Introduce channel Attributes; Use them to pass the binder target user to the NameResolver

api/src/main/java/io/grpc/Grpc.java

@@ -65,6 +65,15 @@ private Grpc() {
   @Documented
   public @interface TransportAttr {}
 
+  /**
+   * Annotation for Channel attributes. It follows the annotation semantics defined by {@link
+   * Attributes}.
+   */
+  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/00000000")
+  @Retention(RetentionPolicy.SOURCE)
+  @Documented
+  public @interface ChannelAttr {}
+
   /**
    * Creates a channel builder with a target string and credentials. The target can be either a
    * valid {@link NameResolver}-compliant URI, or an authority string.

api/src/main/java/io/grpc/NameResolver.java

@@ -22,6 +22,7 @@
 import com.google.common.base.MoreObjects;
 import com.google.common.base.Objects;
 import com.google.errorprone.annotations.InlineMe;
+import io.grpc.Grpc.ChannelAttr;
 import java.lang.annotation.Documented;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
@@ -281,6 +282,7 @@
     private final ProxyDetector proxyDetector;
     private final SynchronizationContext syncContext;
     private final ServiceConfigParser serviceConfigParser;
+    @ChannelAttr @Nullable private final Attributes channelAttributes;
     @Nullable private final ScheduledExecutorService scheduledExecutorService;
     @Nullable private final ChannelLogger channelLogger;
     @Nullable private final Executor executor;
@@ -291,6 +293,7 @@
         ProxyDetector proxyDetector,
         SynchronizationContext syncContext,
         ServiceConfigParser serviceConfigParser,
+        @ChannelAttr Attributes channelAttributes,
         @Nullable ScheduledExecutorService scheduledExecutorService,
         @Nullable ChannelLogger channelLogger,
         @Nullable Executor executor,
@@ -299,6 +302,7 @@
       this.proxyDetector = checkNotNull(proxyDetector, "proxyDetector not set");
       this.syncContext = checkNotNull(syncContext, "syncContext not set");
       this.serviceConfigParser = checkNotNull(serviceConfigParser, "serviceConfigParser not set");
+      this.channelAttributes = checkNotNull(channelAttributes, "channelAttributes not set");
       this.scheduledExecutorService = scheduledExecutorService;
       this.channelLogger = channelLogger;
       this.executor = executor;
@@ -363,6 +367,12 @@
       return serviceConfigParser;
     }
 
+    /** Returns {@link Attributes} of the associated Channel. */
+    @ChannelAttr
+    public Attributes getChannelAttributes() {
+      return channelAttributes;
+    }
+
     /**
      * Returns the {@link ChannelLogger} for the Channel served by this NameResolver.
      *
@@ -452,6 +462,7 @@
       private ProxyDetector proxyDetector;
       private SynchronizationContext syncContext;
       private ServiceConfigParser serviceConfigParser;
+      private Attributes channelAttributes = Attributes.EMPTY;
       private ScheduledExecutorService scheduledExecutorService;
       private ChannelLogger channelLogger;
       private Executor executor;
@@ -510,6 +521,12 @@
         return this;
       }
 
+      /** See {@link Args#getChannelAttributes}. This is a required field, default empty. */
+      public Builder setChannelAttributes(@ChannelAttr Attributes channelAttributes) {
+        this.channelAttributes = checkNotNull(channelAttributes);
+        return this;
+      }
+
       /**
        * See {@link Args#getChannelLogger}.
        *
@@ -548,10 +565,16 @@
        * @since 1.21.0
        */
       public Args build() {
-        return
-            new Args(
-                defaultPort, proxyDetector, syncContext, serviceConfigParser,
-                scheduledExecutorService, channelLogger, executor, overrideAuthority);
+        return new Args(
+            defaultPort,
+            proxyDetector,
+            syncContext,
+            serviceConfigParser,
+            channelAttributes,
+            scheduledExecutorService,
+            channelLogger,
+            executor,
+            overrideAuthority);
       }
     }
   }

binder/src/androidTest/java/io/grpc/binder/BinderChannelSmokeTest.java

@@ -25,6 +25,8 @@
 import android.content.Intent;
 import android.os.Parcel;
 import android.os.Parcelable;
+import android.os.UserHandle;
+import android.os.UserManager;
 import androidx.test.core.app.ApplicationProvider;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 import com.google.common.io.ByteStreams;
@@ -50,6 +52,7 @@
 import io.grpc.StatusRuntimeException;
 import io.grpc.internal.GrpcUtil;
 import io.grpc.internal.testing.FakeNameResolverProvider;
+import io.grpc.internal.testing.FakeNameResolverProvider.FakeNameResolver;
 import io.grpc.stub.ClientCalls;
 import io.grpc.stub.MetadataUtils;
 import io.grpc.stub.ServerCalls;
@@ -59,6 +62,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
+import java.util.List;
 import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicReference;
 import org.junit.After;
@@ -251,6 +255,26 @@ public void testConnectViaIntentFilter() throws Exception {
     assertThat(doCall("Hello").get()).isEqualTo("Hello");
   }
 
