core: narrow SharedResourceHolder types, and make the scheduler unconfigurable

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

@@ -44,7 +44,7 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Random;
 import java.util.Set;
-import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executor;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.Level;
@@ -135,11 +135,11 @@ final class DnsNameResolver extends NameResolver {
   private final String authority;
   private final String host;
   private final int port;
-  private final Resource<ExecutorService> executorResource;
+  private final Resource<Executor> executorResource;
   @GuardedBy("this")
   private boolean shutdown;
   @GuardedBy("this")
-  private ExecutorService executor;
+  private Executor executor;
   @GuardedBy("this")
   private boolean resolving;
   @GuardedBy("this")
@@ -148,7 +148,7 @@ final class DnsNameResolver extends NameResolver {
   private final Runnable resolveRunnable;
 
   DnsNameResolver(@Nullable String nsAuthority, String name, Attributes params,
-      Resource<ExecutorService> executorResource, ProxyDetector proxyDetector,
+      Resource<Executor> executorResource, ProxyDetector proxyDetector,
       Stopwatch stopwatch, boolean isAndroid) {
     // TODO: if a DNS server is provided as nsAuthority, use it.
     // https://www.captechconsulting.com/blogs/accessing-the-dusty-corners-of-dns-with-java

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

@@ -501,17 +501,17 @@ public final class GrpcUtil {
   /**
    * Shared executor for channels.
    */
-  public static final Resource<ExecutorService> SHARED_CHANNEL_EXECUTOR =
+  public static final Resource<Executor> SHARED_CHANNEL_EXECUTOR =
-      new Resource<ExecutorService>() {
+      new Resource<Executor>() {
         private static final String NAME = "grpc-default-executor";
         @Override
-        public ExecutorService create() {
+        public Executor create() {
           return Executors.newCachedThreadPool(getThreadFactory(NAME + "-%d", true));
         }
 
         @Override
-        public void close(ExecutorService instance) {
+        public void close(Executor instance) {
-          instance.shutdown();
+          ((ExecutorService) instance).shutdown();
         }
 
         @Override
@@ -549,7 +549,7 @@ public final class GrpcUtil {
             throw new RuntimeException(e);
           }
 
-          return service;
+          return Executors.unconfigurableScheduledExecutorService(service);
         }
 
         @Override

core/src/test/java/io/grpc/internal/DnsNameResolverTest.java

@@ -58,7 +58,7 @@ import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Random;
-import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executor;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.Nullable;
 import org.junit.After;
@@ -96,15 +96,15 @@ public class DnsNameResolverTest {
   private final FakeClock fakeClock = new FakeClock();
   private final FakeClock fakeExecutor = new FakeClock();
 
-  private final Resource<ExecutorService> fakeExecutorResource =
+  private final Resource<Executor> fakeExecutorResource =
-      new Resource<ExecutorService>() {
+      new Resource<Executor>() {
         @Override
-        public ExecutorService create() {
+        public Executor create() {
           return fakeExecutor.getScheduledExecutorService();
         }
 
         @Override
-        public void close(ExecutorService instance) {
+        public void close(Executor instance) {
         }
       };
 

okhttp/src/main/java/io/grpc/okhttp/OkHttpChannelBuilder.java

@@ -124,16 +124,16 @@ public class OkHttpChannelBuilder extends
           .build();
 
   private static final long AS_LARGE_AS_INFINITE = TimeUnit.DAYS.toNanos(1000L);
-  private static final Resource<ExecutorService> SHARED_EXECUTOR =
+  private static final Resource<Executor> SHARED_EXECUTOR =
-      new Resource<ExecutorService>() {
+      new Resource<Executor>() {
         @Override
-        public ExecutorService create() {
+        public Executor create() {
           return Executors.newCachedThreadPool(GrpcUtil.getThreadFactory("grpc-okhttp-%d", true));
         }
 
         @Override
-        public void close(ExecutorService executor) {
+        public void close(Executor executor) {
-          executor.shutdown();
+          ((ExecutorService) executor).shutdown();
         }
       };
 
@@ -606,7 +606,7 @@ public class OkHttpChannelBuilder extends
       }
 
       if (usingSharedExecutor) {
-        SharedResourceHolder.release(SHARED_EXECUTOR, (ExecutorService) executor);
+        SharedResourceHolder.release(SHARED_EXECUTOR, executor);
       }
     }
   }