS (SSL not supported)"/>
+    <EditText
+        android:id="@+id/uds_proxy_edit_text"
+        android:layout_height="wrap_content"
+        android:layout_width="match_parent"
+        android:enabled="false"
+        android:hint="Enter Unix Domain Socket Abstract Namespace Address"
+        />
+  </LinearLayout>
+
   <LinearLayout
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
@@ -36,6 +55,7 @@
     <CheckBox android:id="@+id/test_cert_checkbox"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
+        android:textColor="@color/focus"
         android:text="Use Test Cert"
         />
   </LinearLayout>

android/src/main/java/io/grpc/android/UdsChannelBuilder.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2021 The gRPC Authors
+ *
+ * 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 io.grpc.android;
+
+import android.net.LocalSocketAddress.Namespace;
+import io.grpc.ChannelCredentials;
+import io.grpc.ExperimentalApi;
+import io.grpc.InsecureChannelCredentials;
+import io.grpc.ManagedChannelBuilder;
+import java.lang.reflect.InvocationTargetException;
+import javax.annotation.Nullable;
+import javax.net.SocketFactory;
+
+/**
+ * Creates a UDS channel by passing in a specialized SocketFactory into an OkHttpChannelBuilder. The
+ * UdsSockets produced by this factory communicate over Android's LocalSockets.
+ *
+ * <p>Example Usage <code>
+ *   Channel channel = UdsChannelBuilder.forPath("/data/data/my.app/app.socket",
+ *     Namespace.FILESYSTEM).build();
+ *   stub = MyService.newStub(channel);
+ * </code>
+ *
+ * <p>This class uses a safe-for-production hack to workaround NameResolver's inability to safely
+ * return non-IP SocketAddress types. The hack simply ignores the name resolver results and connects
+ * to the UDS name provided during construction instead. This class is expected to be replaced with
+ * a `unix:` name resolver when possible.
+ */
+@ExperimentalApi("A stopgap. Not intended to be stabilized")
+public final class UdsChannelBuilder {
+  @Nullable
+  @SuppressWarnings("rawtypes")
+  private static final Class<? extends ManagedChannelBuilder> OKHTTP_CHANNEL_BUILDER_CLASS =
+      findOkHttp();
+
+  @SuppressWarnings("rawtypes")
+  private static Class<? extends ManagedChannelBuilder> findOkHttp() {
+    try {
+      return Class.forName("io.grpc.okhttp.OkHttpChannelBuilder")
+          .asSubclass(ManagedChannelBuilder.class);
+    } catch (ClassNotFoundException e) {
+      return null;
+    }
+  }
+
+  /**
+   * Returns a channel to the UDS endpoint specified by the file-path.
+   *
+   * @param path unix file system path to use for Unix Domain Socket.
+   * @param namespace the type of the namespace that the path belongs to.
+   */
+  public static ManagedChannelBuilder<?> forPath(String path, Namespace namespace) {
+    if (OKHTTP_CHANNEL_BUILDER_CLASS == null) {
+      throw new UnsupportedOperationException("OkHttpChannelBuilder not found on the classpath");
+    }
+    try {
+      // Target 'dns:///localhost' is unused, but necessary as an argument for OkHttpChannelBuilder.
+      // TLS is unsupported because Conscrypt assumes the platform Socket implementation to improve
+      // performance by using the file descriptor directly.
+      Object o = OKHTTP_CHANNEL_BUILDER_CLASS
+          .getMethod("forTarget", String.class, ChannelCredentials.class)
+          .invoke(null, "dns:///localhost", InsecureChannelCredentials.create());
+      ManagedChannelBuilder<?> builder = OKHTTP_CHANNEL_BUILDER_CLASS.cast(o);
+      OKHTTP_CHANNEL_BUILDER_CLASS
+          .getMethod("socketFactory", SocketFactory.class)
+          .invoke(builder, new UdsSocketFactory(path, namespace));
+      return builder;
+    } catch (IllegalAccessException e) {
+      throw new RuntimeException("Failed to create OkHttpChannelBuilder", e);
+    } catch (NoSuchMethodException e) {
+      throw new RuntimeException("Failed to create OkHttpChannelBuilder", e);
+    } catch (InvocationTargetException e) {
+      throw new RuntimeException("Failed to create OkHttpChannelBuilder", e);
+    }
+  }
+
+  private UdsChannelBuilder() {}
+}

android/src/main/java/io/grpc/android/UdsSocket.java

@@ -0,0 +1,312 @@
+/*
+ * Copyright 2021 The gRPC Authors
+ *
+ * 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 io.grpc.android;
+
+import android.net.LocalSocket;
+import android.net.LocalSocketAddress;
+import com.google.errorprone.annotations.concurrent.GuardedBy;
+import java.io.FilterInputStream;
+import java.io.FilterOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.net.InetAddress;
+import java.net.Socket;
+import java.net.SocketAddress;
+import java.net.SocketException;
+import java.nio.channels.SocketChannel;
+
+/**
+ * Adapter from Android's LocalSocket to Socket. This class is only needed by grpc-okhttp, so the
+ * adapter only has to support the things that grcp-okhttp uses. It is fine to support trivial
+ * things unused by the transport, to be less likely to break as the transport usage changes, but it
+ * is also unnecessary. It's okay to stretch the truth or lie when necessary. For example, little
+ * hurts with {@link #setTcpNoDelay(boolean)} being a noop since unix domain sockets don't have such
+ * unnecessary delays.