+  @Test
+  public void testTargetUserChannelAttribute() throws Exception {
+    UserManager userManager = appContext.getSystemService(UserManager.class);
+    List<UserHandle> userProfiles = userManager.getUserProfiles();
+    UserHandle someUser = userProfiles.get(0); // Guaranteed to be non-empty.
+
+    channel =
+        BinderChannelBuilder.forTarget(SERVER_TARGET_URI, appContext).bindAsUser(someUser).build();
+    assertThat(doCall("Hello").get()).isNotNull();
+
+    FakeNameResolver fakeNameResolver =
+        fakeNameResolverProvider.pollNextResolverCreated(5, SECONDS);
+    assertThat(
+            fakeNameResolver
+                .getArgs()
+                .getChannelAttributes()
+                .get(ApiConstants.CHANNEL_ATTR_TARGET_USER))
+        .isEqualTo(someUser);
+  }
+
   @Test
   public void testUncaughtServerException() throws Exception {
     // Use a poison parcelable to cause an unexpected Exception in the server's onTransact().

binder/src/main/java/io/grpc/binder/ApiConstants.java

@@ -17,7 +17,10 @@
 package io.grpc.binder;
 
 import android.content.Intent;
+import android.os.UserHandle;
+import io.grpc.Attributes;
 import io.grpc.ExperimentalApi;
+import io.grpc.Grpc.ChannelAttr;
 
 /** Constant parts of the gRPC binder transport public API. */
 @ExperimentalApi("https://github.com/grpc/grpc-java/issues/8022")
@@ -29,4 +32,20 @@ private ApiConstants() {}
    * themselves in a {@link android.app.Service#onBind(Intent)} call.
    */
   public static final String ACTION_BIND = "grpc.io.action.BIND";
+
+  /**
+   * The target Android user of a binder Channel.
+   *
+   * <p>In multi-user Android, the target user for an Intent is unfortunately not part of the intent
+   * itself. Instead, it's passed around as a separate argument wherever that Intent is needed.
+   * Following suit, this implementation of the binder transport accepts the target Android user as
+   * a parameter to BinderChannelBuilder -- unfortunately it's not in the target URI or the
+   * SocketAddress. Instead, downstream plugins such as {@link io.grpc.NameResolver}s and {@link
+   * io.grpc.LoadBalancer}s can use this attribute to obtain the Channel's target UserHandle. If the
+   * attribute is not set, the Channel's target is the Android user hosting the current process (the
+   * default).
+   */
+  @ChannelAttr
+  public static final Attributes.Key<UserHandle> CHANNEL_ATTR_TARGET_USER =
+      Attributes.Key.create("io.grpc.binder.CHANNEL_ATTR_TARGET_USER");
 }

binder/src/main/java/io/grpc/binder/BinderChannelBuilder.java

@@ -23,6 +23,7 @@
 import android.os.UserHandle;
 import androidx.annotation.RequiresApi;
 import com.google.errorprone.annotations.DoNotCall;
+import io.grpc.Attributes;
 import io.grpc.ExperimentalApi;
 import io.grpc.ForwardingChannelBuilder;
 import io.grpc.ManagedChannel;
@@ -159,6 +160,7 @@ public static BinderChannelBuilder forTarget(String target) {
 
   private final ManagedChannelImplBuilder managedChannelImplBuilder;
   private final BinderClientTransportFactory.Builder transportFactoryBuilder;
+  private final Attributes.Builder channelAttrBuilder = Attributes.newBuilder();
 
   private boolean strictLifecycleManagement;
 
@@ -250,6 +252,7 @@ public BinderChannelBuilder securityPolicy(SecurityPolicy securityPolicy) {
   @RequiresApi(30)
   public BinderChannelBuilder bindAsUser(UserHandle targetUserHandle) {
     transportFactoryBuilder.setTargetUserHandle(targetUserHandle);
+    channelAttrBuilder.set(ApiConstants.CHANNEL_ATTR_TARGET_USER, targetUserHandle);
     return this;
   }
 
