Non existing synonyms sets do not fail shard recovery

modules/analysis-common/src/main/java/org/elasticsearch/analysis/common/SynonymTokenFilterFactory.java

@@ -72,7 +72,7 @@ public ReaderWithOrigin getRulesReader(SynonymTokenFilterFactory factory, IndexC
                     );
                 } else {
                     reader = new ReaderWithOrigin(
-                        Analysis.getReaderFromIndex(synonymsSet, factory.synonymsManagementAPIService),
+                        Analysis.getReaderFromIndex(synonymsSet, factory.synonymsManagementAPIService, factory.lenient),
                         "[" + synonymsSet + "] synonyms_set in .synonyms index",
                         synonymsSet
                     );

qa/mixed-cluster/build.gradle

@@ -70,9 +70,6 @@ excludeList.add('aggregations/percentiles_hdr_metric/Negative values test')
 // sync_id is removed in 9.0
 excludeList.add("cat.shards/10_basic/Help")
 
-// Can't work until auto-expand replicas is 0-1 for synonyms index
-excludeList.add("synonyms/90_synonyms_reloading_for_synset/Reload analyzers for specific synonym set")
-
 def clusterPath = getPath()
 
 buildParams.bwcVersions.withWireCompatible { bwcVersion, baseName ->

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/synonyms/110_synonyms_invalid.yml

@@ -313,3 +313,158 @@ setup:
       indices.stats: { index: test_index }
 
   - length: { indices: 0 }
+
+---
+"Load index with non existent synonyms set":
+  - requires:
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: PUT
+          path: /{index}
+          capabilities: [ synonyms_set_lenient_on_non_existing ]
+      reason: "requires synonyms_set_lenient_on_non_existing bug fix"
+  - do:
+      indices.create:
+        index: test_index
+        body:
+          settings:
+            index:
+              number_of_shards: 1
+              number_of_replicas: 0
+            analysis:
+              filter:
+                my_synonym_filter:
+                  type: synonym
+                  synonyms_set: set1
+                  updateable: true
+              analyzer:
+                my_analyzer:
+                  type: custom
+                  tokenizer: whitespace
+                  filter: [ lowercase, my_synonym_filter ]
+          mappings:
+            properties:
+              my_field:
+                type: text
+                search_analyzer: my_analyzer
+
+  - match: { acknowledged: true }
+  - match: { shards_acknowledged: true }
+
+  - do:
+      indices.stats: { index: test_index }
+
+  - match: { indices.test_index.health: "green" }
+
+  # Synonyms are not applied
+  - do:
+      indices.analyze:
+        index: test_index
+        body:
+          analyzer: my_analyzer
+          text: foo
+
+  - length: { tokens: 1 }
+  - match: { tokens.0.token: foo }
+
+
+  # Create synonyms set and check synonyms are applied
+  - do:
+      synonyms.put_synonym:
+        id: set1
+        body:
+          synonyms_set:
+            synonyms: "foo => bar, baz"
+
+  # This is to ensure that all index shards (write and read) are available. In serverless this can take some time.
+  - do:
+      cluster.health:
+        index: .synonyms
+        wait_for_status: green
+
+
+  - do:
+      indices.stats: { index: test_index }
+
+  - match: { indices.test_index.health: "green" }
+
+  # Synonyms are applied
+  - do:
+      indices.analyze:
+        index: test_index
+        body:
+          analyzer: my_analyzer
+          text: foo
+
+  - length: { tokens: 2 }
+
+---
+"Load index with non existent synonyms set and lenient set to false":
+
+  - do:
+      indices.create:
+        index: test_index
+        body:
+          settings:
+            index:
+              number_of_shards: 1
+              number_of_replicas: 0
+            analysis:
+              filter:
+                my_synonym_filter:
+                  type: synonym
+                  synonyms_set: set1
+                  updateable: true
+                  lenient: false
+              analyzer:
+                my_analyzer:
+                  type: custom
+                  tokenizer: whitespace
+                  filter: [ lowercase, my_synonym_filter ]
+          mappings:
+            properties:
+              my_field:
+                type: text
+                search_analyzer: my_analyzer
+
+  - match: { acknowledged: true }
+  - match: { shards_acknowledged: false }
+
+  - do:
+      indices.stats: { index: test_index }
+
+  - length: { indices: 0 }
+
+  # Create synonyms set and check synonyms are applied
+  - do:
+      synonyms.put_synonym:
+        id: set1
+        body:
+          synonyms_set:
+            synonyms: "foo => bar, baz"
+
+  # This is to ensure that all index shards (write and read) are available. In serverless this can take some time.
+  - do:
+      cluster.health:
+        index: .synonyms
+        wait_for_status: green
+
+  - do:
+      cluster.reroute:
+        retry_failed: true
+
+  - do:
+      cluster.health:
+        index: test_index
+        wait_for_status: green
+
+  # Synonyms are applied
+  - do:
+      indices.analyze:
+        index: test_index
+        body:
+          analyzer: my_analyzer
+          text: foo
+
+  - length: { tokens: 2 }
+

