SOLR-17725: Add merge policy to block older segments from participating in merges

solr/core/src/java/org/apache/solr/index/LatestVersionFilterMergePolicy.java

@@ -0,0 +1,52 @@
+package org.apache.solr.index;
+
+import java.io.IOException;
+import java.util.Map;
+import org.apache.lucene.index.MergePolicy;
+import org.apache.lucene.index.MergeTrigger;
+import org.apache.lucene.index.SegmentCommitInfo;
+import org.apache.lucene.index.SegmentInfos;
+import org.apache.lucene.index.TieredMergePolicy;
+import org.apache.lucene.util.Version;
+
+/**
+ * Only allows latest version segments to be considered for merges. That way a snapshot of older
+ * segments can remain consistent
+ */
+public class LatestVersionFilterMergePolicy extends MergePolicy {
+  MergePolicy delegatePolicy = new TieredMergePolicy();
+
+  @Override
+  public MergeSpecification findMerges(
+      MergeTrigger mergeTrigger, SegmentInfos infos, MergeContext mergeContext) throws IOException {
+    /*we don't want to remove from the original SegmentInfos, else the segments may not carry forward upon a commit.
+    That would be catastrophic. Hence we clone.*/
+    SegmentInfos infosClone = infos.clone();
+    infosClone.clear();
+    for (SegmentCommitInfo info : infos) {
+      if (info.info.getMinVersion() != null
+          && info.info.getMinVersion().major == Version.LATEST.major) {
+        infosClone.add(info);
+      }
+    }
+
+    return delegatePolicy.findMerges(mergeTrigger, infosClone, mergeContext);
+  }
+
+  @Override
+  public MergeSpecification findForcedMerges(
+      SegmentInfos segmentInfos,
+      int maxSegmentCount,
+      Map<SegmentCommitInfo, Boolean> segmentsToMerge,
+      MergeContext mergeContext)
+      throws IOException {
+    return delegatePolicy.findForcedMerges(
+        segmentInfos, maxSegmentCount, segmentsToMerge, mergeContext);
+  }
+
+  @Override
+  public MergeSpecification findForcedDeletesMerges(
+      SegmentInfos segmentInfos, MergeContext mergeContext) throws IOException {
+    return delegatePolicy.findForcedDeletesMerges(segmentInfos, mergeContext);
+  }
+}