HBASE-21888 Add a isClosed method to AsyncConnection

Signed-off-by: Guanghao Zhang <zghao@apache.org>

hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncConnection.java

@@ -21,7 +21,6 @@ import java.io.Closeable;
 import java.io.IOException;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutorService;
-
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.hbase.HBaseInterfaceAudience;
 import org.apache.hadoop.hbase.ServerName;
@@ -196,6 +195,12 @@ public interface AsyncConnection extends Closeable {
    */
   AsyncBufferedMutatorBuilder getBufferedMutatorBuilder(TableName tableName, ExecutorService pool);
 
+  /**
+   * Returns whether the connection is closed or not.
+   * @return true if this connection is closed
+   */
+  boolean isClosed();
+
   /**
    * Retrieve an Hbck implementation to fix an HBase cluster. The returned Hbck is not guaranteed to
    * be thread-safe. A new instance should be created by each thread. This is a lightweight

hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncConnectionImpl.java

@@ -105,6 +105,8 @@ class AsyncConnectionImpl implements AsyncConnection {
 
   private ChoreService authService;
 
+  private volatile boolean closed = false;
+
   public AsyncConnectionImpl(Configuration conf, AsyncRegistry registry, String clusterId,
       User user) {
     this.conf = conf;
@@ -140,11 +142,22 @@ class AsyncConnectionImpl implements AsyncConnection {
 
   @Override
   public void close() {
+    // As the code below is safe to be executed in parallel, here we do not use CAS or lock, just a
+    // simple volatile flag.
+    if (closed) {
+      return;
+    }
     IOUtils.closeQuietly(rpcClient);
     IOUtils.closeQuietly(registry);
     if (authService != null) {
       authService.shutdown();
     }
+    closed = true;
+  }
+
+  @Override
+  public boolean isClosed() {
+    return closed;
   }
 
   @Override

hbase-server/src/test/java/org/apache/hadoop/hbase/client/TestAsyncTable.java

@@ -111,11 +111,13 @@ public class TestAsyncTable {
     TEST_UTIL.createTable(TABLE_NAME, FAMILY);
     TEST_UTIL.waitTableAvailable(TABLE_NAME);
     ASYNC_CONN = ConnectionFactory.createAsyncConnection(TEST_UTIL.getConfiguration()).get();
+    assertFalse(ASYNC_CONN.isClosed());
   }
 
   @AfterClass
   public static void tearDownAfterClass() throws Exception {
     IOUtils.closeQuietly(ASYNC_CONN);
+    assertTrue(ASYNC_CONN.isClosed());
     TEST_UTIL.shutdownMiniCluster();
   }