Add changelog and REST test

Signed-off-by: Michael Froh <[email protected]>

CHANGELOG.md

@@ -46,6 +46,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Changed
 - Indexed IP field supports `terms_query` with more than 1025 IP masks [#16391](https://github.com/opensearch-project/OpenSearch/pull/16391)
 - Make entries for dependencies from server/build.gradle to gradle version catalog ([#16707](https://github.com/opensearch-project/OpenSearch/pull/16707))
+- Sliced search only fans out to shards matched by the selected slice, reducing open search contexts ([#16771](https://github.com/opensearch-project/OpenSearch/pull/16771))
 
 ### Deprecated
 - Performing update operation with default pipeline or final pipeline is deprecated ([#16712](https://github.com/opensearch-project/OpenSearch/pull/16712))

rest-api-spec/src/main/resources/rest-api-spec/api/search_shards.json

@@ -62,6 +62,9 @@
         "default":"open",
         "description":"Whether to expand wildcard expression to concrete indices that are open, closed or both."
       }
+    },
+    "body":{
+      "description":"The search source (in order to specify slice parameters)"
     }
   }
 }

rest-api-spec/src/main/resources/rest-api-spec/test/search_shards/20_slice.yml

@@ -0,0 +1,85 @@
+---
+"Search shards with slice specified in body":
+  - do:
+      indices.create:
+        index: test_index
+        body:
+          settings:
+            index:
+              number_of_shards: 7
+              number_of_replicas: 0
+
+  - do:
+      search_shards:
+        index: test_index
+        body:
+          slice:
+            id: 0
+            max: 3
+  - length: { shards: 3 }
+  - match: { shards.0.0.index: "test_index" }
+  - match: { shards.0.0.shard: 0 }
+  - match: { shards.1.0.shard: 3 }
+  - match: { shards.2.0.shard: 6 }
+
+  - do:
+      search_shards:
+        index: test_index
+        body:
+          slice:
+            id: 1
+            max: 3
+  - length: { shards: 2 }
+  - match: { shards.0.0.index: "test_index" }
+  - match: { shards.0.0.shard: 1 }
+  - match: { shards.1.0.shard: 4 }
+
+  - do:
+      search_shards:
+        index: test_index
+        body:
+          slice:
+            id: 2
+            max: 3
+  - length: { shards: 2 }
+  - match: { shards.0.0.index: "test_index" }
+  - match: { shards.0.0.shard: 2 }
+  - match: { shards.1.0.shard: 5 }
+
+
+  - do:
+      search_shards:
+        index: test_index
+        preference: "_shards:0,2,4,6"
+        body:
+          slice:
+            id: 0
+            max: 3
+  - length: { shards: 2 }
+  - match: { shards.0.0.index: "test_index" }
+  - match: { shards.0.0.shard: 0 }
+  - match: { shards.1.0.shard: 6 }
+
+  - do:
+      search_shards:
+        index: test_index
+        preference: "_shards:0,2,4,6"
+        body:
+          slice:
+            id: 1
+            max: 3
+  - length: { shards: 1 }
+  - match: { shards.0.0.index: "test_index" }
+  - match: { shards.0.0.shard: 2 }
+
+  - do:
+      search_shards:
+        index: test_index
+        preference: "_shards:0,2,4,6"
+        body:
+          slice:
+            id: 2
+            max: 3
+  - length: { shards: 1 }
+  - match: { shards.0.0.index: "test_index" }
+  - match: { shards.0.0.shard: 4 }

server/src/main/java/org/opensearch/rest/action/admin/cluster/RestClusterSearchShardsAction.java

@@ -82,9 +82,9 @@ public RestChannelConsumer prepareRequest(final RestRequest request, final NodeC
         clusterSearchShardsRequest.routing(request.param("routing"));
         clusterSearchShardsRequest.preference(request.param("preference"));
         clusterSearchShardsRequest.indicesOptions(IndicesOptions.fromRequest(request, clusterSearchShardsRequest.indicesOptions()));
-        if (request.hasContent()) {
-            SearchSourceBuilder sourceBuilder = new SearchSourceBuilder();
-            request.withContentOrSourceParamParserOrNull(sourceBuilder::parseXContent);
+        if (request.hasContentOrSourceParam()) {
+           SearchSourceBuilder sourceBuilder = new SearchSourceBuilder();
+           sourceBuilder.parseXContent(request.contentOrSourceParamParser());
             if (sourceBuilder.slice() != null) {
                 clusterSearchShardsRequest.slice(sourceBuilder.slice());
             }