Add deprecation warning for Force Merge API (#44903)

This commit adds a deprecation warning in 7.x for the Force Merge API 
when both only_expunge_deletes and max_num_segments are set in a request.

Relates #44761

client/rest-high-level/src/test/java/org/elasticsearch/client/documentation/IndicesClientDocumentationIT.java

@@ -1313,6 +1313,10 @@ public class IndicesClientDocumentationIT extends ESRestHighLevelClientTestCase
             request.onlyExpungeDeletes(true); // <1>
             // end::force-merge-request-only-expunge-deletes
 
+            // set only expunge deletes back to its default value
+            // as it is mutually exclusive with max. num. segments
+            request.onlyExpungeDeletes(ForceMergeRequest.Defaults.ONLY_EXPUNGE_DELETES);
+
             // tag::force-merge-request-flush
             request.flush(true); // <1>
             // end::force-merge-request-flush

rest-api-spec/src/main/resources/rest-api-spec/test/indices.forcemerge/10_basic.yml

@@ -8,3 +8,24 @@
       indices.forcemerge:
         index: testing
         max_num_segments: 1
+
+---
+"Check deprecation warning when incompatible only_expunge_deletes and max_num_segments values are both set":
+  - skip:
+      version: " - 7.3.99"
+      reason: "deprecation warning about only_expunge_deletes and max_num_segments added in 7.4"
+      features: "warnings"
+
+  - do:
+      indices.create:
+        index: test
+
+  - do:
+      warnings:
+        - 'setting only_expunge_deletes and max_num_segments at the same time is deprecated and will be rejected in a future version'
+      indices.forcemerge:
+        index: test
+        max_num_segments: 10
+        only_expunge_deletes: true
+
+

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

@@ -19,10 +19,12 @@
 
 package org.elasticsearch.rest.action.admin.indices;
 
+import org.apache.logging.log4j.LogManager;
 import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeRequest;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.client.node.NodeClient;
 import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.RestController;
@@ -35,6 +37,8 @@ import static org.elasticsearch.rest.RestRequest.Method.POST;
 
 public class RestForceMergeAction extends BaseRestHandler {
 
+    private static final DeprecationLogger deprecationLogger = new DeprecationLogger(LogManager.getLogger(RestForceMergeAction.class));
+
     public RestForceMergeAction(final Settings settings, final RestController controller) {
         super(settings);
         controller.registerHandler(POST, "/_forcemerge", this);
@@ -53,6 +57,10 @@ public class RestForceMergeAction extends BaseRestHandler {
         mergeRequest.maxNumSegments(request.paramAsInt("max_num_segments", mergeRequest.maxNumSegments()));
         mergeRequest.onlyExpungeDeletes(request.paramAsBoolean("only_expunge_deletes", mergeRequest.onlyExpungeDeletes()));
         mergeRequest.flush(request.paramAsBoolean("flush", mergeRequest.flush()));
+        if (mergeRequest.onlyExpungeDeletes() && mergeRequest.maxNumSegments() != ForceMergeRequest.Defaults.MAX_NUM_SEGMENTS) {
+            deprecationLogger.deprecatedAndMaybeLog("force_merge_expunge_deletes_and_max_num_segments_deprecation",
+               "setting only_expunge_deletes and max_num_segments at the same time is deprecated and will be rejected in a future version");
+        }
         return channel -> client.admin().indices().forceMerge(mergeRequest, new RestToXContentListener<>(channel));
     }
 

server/src/test/java/org/elasticsearch/action/admin/indices/forcemerge/RestForceMergeActionTests.java

@@ -26,15 +26,25 @@ import org.elasticsearch.common.xcontent.NamedXContentRegistry;
 import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.common.xcontent.json.JsonXContent;
 import org.elasticsearch.rest.RestController;
+import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.rest.action.admin.indices.RestForceMergeAction;
-import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.test.rest.FakeRestChannel;
 import org.elasticsearch.test.rest.FakeRestRequest;
+import org.elasticsearch.test.rest.RestActionTestCase;
+import org.junit.Before;
+
+import java.util.HashMap;
+import java.util.Map;
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.mockito.Mockito.mock;
 
-public class RestForceMergeActionTests extends ESTestCase {
+public class RestForceMergeActionTests extends RestActionTestCase {
+
+    @Before
+    public void setUpAction() {
+        new RestForceMergeAction(Settings.EMPTY, controller());
+    }
 
     public void testBodyRejection() throws Exception {
         final RestForceMergeAction handler = new RestForceMergeAction(Settings.EMPTY, mock(RestController.class));
@@ -48,4 +58,20 @@ public class RestForceMergeActionTests extends ESTestCase {
         assertThat(e.getMessage(), equalTo("request [GET /_forcemerge] does not support having a body"));
     }
 
+    public void testDeprecationMessage() {
+        final Map<String, String> params = new HashMap<>();
+        params.put("only_expunge_deletes", Boolean.TRUE.toString());
+        params.put("max_num_segments", Integer.toString(randomIntBetween(0, 10)));
+        params.put("flush", Boolean.toString(randomBoolean()));
+
+        final RestRequest request = new FakeRestRequest.Builder(NamedXContentRegistry.EMPTY)
+            .withPath("/_forcemerge")
+            .withMethod(RestRequest.Method.POST)
+            .withParams(params)
+            .build();
+
+        dispatchRequest(request);
+        assertWarnings("setting only_expunge_deletes and max_num_segments at the same time is deprecated " +
+            "and will be rejected in a future version");
+    }
 }