make ClientContext work properly with old surface (#312)

This fixes a bug introduced in #305.
UnaryCallable uses ClientContext to pass values.
To ease transition, old surfaces are marked deprecated
but reimplemented using ClientContext.

These old surfaces did not populate closeables,
so ClientContext cannot be initialized.
This PR side steps this problem by initializing closeables
to empty list.

gax-grpc/src/main/java/com/google/api/gax/grpc/ClientContext.java

@@ -37,6 +37,7 @@
 import io.grpc.ManagedChannel;
 import java.io.IOException;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.concurrent.ScheduledExecutorService;
 import javax.annotation.Nullable;
 
@@ -59,7 +60,8 @@ public abstract class ClientContext {
   public abstract Credentials getCredentials();
 
   static Builder newBuilder() {
-    return new AutoValue_ClientContext.Builder();
+    return new AutoValue_ClientContext.Builder()
+        .setCloseables(Collections.<AutoCloseable>emptyList());
   }
 
   public static ClientContext create(ClientSettings settings) throws IOException {

gax-grpc/src/test/java/com/google/api/gax/grpc/UnaryCallableTest.java

@@ -39,6 +39,7 @@
 import com.google.api.gax.batching.RequestBuilder;
 import com.google.api.gax.batching.TrackedFlowController;
 import com.google.api.gax.core.FakeApiClock;
+import com.google.api.gax.grpc.testing.FakeMethodDescriptor;
 import com.google.api.gax.paging.FixedSizeCollection;
 import com.google.api.gax.paging.Page;
 import com.google.api.gax.retrying.RetrySettings;
@@ -137,6 +138,27 @@ public ApiFuture<ResponseT> futureCall(RequestT request, CallContext context) {
     }
   }
 
+  @Test
+  public void createNoThrow() {
+    SimpleCallSettings settings =
+        SimpleCallSettings.newBuilder(FakeMethodDescriptor.create())
+            .setRetryableCodes()
+            .setRetrySettingsBuilder(
+                RetrySettings.newBuilder()
+                    .setTotalTimeout(Duration.ZERO)
+                    .setInitialRetryDelay(Duration.ZERO)
+                    .setRetryDelayMultiplier(1)
+                    .setMaxRetryDelay(Duration.ZERO)
+                    .setMaxAttempts(1)
+                    .setInitialRpcTimeout(Duration.ZERO)
+                    .setRpcTimeoutMultiplier(1)
+                    .setMaxRpcTimeout(Duration.ZERO))
+            .build();
+    Channel channel = Mockito.mock(Channel.class);
+    ScheduledExecutorService executor = Mockito.mock(ScheduledExecutorService.class);
+    UnaryCallable.<Integer, Integer>create(settings, channel, executor);
+  }
+
   @Test
   public void simpleCall() throws Exception {
     StashCallable<Integer, Integer> stash = new StashCallable<>(1);