Show concrete error when enrich index not exist rather than NPE

docs/changelog/99604.yaml

@@ -0,0 +1,5 @@
+pr: 99604
+summary: Show concrete error when enrich index not exist rather than NPE
+area: Ingest Node
+type: enhancement
+issues: []

x-pack/plugin/enrich/src/main/java/org/elasticsearch/xpack/enrich/EnrichCache.java

@@ -14,6 +14,7 @@
 import org.elasticsearch.common.cache.Cache;
 import org.elasticsearch.common.cache.CacheBuilder;
 import org.elasticsearch.common.util.Maps;
+import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.search.SearchHit;
 import org.elasticsearch.xpack.core.enrich.action.EnrichStatsAction;
 
@@ -88,6 +89,9 @@ public EnrichStatsAction.Response.CacheStats getStats(String localNodeId) {
     private String getEnrichIndexKey(SearchRequest searchRequest) {
         String alias = searchRequest.indices()[0];
         IndexAbstraction ia = metadata.getIndicesLookup().get(alias);
+        if (ia == null) {
+            throw new IndexNotFoundException("no generated enrich index [" + alias + "]");
+        }
         return ia.getIndices().get(0).getName();
     }
 

x-pack/plugin/enrich/src/test/java/org/elasticsearch/xpack/enrich/EnrichCacheTests.java

@@ -10,6 +10,7 @@
 import org.elasticsearch.cluster.metadata.AliasMetadata;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.metadata.Metadata;
+import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.index.query.MatchQueryBuilder;
 import org.elasticsearch.search.builder.SearchSourceBuilder;
@@ -21,6 +22,7 @@
 import java.util.List;
 import java.util.Map;
 
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.notNullValue;
@@ -183,4 +185,22 @@ public void testDeepCopy() {
         assertArrayEquals(new byte[] { 1, 2, 3 }, (byte[]) result.get("embedded_object"));
     }
 
+    public void testEnrichIndexNotExist() {
+        // Emulate cluster metadata:
+        var metadata = Metadata.builder().build();
+
+        // Emulated search request on a non-exist enrich index that an enrich processor could generate
+        var searchRequest = new SearchRequest(EnrichPolicy.getBaseName("policy-enrich-index-not-generated")).source(
+            new SearchSourceBuilder().query(new MatchQueryBuilder("test", "query"))
+        );
+        // Emulated search response (content doesn't matter, since it isn't used, it just a cache entry)
+        List<Map<?, ?>> searchResponse = List.of(Map.of("test", "entry"));
+
+        EnrichCache enrichCache = new EnrichCache(1);
+        enrichCache.setMetadata(metadata);
+
+        IndexNotFoundException e = expectThrows(IndexNotFoundException.class, () -> enrichCache.put(searchRequest, searchResponse));
+        assertThat(e.getMessage(), containsString("no generated enrich index [.enrich-policy-enrich-index-not-generated]"));
+    }
+
 }