Fix SegmentInfos replace doesn't update userData

src/Lucene.Net.Tests/Index/TestIndexWriter.cs

@@ -816,6 +816,65 @@ public virtual void TestMaxThreadPriority()
             }
         }
 
+        private Dictionary<String, String> GetCommitData(IndexWriter writer)
+        {
+            Dictionary<String, String> data = new Dictionary<String, String>();            
+            foreach (var ent in writer.CommitData)
+            {
+                data.Put(ent.Key, ent.Value);
+            }            
+            return data;
+        }
+
+        [Test]
+        public void testGetCommitDataFromOldSnapshot() 
+        {
+            Directory dir = NewDirectory();
+            IndexWriterConfig conf = NewIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(Random)).SetMaxBufferedDocs(2).SetMergePolicy(NewLogMergePolicy());
+            conf.SetIndexDeletionPolicy(new SnapshotDeletionPolicy(NoDeletionPolicy.INSTANCE));
+            IndexWriter writer = new IndexWriter(dir, conf);
+            writer.SetCommitData(
+            new Dictionary<String, String>() 
+            {
+                { "key", "value" },
+            });
+            assertEquals("value", GetCommitData(writer).GetValueOrDefault("key"));
+            writer.Commit();
+            // Snapshot this commit to open later
+            IndexCommit indexCommit =
+                ((SnapshotDeletionPolicy)writer.Config.IndexDeletionPolicy).Snapshot();
+            writer.Dispose();
+
+            // Modify the commit data and commit on close so the most recent commit data is different
+            conf = NewIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(Random)).SetMaxBufferedDocs(2).SetMergePolicy(NewLogMergePolicy());
+            conf.SetIndexDeletionPolicy(new SnapshotDeletionPolicy(NoDeletionPolicy.INSTANCE));
+            writer = new IndexWriter(dir, conf);
+            writer.SetCommitData(
+            new Dictionary<String, String>() 
+              {
+                    {"key", "value2" },
+              });
+
+            assertEquals("value2", GetCommitData(writer).GetValueOrDefault("key"));
+            writer.Dispose();
+
+            // validate that when opening writer from older snapshotted index commit, the old commit data is
+            // visible
+            conf = NewIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(Random)).SetMaxBufferedDocs(2).SetMergePolicy(NewLogMergePolicy());
+            conf.SetIndexDeletionPolicy(new SnapshotDeletionPolicy(NoDeletionPolicy.INSTANCE));
+            writer =
+                new IndexWriter(
+                    dir,
+                    conf
+                        .SetOpenMode(OpenMode.APPEND)
+                        .SetIndexCommit(indexCommit));
+            assertEquals("value", GetCommitData(writer).GetValueOrDefault("key"));
+            writer.Dispose();
+
+            dir.Dispose();
+         }
+
+
         [Test]
         public virtual void TestVariableSchema()
         {

src/Lucene.Net/Index/SegmentInfos.cs

@@ -1452,6 +1452,7 @@ internal void Replace(SegmentInfos other)
         {
             RollbackSegmentInfos(other.AsList());
             lastGeneration = other.lastGeneration;
+            userData = other.userData;
         }
 
         /// <summary>
@@ -1632,4 +1633,4 @@ internal int IndexOf(SegmentCommitInfo si)
             return segments.IndexOf(si);
         }
     }
-}
+}