Revert "Remove PreShardSyncedFlush (#136409)"

server/src/main/java/org/elasticsearch/TransportVersions.java

@@ -60,6 +60,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion V_7_8_1 = def(7_08_01_99);
     public static final TransportVersion V_7_9_0 = def(7_09_00_99);
     public static final TransportVersion V_7_10_0 = def(7_10_00_99);
+    public static final TransportVersion V_8_0_0 = def(8_00_00_99);
     public static final TransportVersion V_8_8_0 = def(8_08_00_99);
     public static final TransportVersion V_8_8_1 = def(8_08_01_99);
     /*

server/src/main/java/org/elasticsearch/action/admin/indices/flush/TransportShardFlushAction.java

@@ -9,6 +9,7 @@
 
 package org.elasticsearch.action.admin.indices.flush;
 
+import org.elasticsearch.TransportVersions;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.ActionType;
 import org.elasticsearch.action.support.ActionFilters;
@@ -17,11 +18,17 @@
 import org.elasticsearch.cluster.action.shard.ShardStateAction;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.shard.IndexShard;
+import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.indices.IndicesService;
 import org.elasticsearch.injection.guice.Inject;
+import org.elasticsearch.tasks.Task;
 import org.elasticsearch.threadpool.ThreadPool;
+import org.elasticsearch.transport.AbstractTransportRequest;
+import org.elasticsearch.transport.TransportChannel;
+import org.elasticsearch.transport.TransportRequestHandler;
 import org.elasticsearch.transport.TransportService;
 
 import java.io.IOException;
@@ -57,6 +64,12 @@ public TransportShardFlushAction(
             PrimaryActionExecution.RejectOnOverload,
             ReplicaActionExecution.SubjectToCircuitBreaker
         );
+        transportService.registerRequestHandler(
+            PRE_SYNCED_FLUSH_ACTION_NAME,
+            threadPool.executor(ThreadPool.Names.FLUSH),
+            PreShardSyncedFlushRequest::new,
+            new PreSyncedFlushTransportHandler(indicesService)
+        );
     }
 
     @Override
@@ -83,4 +96,43 @@ protected void shardOperationOnReplica(ShardFlushRequest request, IndexShard rep
             return new ReplicaResult();
         }));
     }
+
+    // TODO: Remove this transition in 9.0
+    private static final String PRE_SYNCED_FLUSH_ACTION_NAME = "internal:indices/flush/synced/pre";
+
+    private static class PreShardSyncedFlushRequest extends AbstractTransportRequest {
+        private final ShardId shardId;
+
+        private PreShardSyncedFlushRequest(StreamInput in) throws IOException {
+            super(in);
+            assert in.getTransportVersion().before(TransportVersions.V_8_0_0) : "received pre_sync request from a new node";
+            this.shardId = new ShardId(in);
+        }
+
+        @Override
+        public String toString() {
+            return "PreShardSyncedFlushRequest{" + "shardId=" + shardId + '}';
+        }
+
+        @Override
+        public void writeTo(StreamOutput out) throws IOException {
+            assert false : "must not send pre_sync request from a new node";
+            throw new UnsupportedOperationException("");
+        }
+    }
+
+    private static final class PreSyncedFlushTransportHandler implements TransportRequestHandler<PreShardSyncedFlushRequest> {
+        private final IndicesService indicesService;
+
+        PreSyncedFlushTransportHandler(IndicesService indicesService) {
+            this.indicesService = indicesService;
+        }
+
+        @Override
+        public void messageReceived(PreShardSyncedFlushRequest request, TransportChannel channel, Task task) {
+            IndexShard indexShard = indicesService.indexServiceSafe(request.shardId.getIndex()).getShard(request.shardId.id());
+            indexShard.flush(new FlushRequest().force(false).waitIfOngoing(true));
+            throw new UnsupportedOperationException("Synced flush was removed and a normal flush was performed instead.");
+        }
+    }
 }