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 +1,75 @@
+package org.apache.solr.index;
+
+import java.io.IOException;
+import java.util.Map;
+import org.apache.lucene.index.FilterMergePolicy;
+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.util.Version;
+
+/**
+ * Prevents any older version segment (< {@link Version.LATEST}), either original or one derived as
+ * a result of merging with an older version segment, from being considered for merges. That way a
+ * snapshot of older segments remains consistent. This assists in upgrading to a future Lucene major
+ * version if existing documents are reindexed in the current version with this merge policy in
+ * place.
+ */
+public class LatestVersionFilterMergePolicy extends FilterMergePolicy {
+
+  public LatestVersionFilterMergePolicy(MergePolicy in) {
+    super(in);
+  }
+
+  @Override
+  public MergeSpecification findMerges(
+      MergeTrigger mergeTrigger, SegmentInfos infos, MergeContext mergeContext) throws IOException {
+    return in.findMerges(mergeTrigger, getFilteredInfosClone(infos), mergeContext);
+  }
+
+  @Override
+  public MergeSpecification findForcedMerges(
+      SegmentInfos infos,
+      int maxSegmentCount,
+      Map<SegmentCommitInfo, Boolean> segmentsToMerge,
+      MergeContext mergeContext)
+      throws IOException {
+    return in.findForcedMerges(
+        getFilteredInfosClone(infos), maxSegmentCount, segmentsToMerge, mergeContext);
+  }
+
+  @Override
+  public MergeSpecification findForcedDeletesMerges(SegmentInfos infos, MergeContext mergeContext)
+      throws IOException {
+    return in.findForcedDeletesMerges(getFilteredInfosClone(infos), mergeContext);
+  }
+
+  @Override
+  public MergeSpecification findFullFlushMerges(
+      MergeTrigger mergeTrigger, SegmentInfos infos, MergeContext mergeContext) throws IOException {
+    return in.findFullFlushMerges(mergeTrigger, getFilteredInfosClone(infos), mergeContext);
+  }
+
+  private SegmentInfos getFilteredInfosClone(SegmentInfos infos) {
+    // We should not remove from the original SegmentInfos. Hence we clone.
+    SegmentInfos infosClone = infos.clone();
+    infosClone.clear();
+    for (SegmentCommitInfo info : infos) {
+      if (allowSegmentForMerge(info)) {
+        infosClone.add(info);
+      }
+    }
+    return infosClone;
+  }
+
+  /**
+   * Determines if a SegmentCommitInfo should be part of the candidate set of segments that will be
+   * considered for merges. By default, we only allow LATEST version segments to participate in
+   * merges.
+   */
+  protected boolean allowSegmentForMerge(SegmentCommitInfo info) {
+    return info.info.getMinVersion() != null
+        && info.info.getMinVersion().major == Version.LATEST.major;
+  }
+}