LUCENE-8334: Ensure SR#getSementInfo() returns snapshot

The SegmentCommitInfo passed to the segment reader is mutated concurrently.
An instance obtained from SR#getSegmentInfo() might return wrong delete counts
or generation ids. This ensures that the SR will use a clone internally while stil
maintaining the original SI since it's needed inside IW for maintainance like
accessing pooled readers etc.

lucene/core/src/java/org/apache/lucene/index/FrozenBufferedUpdates.java

@@ -397,7 +397,7 @@ final class FrozenBufferedUpdates {
           if (allDeleted == null) {
             allDeleted = new ArrayList<>();
           }
-          allDeleted.add(segState.reader.getSegmentInfo());
+          allDeleted.add(segState.reader.getOriginalSegmentInfo());
         }
       }
     }
@@ -452,7 +452,7 @@ final class FrozenBufferedUpdates {
 
     if (privateSegment != null) {
       assert segStates.length == 1;
-      assert privateSegment == segStates[0].reader.getSegmentInfo();
+      assert privateSegment == segStates[0].reader.getOriginalSegmentInfo();
     }
 
     totalDelCount += applyTermDeletes(segStates);

lucene/core/src/java/org/apache/lucene/index/IndexWriter.java

@@ -1457,7 +1457,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit, Accountable,
       throw new IllegalArgumentException("the reader must be a SegmentReader or composite reader containing only SegmentReaders");
     }
 
-    final SegmentCommitInfo info = ((SegmentReader) reader).getSegmentInfo();
+    final SegmentCommitInfo info = ((SegmentReader) reader).getOriginalSegmentInfo();
 
     // TODO: this is a slow linear search, but, number of
     // segments should be contained unless something is
@@ -4306,7 +4306,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit, Accountable,
     final boolean drop = suppressExceptions == false;
     try (Closeable finalizer = merge::mergeFinished) {
       IOUtils.applyToAll(merge.readers, sr -> {
-        final ReadersAndUpdates rld = getPooledInstance(sr.getSegmentInfo(), false);
+        final ReadersAndUpdates rld = getPooledInstance(sr.getOriginalSegmentInfo(), false);
         // We still hold a ref so it should not have been removed:
         assert rld != null;
         if (drop) {

lucene/core/src/java/org/apache/lucene/index/ReaderPool.java

@@ -87,8 +87,8 @@ final class ReaderPool implements Closeable {
         SegmentReader segReader = (SegmentReader) leaf.reader();
         SegmentReader newReader = new SegmentReader(segmentInfos.info(i), segReader, segReader.getLiveDocs(),
             segReader.numDocs());
-        readerMap.put(newReader.getSegmentInfo(), new ReadersAndUpdates(segmentInfos.getIndexCreatedVersionMajor(),
+        readerMap.put(newReader.getOriginalSegmentInfo(), new ReadersAndUpdates(segmentInfos.getIndexCreatedVersionMajor(),
-            newReader, newPendingDeletes(newReader, newReader.getSegmentInfo())));
+            newReader, newPendingDeletes(newReader, newReader.getOriginalSegmentInfo())));
       }
     }
   }

lucene/core/src/java/org/apache/lucene/index/ReadersAndUpdates.java

@@ -99,7 +99,7 @@ final class ReadersAndUpdates {
    *
    * <p>NOTE: steals incoming ref from reader. */
   ReadersAndUpdates(int indexCreatedVersionMajor, SegmentReader reader, PendingDeletes pendingDeletes) throws IOException {
-    this(indexCreatedVersionMajor, reader.getSegmentInfo(), pendingDeletes);
+    this(indexCreatedVersionMajor, reader.getOriginalSegmentInfo(), pendingDeletes);
     assert pendingDeletes.numPendingDeletes() >= 0
         : "got " + pendingDeletes.numPendingDeletes() + " reader.numDeletedDocs()=" + reader.numDeletedDocs() + " info.getDelCount()=" + info.getDelCount() + " maxDoc=" + reader.maxDoc() + " numDocs=" + reader.numDocs();
     this.reader = reader;
@@ -200,7 +200,7 @@ final class ReadersAndUpdates {
   }
 
   public synchronized void release(SegmentReader sr) throws IOException {
-    assert info == sr.getSegmentInfo();
+    assert info == sr.getOriginalSegmentInfo();
     sr.decRef();
   }
 
