HDFS-13662. TestBlockReaderLocal#testStatisticsForErasureCodingRead is flaky

(cherry picked from commit 533138718cc05b78e0afe583d7a9bd30e8a48fdc)

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/client/impl/TestBlockReaderLocal.java

@@ -28,6 +28,7 @@ import java.nio.ByteBuffer;
 import java.util.UUID;
 import java.util.concurrent.TimeoutException;
 
+import org.apache.hadoop.fs.BlockLocation;
 import org.apache.hadoop.fs.ChecksumException;
 import org.apache.hadoop.fs.FSDataInputStream;
 import org.apache.hadoop.fs.FileSystem;
@@ -827,9 +828,12 @@ public class TestBlockReaderLocal {
 
       Path ecFile = new Path(ecDir, "file2");
       DFSTestUtil.createFile(fs, ecFile, length, repl, randomSeed);
-      // Shutdown one DataNode so that erasure coding decoding process can kick
-      // in.
-      cluster.shutdownDataNode(0);
+
+      // Shutdown a DataNode that holds a data block, to trigger EC decoding.
+      final BlockLocation[] locs = fs.getFileBlockLocations(ecFile, 0, length);
+      final String[] nodes = locs[0].getNames();
+      cluster.stopDataNode(nodes[0]);
+
       try (HdfsDataInputStream in = (HdfsDataInputStream) fs.open(ecFile)) {
         IOUtils.readFully(in, buf, 0, length);