Fix upgrade of custom similarity (#50851)

This change fixes the upgrade of index metadata that contain
a custom similarity with options that are not compatible with BM25.
The upgrade doesn't need a real similarity service so we fake one that
resolves all custom similarity to BM25 but this logic fails because the
BM25 provider checks that all options are compatible. This commit removes
the verification step as it is not needed during the upgrade (the verification
is done when the index is restored/opened).

Closes #50763

Author: jimczi

server/src/main/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeService.java

@@ -22,6 +22,7 @@
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.apache.lucene.analysis.Analyzer;
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.Similarity;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.TriFunction;
@@ -145,7 +146,7 @@ public boolean containsKey(Object key) {
                 @Override
                 public TriFunction<Settings, Version, ScriptService, Similarity> get(Object key) {
                     assert key instanceof String : "key must be a string but was: " + key.getClass();
-                    return SimilarityService.BUILT_IN.get(SimilarityService.DEFAULT_SIMILARITY);
+                    return (settings, version, scriptService) -> new BM25Similarity();
                 }
 
                 // this entrySet impl isn't fully correct but necessary as SimilarityService will iterate

server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataIndexUpgradeServiceTests.java

@@ -82,6 +82,18 @@ public void testUpgrade() {
         assertSame(src, service.upgradeIndexMetaData(src, Version.CURRENT.minimumIndexCompatibilityVersion())); // no double upgrade
     }
 
+    public void testUpgradeCustomSimilarity() {
+        MetaDataIndexUpgradeService service = getMetaDataIndexUpgradeService();
+        IndexMetaData src = newIndexMeta("foo",
+            Settings.builder()
+                .put("index.similarity.my_similarity.type", "DFR")
+                .put("index.similarity.my_similarity.after_effect", "l")
+                .build());
+        assertFalse(service.isUpgraded(src));
+        src = service.upgradeIndexMetaData(src, Version.CURRENT.minimumIndexCompatibilityVersion());
+        assertTrue(service.isUpgraded(src));
+    }
+
     public void testIsUpgraded() {
         MetaDataIndexUpgradeService service = getMetaDataIndexUpgradeService();
         IndexMetaData src = newIndexMeta("foo", Settings.builder().put("index.refresh_interval", "-200").build());