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/synonyms/80_synonyms_from_index.yml

@@ -165,34 +165,6 @@ setup:
                 query: hola
   - match: { hits.total.value: 1 }
 
----
-"Fail loading synonyms from index if synonyms_set doesn't exist":
-  - do:
-      indices.create:
-        index: another_index
-        body:
-          settings:
-            index:
-              number_of_shards: 1
-            analysis:
-              filter:
-                my_synonym_filter:
-                  type: synonym
-                  synonyms_set: set_missing
-                  updateable: true
-              analyzer:
-                my_analyzer:
-                  type: custom
-                  tokenizer: standard
-                  filter: [ lowercase, my_synonym_filter ]
-          mappings:
-            properties:
-              my_field:
-                type: text
-                search_analyzer: my_analyzer
-  - match: { acknowledged: true }
-  - match: { shards_acknowledged: false }
-
 ---
 "Load empty synonyms set from index for an analyzer":
   - do:

server/src/main/java/org/elasticsearch/index/analysis/Analysis.java

@@ -48,6 +48,7 @@
 import org.apache.lucene.analysis.th.ThaiAnalyzer;
 import org.apache.lucene.analysis.tr.TurkishAnalyzer;
 import org.apache.lucene.analysis.util.CSVUtil;
+import org.elasticsearch.ResourceNotFoundException;
 import org.elasticsearch.action.support.PlainActionFuture;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.settings.Settings;
@@ -353,10 +354,27 @@ public static Reader getReaderFromFile(Environment env, String filePath, String
         }
     }
 
-    public static Reader getReaderFromIndex(String synonymsSet, SynonymsManagementAPIService synonymsManagementAPIService) {
+    public static Reader getReaderFromIndex(
+        String synonymsSet,
+        SynonymsManagementAPIService synonymsManagementAPIService,
+        boolean ignoreMissing
+    ) {
         final PlainActionFuture<PagedResult<SynonymRule>> synonymsLoadingFuture = new PlainActionFuture<>();
         synonymsManagementAPIService.getSynonymSetRules(synonymsSet, synonymsLoadingFuture);
-        PagedResult<SynonymRule> results = synonymsLoadingFuture.actionGet();
+
+        PagedResult<SynonymRule> results;
+        try {
+            results = synonymsLoadingFuture.actionGet();
+        } catch (ResourceNotFoundException e) {
+            if (ignoreMissing == false) {
+                throw e;
+            }
+            logger.warn(
+                "Synonyms set {} not found - synonyms will not be applied to search results on indices that use this synonym set",
+                synonymsSet
+            );
+            results = new PagedResult<>(0, new SynonymRule[0]);
+        }
 
         SynonymRule[] synonymRules = results.pageResults();
         StringBuilder sb = new StringBuilder();

server/src/main/java/org/elasticsearch/rest/action/admin/indices/CreateIndexCapabilities.java

@@ -32,11 +32,14 @@ public class CreateIndexCapabilities {
 
     private static final String HUNSPELL_DICT_400 = "hunspell_dict_400";
 
+    private static final String SYNONYMS_SET_LENIENT_ON_NON_EXISTING = "synonyms_set_lenient_on_non_existing";
+
     public static final Set<String> CAPABILITIES = Set.of(
         LOGSDB_INDEX_MODE_CAPABILITY,
         LOOKUP_INDEX_MODE_CAPABILITY,
         NESTED_DENSE_VECTOR_SYNTHETIC_TEST,
         POORLY_FORMATTED_BAD_REQUEST,
-        HUNSPELL_DICT_400
+        HUNSPELL_DICT_400,
+        SYNONYMS_SET_LENIENT_ON_NON_EXISTING
     );
 }