HDFS-7926. NameNode implementation of ClientProtocol.truncate(..) is not idempotent. Contributed by Tsz Wo Nicholas Sze

(cherry picked from commit f446669afb5c3d31a00c65449f27088b39e11ae3)
(cherry picked from commit af80a98ace50934284dde417efc802bb094c8b4e)

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

@@ -827,6 +827,9 @@ Release 2.7.0 - UNRELEASED
     HDFS-6833.  DirectoryScanner should not register a deleting block with
     memory of DataNode.  (Shinichi Yamashita via szetszwo)
 
+    HDFS-7926. NameNode implementation of ClientProtocol.truncate(..) is not 
+    idempotent (Tsz Wo Nicholas Sze via brandonli)
+
     BREAKDOWN OF HDFS-7584 SUBTASKS AND RELATED JIRAS
 
       HDFS-7720. Quota by Storage Type API, tools and ClientNameNode

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

@@ -383,6 +383,7 @@ public class BlockInfoContiguousUnderConstruction extends BlockInfoContiguous {
 
   private void appendUCParts(StringBuilder sb) {
     sb.append("{UCState=").append(blockUCState)
+      .append(", truncateBlock=" + truncateBlock)
       .append(", primaryNodeIndex=").append(primaryNodeIndex)
       .append(", replicas=[");
     if (replicas != null) {

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java

@@ -2021,6 +2021,21 @@ public class FSNamesystem implements Namesystem, FSNamesystemMBean,
       throw new UnsupportedOperationException(
           "Cannot truncate lazy persist file " + src);
     }
+
+    // Check if the file is already being truncated with the same length
+    final BlockInfoContiguous last = file.getLastBlock();
+    if (last != null && last.getBlockUCState() == BlockUCState.UNDER_RECOVERY) {
+      final Block truncateBlock
+          = ((BlockInfoContiguousUnderConstruction)last).getTruncateBlock();
+      if (truncateBlock != null) {
+        final long truncateLength = file.computeFileSize(false, false)
+            + truncateBlock.getNumBytes();
+        if (newLength == truncateLength) {
+          return false;
+        }
+      }
+    }
+
     // Opening an existing file for truncate. May need lease recovery.
     recoverLeaseInternal(RecoverLeaseOp.TRUNCATE_FILE,
         iip, src, clientName, clientMachine, false);

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFileTruncate.java

@@ -166,6 +166,8 @@ public class TestFileTruncate {
       LOG.info("newLength=" + newLength + ", isReady=" + isReady);
       assertEquals("File must be closed for truncating at the block boundary",
           isReady, newLength % BLOCK_SIZE == 0);
+      assertEquals("Truncate is not idempotent",
+          isReady, fs.truncate(p, newLength));
       if (!isReady) {
         checkBlockRecovery(p);
       }