+ */
+@SuppressWarnings("UnsynchronizedOverridesSynchronized") // Rely on LocalSocket's synchronization
+class UdsSocket extends Socket {
+
+  private final LocalSocket localSocket;
+  private final LocalSocketAddress localSocketAddress;
+
+  @GuardedBy("this")
+  private boolean closed = false;
+
+  @GuardedBy("this")
+  private boolean inputShutdown = false;
+
+  @GuardedBy("this")
+  private boolean outputShutdown = false;
+
+  public UdsSocket(LocalSocketAddress localSocketAddress) {
+    this.localSocketAddress = localSocketAddress;
+    localSocket = new LocalSocket();
+  }
+
+  @Override
+  public void bind(SocketAddress bindpoint) {
+    // no-op
+  }
+
+  @Override
+  public synchronized void close() throws IOException {
+    if (closed) {
+      return;
+    }
+    if (!inputShutdown) {
+      shutdownInput();
+    }
+    if (!outputShutdown) {
+      shutdownOutput();
+    }
+    localSocket.close();
+    closed = true;
+  }
+
+  @Override
+  public void connect(SocketAddress endpoint) throws IOException {
+    localSocket.connect(localSocketAddress);
+  }
+
+  @Override
+  public void connect(SocketAddress endpoint, int timeout) throws IOException {
+    localSocket.connect(localSocketAddress, timeout);
+  }
+
+  @Override
+  public SocketChannel getChannel() {
+    throw new UnsupportedOperationException("getChannel() not supported");
+  }
+
+  @Override
+  public InetAddress getInetAddress() {
+    throw new UnsupportedOperationException("getInetAddress() not supported");
+  }
+
+  @Override
+  public InputStream getInputStream() throws IOException {
+    return new FilterInputStream(localSocket.getInputStream()) {
+      @Override
+      public void close() throws IOException {
+        UdsSocket.this.close();
+      }
+    };
+  }
+
+  @Override
+  public boolean getKeepAlive() {
+    throw new UnsupportedOperationException("Unsupported operation getKeepAlive()");
+  }
+
+  @Override
+  public InetAddress getLocalAddress() {
+    throw new UnsupportedOperationException("Unsupported operation getLocalAddress()");
+  }
+
+  @Override
+  public int getLocalPort() {
+    throw new UnsupportedOperationException("Unsupported operation getLocalPort()");
+  }
+
+  @Override
+  public SocketAddress getLocalSocketAddress() {
+    return new SocketAddress() {};
+  }
+
+  @Override
+  public boolean getOOBInline() {
+    throw new UnsupportedOperationException("Unsupported operation getOOBInline()");
+  }
+
+  @Override
+  public OutputStream getOutputStream() throws IOException {
+    return new FilterOutputStream(localSocket.getOutputStream()) {
+      @Override
+      public void close() throws IOException {
+        UdsSocket.this.close();
+      }
+    };
+  }
+
+  @Override
+  public int getPort() {
+    throw new UnsupportedOperationException("Unsupported operation getPort()");
+  }
+
+  @Override
+  public int getReceiveBufferSize() throws SocketException {
+    try {
+      return localSocket.getReceiveBufferSize();
+    } catch (IOException e) {
+      throw toSocketException(e);
+    }
+  }
+
+  @Override
+  public SocketAddress getRemoteSocketAddress() {
+    return new SocketAddress() {};
+  }
+
+  @Override
+  public boolean getReuseAddress() {
+    throw new UnsupportedOperationException("Unsupported operation getReuseAddress()");
+  }
+
+  @Override
+  public int getSendBufferSize() throws SocketException {
+    try {
+      return localSocket.getSendBufferSize();
+    } catch (IOException e) {
+      throw toSocketException(e);
+    }
+  }
+
+  @Override
+  public int getSoLinger() {
+    return -1; // unsupported
+  }
+
+  @Override
+  public int getSoTimeout() throws SocketException {
+    try {
+      return localSocket.getSoTimeout();
+    } catch (IOException e) {
+      throw toSocketException(e);
+    }
+  }
+
+  @Override
+  public boolean getTcpNoDelay() {
+    return true;
+  }
+
+  @Override
+  public int getTrafficClass() {
+    throw new UnsupportedOperationException("Unsupported operation getTrafficClass()");
+  }
+
+  @Override
+  public boolean isBound() {
+    return localSocket.isBound();
+  }
+
+  @Override
+  public synchronized boolean isClosed() {
+    return closed;
+  }
+
+  @Override
+  public boolean isConnected() {
+    return localSocket.isConnected();
+  }
+
+  @Override
+  public synchronized boolean isInputShutdown() {
