HDFS-8850. VolumeScanner thread exits with exception if there is no block pool to be scanned but there are suspicious blocks. (Colin Patrick McCabe via yliu)

(cherry picked from commit d2b941f94a835f7bdde7714d21a470b505aa582b)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -53,6 +53,10 @@ Release 2.7.2 - UNRELEASED
     HDFS-9106. Transfer failure during pipeline recovery causes permanent
     write failures (kihwal)
 
+    HDFS-8850. VolumeScanner thread exits with exception if there is no block
+    pool to be scanned but there are suspicious blocks. (Colin Patrick McCabe
+    via yliu)
+
 Release 2.7.1 - 2015-07-06
 
   INCOMPATIBLE CHANGES

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/VolumeScanner.java

@@ -536,11 +536,13 @@ public class VolumeScanner extends Thread {
           return 0;
         }
       }
-      long saveDelta = monotonicMs - curBlockIter.getLastSavedMs();
-      if (saveDelta >= conf.cursorSaveMs) {
-        LOG.debug("{}: saving block iterator {} after {} ms.",
-            this, curBlockIter, saveDelta);
-        saveBlockIterator(curBlockIter);
+      if (curBlockIter != null) {
+        long saveDelta = monotonicMs - curBlockIter.getLastSavedMs();
+        if (saveDelta >= conf.cursorSaveMs) {
+          LOG.debug("{}: saving block iterator {} after {} ms.",
+              this, curBlockIter, saveDelta);
+          saveBlockIterator(curBlockIter);
+        }
       }
       bytesScanned = scanBlock(block, conf.targetBytesPerSec);
       if (bytesScanned >= 0) {