[HBASE-20141] Fix TooManyFiles exception when RefreshingChannels

HBASE-19435 implements a fix for reopening file channels when they are unnexpected closed
to avoid disabling the BucketCache. However, it was missed that the the channels might not
actually be completely closed (the write or read channel might still be open
(see https://docs.oracle.com/javase/7/docs/api/java/nio/channels/ClosedChannelException.html)
This commit closes any open channels before creating a new channel.

hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/FileIOEngine.java

@@ -19,7 +19,6 @@
 package org.apache.hadoop.hbase.io.hfile.bucket;
 
 import java.io.File;
-import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.RandomAccessFile;
 import java.nio.ByteBuffer;
@@ -234,7 +233,17 @@ public class FileIOEngine implements IOEngine {
     return fileNum;
   }
 
-  private void refreshFileConnection(int accessFileNum) throws FileNotFoundException {
+  @VisibleForTesting
+  FileChannel[] getFileChannels() {
+    return fileChannels;
+  }
+
+  @VisibleForTesting
+  void refreshFileConnection(int accessFileNum) throws IOException {
+    FileChannel fileChannel = fileChannels[accessFileNum];
+    if (fileChannel != null) {
+      fileChannel.close();
+    }
     rafs[accessFileNum] = new RandomAccessFile(filePaths[accessFileNum], "rw");
     fileChannels[accessFileNum] = rafs[accessFileNum].getChannel();
   }

hbase-server/src/test/java/org/apache/hadoop/hbase/io/hfile/bucket/TestFileIOEngine.java

@@ -19,10 +19,13 @@
 package org.apache.hadoop.hbase.io.hfile.bucket;
 
 import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
 
 import java.io.File;
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -129,4 +132,12 @@ public class TestFileIOEngine {
     fileIOEngine.read(ByteBuffer.wrap(data2), offset);
     assertArrayEquals(data1, data2);
   }
+
+  @Test
+  public void testRefreshFileConnectionClosesConnections() throws IOException {
+    FileChannel fileChannel = fileIOEngine.getFileChannels()[0];
+    assertNotNull(fileChannel);
+    fileIOEngine.refreshFileConnection(0);
+    assertFalse(fileChannel.isOpen());
+  }
 }