+    return inputShutdown;
+  }
+
+  @Override
+  public synchronized boolean isOutputShutdown() {
+    return outputShutdown;
+  }
+
+  @Override
+  public void sendUrgentData(int data) {
+    throw new UnsupportedOperationException("Unsupported operation sendUrgentData()");
+  }
+
+  @Override
+  public void setKeepAlive(boolean on) {
+    throw new UnsupportedOperationException("Unsupported operation setKeepAlive()");
+  }
+
+  @Override
+  public void setOOBInline(boolean on) {
+    throw new UnsupportedOperationException("Unsupported operation setOOBInline()");
+  }
+
+  @Override
+  public void setPerformancePreferences(int connectionTime, int latency, int bandwidth) {
+    throw new UnsupportedOperationException("Unsupported operation setPerformancePreferences()");
+  }
+
+  @Override
+  public void setReceiveBufferSize(int size) throws SocketException {
+    try {
+      localSocket.setReceiveBufferSize(size);
+    } catch (IOException e) {
+      throw toSocketException(e);
+    }
+  }
+
+  @Override
+  public void setReuseAddress(boolean on) {
+    throw new UnsupportedOperationException("Unsupported operation setReuseAddress()");
+  }
+
+  @Override
+  public void setSendBufferSize(int size) throws SocketException {
+    try {
+      localSocket.setSendBufferSize(size);
+    } catch (IOException e) {
+      throw toSocketException(e);
+    }
+  }
+
+  @Override
+  public void setSoLinger(boolean on, int linger) {
+    throw new UnsupportedOperationException("Unsupported operation setSoLinger()");
+  }
+
+  @Override
+  public void setSoTimeout(int timeout) throws SocketException {
+    try {
+      localSocket.setSoTimeout(timeout);
+    } catch (IOException e) {
+      throw toSocketException(e);
+    }
+  }
+
+  @Override
+  public void setTcpNoDelay(boolean on) {
+    // no-op
+  }
+
+  @Override
+  public void setTrafficClass(int tc) {
+    throw new UnsupportedOperationException("Unsupported operation setTrafficClass()");
+  }
+
+  @Override
+  public synchronized void shutdownInput() throws IOException {
+    localSocket.shutdownInput();
+    inputShutdown = true;
+  }
+
+  @Override
+  public synchronized void shutdownOutput() throws IOException {
+    localSocket.shutdownOutput();
+    outputShutdown = true;
+  }
+
+  private static SocketException toSocketException(Throwable e) {
+    SocketException se = new SocketException();
+    se.initCause(e);
+    return se;
+  }
+}

android/src/main/java/io/grpc/android/UdsSocketFactory.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright 2021 The gRPC Authors
+ *
+ * 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 io.grpc.android;
+
+import android.net.LocalSocketAddress;
+import android.net.LocalSocketAddress.Namespace;
+import java.io.IOException;
+import java.net.InetAddress;
+import java.net.InetSocketAddress;
+import java.net.Socket;
+import java.net.SocketAddress;
+import javax.net.SocketFactory;
+
+/**
+ * A SocketFactory that produces {@link UdsSocket} instances. This is used to provide support for
+ * gRPC channels with an underlying Unix Domain Socket transport.
+ */
+class UdsSocketFactory extends SocketFactory {
+
+  private final LocalSocketAddress localSocketAddress;
+
+  public UdsSocketFactory(String path, Namespace namespace) {
+    localSocketAddress = new LocalSocketAddress(path, namespace);
+  }
+
+  @Override
+  public Socket createSocket() throws IOException {
+    return create();
+  }
+
+  @Override
+  public Socket createSocket(String host, int port) throws IOException {
+    return createAndConnect();
+  }
+
+  @Override
+  public Socket createSocket(String host, int port, InetAddress localHost, int localPort)
+      throws IOException {
+    return createAndConnect();
+  }
+
+  @Override
+  public Socket createSocket(InetAddress host, int port) throws IOException {
+    return createAndConnect();
+  }
+
+  @Override
+  public Socket createSocket(InetAddress address, int port, InetAddress localAddress, int localPort)
+      throws IOException {
+    return createAndConnect();
+  }
+
+  private Socket create() {
+    return new UdsSocket(localSocketAddress);
+  }
+
+  private Socket createAndConnect() throws IOException {
+    Socket socket = create();
+    SocketAddress unusedAddress = new InetSocketAddress(0);
+    socket.connect(unusedAddress);
+    return socket;
+  }
+}