dsmiley commented on code in PR #3883:
URL: https://github.com/apache/solr/pull/3883#discussion_r2572071834
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solr/core/src/java/org/apache/solr/index/LatestVersionFilterMergePolicy.java:
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@@ -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) {
Review Comment:
nit: clone is an implementation detail that doesn't belong in the method.
For example, maybe we see that rebuilding this thing is expensive-ish and we
decide to first check if all segments are current to not clone. Or if no
segments are updated, and return an empty one.
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