@@ -284,6 +287,7 @@ public BinderChannelBuilder idleTimeout(long value, TimeUnit unit) {
   public ManagedChannel build() {
     transportFactoryBuilder.setOffloadExecutorPool(
         managedChannelImplBuilder.getOffloadExecutorPool());
+    managedChannelImplBuilder.setChannelAttributes(channelAttrBuilder.build());
     return super.build();
   }
 }

core/src/main/java/io/grpc/internal/ManagedChannelImpl.java

@@ -590,6 +590,7 @@ ClientStream newSubstream(
     this.authorityOverride = builder.authorityOverride;
     this.nameResolverArgs =
         NameResolver.Args.newBuilder()
+            .setChannelAttributes(builder.channelAttributes)
             .setDefaultPort(builder.getDefaultPort())
             .setProxyDetector(proxyDetector)
             .setSynchronizationContext(syncContext)

core/src/main/java/io/grpc/internal/ManagedChannelImplBuilder.java

@@ -35,6 +35,7 @@
 import io.grpc.CompressorRegistry;
 import io.grpc.DecompressorRegistry;
 import io.grpc.EquivalentAddressGroup;
+import io.grpc.Grpc.ChannelAttr;
 import io.grpc.InternalChannelz;
 import io.grpc.InternalConfiguratorRegistry;
 import io.grpc.ManagedChannel;
@@ -197,6 +198,8 @@
   @Nullable
   ProxyDetector proxyDetector;
 
+  @ChannelAttr Attributes channelAttributes = Attributes.EMPTY;
+
   private boolean authorityCheckerDisabled;
   private boolean statsEnabled = true;
   private boolean recordStartedRpcs = true;
@@ -663,6 +666,14 @@
     tracingEnabled = value;
   }
 
+  /**
+   * Makes the specified Attributes available to downstream plugins such as resolvers and load
+   * balancers.
+   */
+  public void setChannelAttributes(@ChannelAttr Attributes channelAttributes) {
+    this.channelAttributes = channelAttributes;
+  }
+
   /**
    * Verifies the authority is valid.
    */

testing/src/main/java/io/grpc/internal/testing/FakeNameResolverProvider.java

@@ -25,12 +25,16 @@
 import java.net.URI;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.concurrent.BlockingDeque;
+import java.util.concurrent.LinkedBlockingDeque;
+import java.util.concurrent.TimeUnit;
 
 /** A name resolver to always resolve the given URI into the given address. */
 public final class FakeNameResolverProvider extends NameResolverProvider {
 
   private final URI targetUri;
   private final SocketAddress address;
+  private final BlockingDeque<FakeNameResolver> resolversCreated = new LinkedBlockingDeque<>();
 
   public FakeNameResolverProvider(String targetUri, SocketAddress address) {
     this.targetUri = URI.create(targetUri);
@@ -40,7 +44,9 @@ public FakeNameResolverProvider(String targetUri, SocketAddress address) {
   @Override
   public NameResolver newNameResolver(URI targetUri, NameResolver.Args args) {
     if (targetUri.equals(this.targetUri)) {
-      return new FakeNameResolver(address);
+      FakeNameResolver result = new FakeNameResolver(address, args);
+      resolversCreated.add(result);
+      return result;
     }
     return null;
   }
@@ -65,15 +71,26 @@ public Collection<Class<? extends SocketAddress>> getProducedSocketAddressTypes(
     return Collections.singleton(address.getClass());
   }
 
+  /**
+   * Returns the next FakeNameResolver created by this provider, or null if none were created before
+   * the timeout lapsed.
+   */
+  public FakeNameResolver pollNextResolverCreated(long timeout, TimeUnit unit)
+      throws InterruptedException {
+    return resolversCreated.poll(timeout, unit);
+  }
+
   /** A single name resolver. */
-  private static final class FakeNameResolver extends NameResolver {
+  public static final class FakeNameResolver extends NameResolver {
     private static final String AUTHORITY = "fake-authority";
 
     private final SocketAddress address;
+    private final Args args;
     private volatile boolean shutdown;
 
-    private FakeNameResolver(SocketAddress address) {
+    private FakeNameResolver(SocketAddress address, Args args) {
       this.address = address;
+      this.args = args;
     }
 
     @Override
@@ -97,5 +114,9 @@ public String getServiceAuthority() {
     public void shutdown() {
       shutdown = true;
     }
+
+    public Args getArgs() {
+      return this.args;
+    }
   }
 }