HDFS-13143. SnapshotDiff - snapshotDiffReport might be inconsistent if the snapshotDiff calculation happens between a snapshot and the current tree. Contributed by Shashikant Banerjee

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSClient.java

@@ -139,6 +139,7 @@
 import org.apache.hadoop.hdfs.protocol.SnapshottableDirectoryStatus;
 import org.apache.hadoop.hdfs.protocol.UnresolvedPathException;
 import org.apache.hadoop.hdfs.protocol.ZoneReencryptionStatus;
+import org.apache.hadoop.hdfs.protocol.SnapshotDiffReport;
 import org.apache.hadoop.hdfs.protocol.SnapshotDiffReportListing;
 import org.apache.hadoop.hdfs.protocol.datatransfer.DataTransferProtoUtil;
 import org.apache.hadoop.hdfs.protocol.datatransfer.IOStreamPair;
@@ -2118,6 +2119,20 @@ public void disallowSnapshot(String snapshotRoot) throws IOException {
   /**
    * Get the difference between two snapshots, or between a snapshot and the
    * current tree of a directory.
+   * @see ClientProtocol#getSnapshotDiffReport
+   */
+  public SnapshotDiffReport getSnapshotDiffReport(String snapshotDir,
+      String fromSnapshot, String toSnapshot) throws IOException {
+    checkOpen();
+    try (TraceScope ignored = tracer.newScope("getSnapshotDiffReport")) {
+      return namenode
+          .getSnapshotDiffReport(snapshotDir, fromSnapshot, toSnapshot);
+    } catch (RemoteException re) {
+      throw re.unwrapRemoteException();
+    }
+  }
+  /**
+   * Get the difference between two snapshots of a directory iteratively.
    * @see ClientProtocol#getSnapshotDiffReportListing
    */
   public SnapshotDiffReportListing getSnapshotDiffReportListing(

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DistributedFileSystem.java

@@ -2020,8 +2020,13 @@ <SnapshotDiffReportListing> snapshotDiffReportListingRemoteIterator(
       @Override
       public RemoteIterator<SnapshotDiffReportListing> doCall(final Path p)
           throws IOException {
-        return new SnapshotDiffReportListingIterator(
-            getPathName(p), fromSnapshot, toSnapshot);
+        if (!isValidSnapshotName(fromSnapshot) || !isValidSnapshotName(
+            toSnapshot)) {
+          throw new UnsupportedOperationException("Remote Iterator is"
+              + "supported for snapshotDiffReport between two snapshots");
+        }
+        return new SnapshotDiffReportListingIterator(getPathName(p),
+            fromSnapshot, toSnapshot);
       }
 
       @Override
@@ -2081,9 +2086,23 @@ public SnapshotDiffReportListing next() throws IOException {
     }
   }
 
+  private boolean isValidSnapshotName(String snapshotName) {
+    // If any of the snapshots specified in the getSnapshotDiffReport call
+    // is null or empty, it points to the current tree.
+    return (snapshotName != null && !snapshotName.isEmpty());
+  }
+
   private SnapshotDiffReport getSnapshotDiffReportInternal(
       final String snapshotDir, final String fromSnapshot,
       final String toSnapshot) throws IOException {
+    // In case the diff needs to be computed between a snapshot and the current
+    // tree, we should not do iterative diffReport computation as the iterative
+    // approach might fail if in between the rpc calls the current tree
+    // changes in absence of the global fsn lock.
+    if (!isValidSnapshotName(fromSnapshot) || !isValidSnapshotName(
+        toSnapshot)) {
+      return dfs.getSnapshotDiffReport(snapshotDir, fromSnapshot, toSnapshot);
+    }
     byte[] startPath = DFSUtilClient.EMPTY_BYTES;
     int index = -1;
     SnapshotDiffReportGenerator snapshotDiffReport;

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/protocol/ClientProtocol.java

@@ -1308,8 +1308,7 @@ SnapshotDiffReport getSnapshotDiffReport(String snapshotRoot,
       String fromSnapshot, String toSnapshot) throws IOException;
 
   /**
-   * Get the difference between two snapshots, or between a snapshot and the
-   * current tree of a directory.
+   * Get the difference between two snapshots of a directory iteratively.
    *
    * @param snapshotRoot
    *          full path of the directory where snapshots are taken

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

@@ -1539,4 +1539,17 @@ public void testSnapshotDiffReportRemoteIterator() throws Exception {
         new DiffReportEntry(DiffType.DELETE,
             DFSUtil.string2Bytes("dir3/file3")));
   }
+
+  @Test
+  public void testSnapshotDiffReportRemoteIterator2() throws Exception {
+    final Path root = new Path("/");
+    hdfs.mkdirs(root);
+    SnapshotTestHelper.createSnapshot(hdfs, root, "s0");
+    try {
+      hdfs.snapshotDiffReportListingRemoteIterator(root, "s0", "");
+    } catch (Exception e) {
+      Assert.assertTrue(e.getMessage().contains("Remote Iterator is"
+          + "supported for snapshotDiffReport between two snapshots"));
+    }
+  }
 }