HDFS-11681. DatanodeStorageInfo#getBlockIterator() should return an iterator to an unmodifiable set Contributed by Virajith Jalaparti

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockManager.java

@@ -1409,12 +1409,15 @@ public class BlockManager implements BlockStatsMXBean {
   void removeBlocksAssociatedTo(final DatanodeDescriptor node) {
     for (DatanodeStorageInfo storage : node.getStorageInfos()) {
       final Iterator<BlockInfo> it = storage.getBlockIterator();
+      //add the BlockInfos to a new collection as the
+      //returned iterator is not modifiable.
+      Collection<BlockInfo> toRemove = new ArrayList<>();
       while (it.hasNext()) {
-        BlockInfo block = it.next();
-        // DatanodeStorageInfo must be removed using the iterator to avoid
-        // ConcurrentModificationException in the underlying storage
-        it.remove();
-        removeStoredBlock(block, node);
+        toRemove.add(it.next());
+      }
+
+      for (BlockInfo b : toRemove) {
+        removeStoredBlock(b, node);
       }
     }
     // Remove all pending DN messages referencing this DN.
@@ -1429,11 +1432,11 @@ public class BlockManager implements BlockStatsMXBean {
     assert namesystem.hasWriteLock();
     final Iterator<BlockInfo> it = storageInfo.getBlockIterator();
     DatanodeDescriptor node = storageInfo.getDatanodeDescriptor();
-    while(it.hasNext()) {
-      BlockInfo block = it.next();
-      // DatanodeStorageInfo must be removed using the iterator to avoid
-      // ConcurrentModificationException in the underlying storage
-      it.remove();
+    Collection<BlockInfo> toRemove = new ArrayList<>();
+    while (it.hasNext()) {
+      toRemove.add(it.next());
+    }
+    for (BlockInfo block : toRemove) {
       removeStoredBlock(block, node);
       final Block b = getBlockOnStorage(block, storageInfo);
       if (b != null) {

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeStorageInfo.java

@@ -18,6 +18,7 @@
 package org.apache.hadoop.hdfs.server.blockmanagement;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 
@@ -270,8 +271,12 @@ public class DatanodeStorageInfo {
     return blocks.size();
   }
   
+  /**
+   * @return iterator to an unmodifiable set of blocks
+   * related to this {@link DatanodeStorageInfo}
+   */
   Iterator<BlockInfo> getBlockIterator() {
-    return blocks.iterator();
+    return Collections.unmodifiableSet(blocks).iterator();
   }
 
   void updateState(StorageReport r) {

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestGetBlocks.java

@@ -307,6 +307,13 @@ public class TestGetBlocks {
           BlockManagerTestUtil.getBlockIterator(s);
       while(storageBlockIt.hasNext()) {
         allBlocks[idx++] = storageBlockIt.next();
+        try {
+          storageBlockIt.remove();
+          assertTrue(
+              "BlockInfo iterator should have been unmodifiable", false);
+        } catch (UnsupportedOperationException e) {
+          //expected exception
+        }
       }
     }