HDFS-10477. Stop decommission a rack of DataNodes caused NameNode fail over to standby. Contributed by yunjiong zhao and Wei-Chiu Chuang.

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

@@ -3623,17 +3623,42 @@ public class BlockManager implements BlockStatsMXBean {
     if (!isPopulatingReplQueues()) {
       return;
     }
-    final Iterator<BlockInfo> it = srcNode.getBlockIterator();
+
     int numOverReplicated = 0;
-    while(it.hasNext()) {
-      final BlockInfo block = it.next();
-      short expectedReplication = getExpectedReplicaNum(block);
-      NumberReplicas num = countNodes(block);
-      int numCurrentReplica = num.liveReplicas();
-      if (numCurrentReplica > expectedReplication) {
-        // over-replicated block 
-        processOverReplicatedBlock(block, expectedReplication, null, null);
-        numOverReplicated++;
+    for (DatanodeStorageInfo datanodeStorageInfo : srcNode.getStorageInfos()) {
+      // the namesystem lock is released between iterations. Make sure the
+      // storage is not removed before continuing.
+      if (srcNode.getStorageInfo(datanodeStorageInfo.getStorageID()) == null) {
+        continue;
+      }
+      final Iterator<BlockInfo> it = datanodeStorageInfo.getBlockIterator();
+      while(it.hasNext()) {
+        final BlockInfo block = it.next();
+        if (block.isDeleted()) {
+          //Orphan block, will be handled eventually, skip
+          continue;
+        }
+        short expectedReplication = this.getExpectedReplicaNum(block);
+        NumberReplicas num = countNodes(block);
+        int numCurrentReplica = num.liveReplicas();
+        if (numCurrentReplica > expectedReplication) {
+          // over-replicated block
+          processOverReplicatedBlock(block, expectedReplication, null,
+              null);
+          numOverReplicated++;
+        }
+      }
+      // When called by tests like TestDefaultBlockPlacementPolicy.
+      // testPlacementWithLocalRackNodesDecommissioned, it is not protected by
+      // lock, only when called by DatanodeManager.refreshNodes have writeLock
+      if (namesystem.hasWriteLock()) {
+        namesystem.writeUnlock();
+        try {
+          Thread.sleep(1);
+        } catch (InterruptedException e) {
+          Thread.currentThread().interrupt();
+        }
+        namesystem.writeLock();
       }
     }
     LOG.info("Invalidated " + numOverReplicated + " over-replicated blocks on " +