SOLR-8691: Cache index fingerprints per searcher

solr/CHANGES.txt

@@ -297,6 +297,8 @@ Other Changes
 * SOLR-8690: Make peersync fingerprinting optional with solr.disableFingerprint system
   property. (yonik)
 
+* SOLR-8691: Cache index fingerprints per searcher. (yonik)
+
 ======================= 5.5.0 =======================
 
 Consult the LUCENE_CHANGES.txt file for additional, low level, changes in this release

solr/core/src/java/org/apache/solr/search/SolrIndexSearcher.java

@@ -123,6 +123,7 @@ import org.apache.solr.schema.TrieFloatField;
 import org.apache.solr.schema.TrieIntField;
 import org.apache.solr.search.facet.UnInvertedField;
 import org.apache.solr.search.stats.StatsSource;
+import org.apache.solr.update.IndexFingerprint;
 import org.apache.solr.update.SolrIndexConfig;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -192,6 +193,9 @@ public class SolrIndexSearcher extends IndexSearcher implements Closeable, SolrI
   private final String path;
   private boolean releaseDirectory;
 
+  private volatile IndexFingerprint fingerprint;
+  private final Object fingerprintLock = new Object();
+
   private static DirectoryReader getReader(SolrCore core, SolrIndexConfig config, DirectoryFactory directoryFactory,
       String path) throws IOException {
     final Directory dir = directoryFactory.get(path, DirContext.DEFAULT, config.lockType);
@@ -2398,6 +2402,20 @@ public class SolrIndexSearcher extends IndexSearcher implements Closeable, SolrI
     return super.explain(QueryUtils.makeQueryable(query), doc);
   }
 
+  /** @lucene.internal
+   * gets a cached version of the IndexFingerprint for this searcher
+   **/
+  public IndexFingerprint getIndexFingerprint(long maxVersion) throws IOException {
+    // possibly expensive, so prevent more than one thread from calculating it for this searcher
+    synchronized (fingerprintLock) {
+      if (fingerprint == null) {
+        fingerprint = IndexFingerprint.getFingerprint(this, maxVersion);
+      }
+    }
+
+    return fingerprint;
+  }
+
   /////////////////////////////////////////////////////////////////////
   // SolrInfoMBean stuff: Statistics and Module Info
   /////////////////////////////////////////////////////////////////////

solr/core/src/java/org/apache/solr/update/IndexFingerprint.java

@@ -76,12 +76,12 @@ public class IndexFingerprint {
     return maxDoc;
   }
 
-  /** Opens a new realtime searcher and returns it's fingerprint */
+  /** Opens a new realtime searcher and returns it's (possibly cached) fingerprint */
   public static IndexFingerprint getFingerprint(SolrCore core, long maxVersion) throws IOException {
     core.getUpdateHandler().getUpdateLog().openRealtimeSearcher();
     RefCounted<SolrIndexSearcher> newestSearcher = core.getUpdateHandler().getUpdateLog().uhandler.core.getRealtimeSearcher();
     try {
-      return getFingerprint(newestSearcher.get(), maxVersion);
+      return newestSearcher.get().getIndexFingerprint(maxVersion);
     } finally {
       if (newestSearcher != null) {
         newestSearcher.decref();
@@ -89,6 +89,7 @@ public class IndexFingerprint {
     }
   }
 
+  /** Calculates an index fingerprint */
   public static IndexFingerprint getFingerprint(SolrIndexSearcher searcher, long maxVersion) throws IOException {
     RTimer timer = new RTimer();