HDFS-13868. WebHDFS: GETSNAPSHOTDIFF API NPE when param "snapshotname" is given but "oldsnapshotname" is not. Contributed by Pranay Singh.

(cherry picked from commit 28ceb34a725cd06d28fb51361c49bb45464f5368)
(cherry picked from commit 7442b7941dc57ebf4bd0ded4d46cfbf5af6fea81)

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

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

@@ -2068,6 +2068,10 @@ public class DFSClient implements java.io.Closeable, RemotePeerFactory,
       String fromSnapshot, String toSnapshot) throws IOException {
     checkOpen();
     try (TraceScope ignored = tracer.newScope("getSnapshotDiffReport")) {
+      Preconditions.checkArgument(fromSnapshot != null,
+          "null fromSnapshot");
+      Preconditions.checkArgument(toSnapshot != null,
+          "null toSnapshot");
       return namenode.getSnapshotDiffReport(snapshotDir,
           fromSnapshot, toSnapshot);
     } catch (RemoteException re) {

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

@@ -271,7 +271,7 @@ public class DirectorySnapshottableFeature extends DirectoryWithSnapshotFeature
     Snapshot fromSnapshot = getSnapshotByName(snapshotRootDir, from);
     Snapshot toSnapshot = getSnapshotByName(snapshotRootDir, to);
     // if the start point is equal to the end point, return null
-    if (from.equals(to)) {
+    if (from != null && from.equals(to)) {
       return null;
     }
     SnapshotDiffInfo diffs = new SnapshotDiffInfo(snapshotRootDir,

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

@@ -245,6 +245,12 @@ public class TestSnapshotDiffReport {
     LOG.info(report.toString());
     assertEquals(0, report.getDiffList().size());
 
+    try {
+      report = hdfs.getSnapshotDiffReport(subsubsub1, null, "s2");
+      fail("Expect exception when providing null fromSnapshot ");
+    } catch (IllegalArgumentException e) {
+      GenericTestUtils.assertExceptionContains("null fromSnapshot", e);
+    }
     report = hdfs.getSnapshotDiffReport(subsubsub1, "s0", "s2");
     LOG.info(report.toString());
     assertEquals(0, report.getDiffList().size());

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/web/TestWebHDFS.java

@@ -743,6 +743,12 @@ public class TestWebHDFS {
       Assert.assertTrue(diffReport.getDiffList().contains(entry3));
       Assert.assertTrue(diffReport.getDiffList().contains(entry4));
       Assert.assertEquals(diffReport.getDiffList().size(), 5);
+
+      // Test with fromSnapshot and toSnapshot as null.
+      diffReport = webHdfs.getSnapshotDiffReport(foo, null, "s2");
+      Assert.assertEquals(diffReport.getDiffList().size(), 0);
+      diffReport = webHdfs.getSnapshotDiffReport(foo, "s1", null);
+      Assert.assertEquals(diffReport.getDiffList().size(), 5);
     } finally {
       if (cluster != null) {
         cluster.shutdown();