@@ -235,7 +235,7 @@ final class ReadersAndUpdates {
     // force new liveDocs
     Bits liveDocs = pendingDeletes.getLiveDocs();
     if (liveDocs != null) {
-      return new SegmentReader(reader.getSegmentInfo(), reader, liveDocs,
+      return new SegmentReader(info, reader, liveDocs,
           info.info.maxDoc() - info.getDelCount() - pendingDeletes.numPendingDeletes());
     } else {
       // liveDocs == null and reader != null. That can only be if there are no deletes

lucene/core/src/java/org/apache/lucene/index/SegmentReader.java

@@ -45,6 +45,10 @@ import org.apache.lucene.util.IOUtils;
 public final class SegmentReader extends CodecReader {
        
   private final SegmentCommitInfo si;
+  // this is the original SI that IW uses internally but it's mutated behind the scenes
+  // and we don't want this SI to be used for anything. Yet, IW needs this to do maintainance
+  // and lookup pooled readers etc.
+  private final SegmentCommitInfo originalSi;
   private final LeafMetaData metaData;
   private final Bits liveDocs;
 
@@ -67,9 +71,9 @@ public final class SegmentReader extends CodecReader {
    * @throws CorruptIndexException if the index is corrupt
    * @throws IOException if there is a low-level IO error
    */
-  // TODO: why is this public?
+  SegmentReader(SegmentCommitInfo si, int createdVersionMajor, IOContext context) throws IOException {
-  public SegmentReader(SegmentCommitInfo si, int createdVersionMajor, IOContext context) throws IOException {
+    this.si = si.clone();
-    this.si = si;
+    this.originalSi = si;
     this.metaData = new LeafMetaData(createdVersionMajor, si.info.getMinVersion(), si.info.getIndexSort());
 
     // We pull liveDocs/DV updates from disk:
@@ -133,7 +137,8 @@ public final class SegmentReader extends CodecReader {
     if (liveDocs != null && liveDocs.length() != si.info.maxDoc()) {
       throw new IllegalArgumentException("maxDoc=" + si.info.maxDoc() + " but liveDocs.size()=" + liveDocs.length());
     }
-    this.si = si;
+    this.si = si.clone();
+    this.originalSi = si;
     this.metaData = sr.getMetaData();
     this.liveDocs = liveDocs;
     this.isNRT = isNRT;
@@ -348,4 +353,12 @@ public final class SegmentReader extends CodecReader {
   public LeafMetaData getMetaData() {
     return metaData;
   }
+
+  /**
+   * Returns the original SegmentInfo passed to the segment reader on creation time.
+   * {@link #getSegmentInfo()} returns a clone of this instance.
+   */
+  SegmentCommitInfo getOriginalSegmentInfo() {
+    return originalSi;
+  }
 }

lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java

@@ -3352,4 +3352,28 @@ public class TestIndexWriter extends LuceneTestCase {
     IOUtils.close(writer, dir);
   }
 
+  public void testSegmentInfoIsSnapshot() throws IOException {
+    Directory dir = newDirectory();
+    IndexWriterConfig config = newIndexWriterConfig();
+    config.setRAMBufferSizeMB(Integer.MAX_VALUE);
+    config.setMaxBufferedDocs(2); // no auto flush
+    IndexWriter writer = new IndexWriter(dir, config);
+    Document d = new Document();
+    d.add(new StringField("id", "doc-0", Field.Store.YES));
+    writer.addDocument(d);
+    d = new Document();
+    d.add(new StringField("id", "doc-1", Field.Store.YES));
+    writer.addDocument(d);
+    DirectoryReader reader = writer.getReader();
+    SegmentCommitInfo segmentInfo = ((SegmentReader) reader.leaves().get(0).reader()).getSegmentInfo();
+    SegmentCommitInfo originalInfo = ((SegmentReader) reader.leaves().get(0).reader()).getOriginalSegmentInfo();
+    assertEquals(0, originalInfo.getDelCount());
+    assertEquals(0, segmentInfo.getDelCount());
+    writer.deleteDocuments(new Term("id", "doc-0"));
+    writer.commit();
+    assertEquals(0, segmentInfo.getDelCount());
+    assertEquals(1, originalInfo.getDelCount());
+    IOUtils.close(reader, writer, dir);
+  }
+
 }