Add global checkpoint tracking on the primary

This commit adds local tracking of the global checkpoints on all shard
copies when a global checkpoint tracker is operating in primary
mode. With this, we relay the global checkpoint on a shard copy back to
the primary shard during replication operations. This serves as another
step towards adding a background sync of the global checkpoint to the
shard copies.

Relates #26666

build.gradle

@@ -175,7 +175,7 @@ task verifyVersions {
  * after the backport of the backcompat code is complete.
  */
 allprojects {
-  ext.bwc_tests_enabled = true
+  ext.bwc_tests_enabled = false
 }
 
 task verifyBwcTestsEnabled {

core/src/main/java/org/elasticsearch/action/resync/TransportResyncReplicationAction.java

@@ -93,7 +93,8 @@ protected void sendReplicaRequest(
         if (node.getVersion().onOrAfter(Version.V_6_0_0_alpha1)) {
             super.sendReplicaRequest(replicaRequest, node, listener);
         } else {
-            listener.onResponse(new ReplicaResponse(SequenceNumbersService.PRE_60_NODE_LOCAL_CHECKPOINT));
+            final long pre60NodeCheckpoint = SequenceNumbersService.PRE_60_NODE_CHECKPOINT;
+            listener.onResponse(new ReplicaResponse(pre60NodeCheckpoint, pre60NodeCheckpoint));
         }
     }
 

core/src/main/java/org/elasticsearch/action/support/replication/ReplicationOperation.java

@@ -32,6 +32,7 @@
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.util.set.Sets;
+import org.elasticsearch.index.seqno.SequenceNumbersService;
 import org.elasticsearch.index.shard.ReplicationGroup;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.rest.RestStatus;
@@ -173,6 +174,7 @@ public void onResponse(ReplicaResponse response) {
                 successfulShards.incrementAndGet();
                 try {
                     primary.updateLocalCheckpointForShard(shard.allocationId().getId(), response.localCheckpoint());
+                    primary.updateGlobalCheckpointForShard(shard.allocationId().getId(), response.globalCheckpoint());
                 } catch (final AlreadyClosedException e) {
                     // okay, the index was deleted or this shard was never activated after a relocation; fall through and finish normally
                 } catch (final Exception e) {
@@ -315,6 +317,14 @@ public interface Primary<
          */
         void updateLocalCheckpointForShard(String allocationId, long checkpoint);
 
+        /**
+         * Update the local knowledge of the global checkpoint for the specified allocation ID.
+         *
+         * @param allocationId     the allocation ID to update the global checkpoint for
+         * @param globalCheckpoint the global checkpoint
+         */
+        void updateGlobalCheckpointForShard(String allocationId, long globalCheckpoint);
+
         /**
          * Returns the local checkpoint on the primary shard.
          *
@@ -385,12 +395,24 @@ void markShardCopyAsStaleIfNeeded(ShardId shardId, String allocationId, Runnable
     }
 
     /**
-     * An interface to encapsulate the metadata needed from replica shards when they respond to operations performed on them
+     * An interface to encapsulate the metadata needed from replica shards when they respond to operations performed on them.
      */
     public interface ReplicaResponse {
 
-        /** the local check point for the shard. see {@link org.elasticsearch.index.seqno.SequenceNumbersService#getLocalCheckpoint()} */
+        /**
+         * The local checkpoint for the shard. See {@link SequenceNumbersService#getLocalCheckpoint()}.
+         *
+         * @return the local checkpoint
+         **/
         long localCheckpoint();
+
+        /**
+         * The global checkpoint for the shard. See {@link SequenceNumbersService#getGlobalCheckpoint()}.
+         *
+         * @return the global checkpoint
+         **/
+        long globalCheckpoint();
+
     }
 
     public static class RetryOnPrimaryException extends ElasticsearchException {

core/src/main/java/org/elasticsearch/action/support/replication/TransportReplicationAction.java

@@ -531,7 +531,8 @@ public void onResponse(Releasable releasable) {
             try {
                 final ReplicaResult replicaResult = shardOperationOnReplica(request, replica);
                 releasable.close(); // release shard operation lock before responding to caller
-                final TransportReplicationAction.ReplicaResponse response = new ReplicaResponse(replica.getLocalCheckpoint());
+                final TransportReplicationAction.ReplicaResponse response =
+                        new ReplicaResponse(replica.getLocalCheckpoint(), replica.getGlobalCheckpoint());
                 replicaResult.respond(new ResponseListener(response));
             } catch (final Exception e) {
                 Releasables.closeWhileHandlingException(releasable); // release shard operation lock before responding to caller
@@ -1006,6 +1007,11 @@ public void updateLocalCheckpointForShard(String allocationId, long checkpoint)
             indexShard.updateLocalCheckpointForShard(allocationId, checkpoint);
         }
 
+        @Override
+        public void updateGlobalCheckpointForShard(final String allocationId, final long globalCheckpoint) {
+            indexShard.updateGlobalCheckpointForShard(allocationId, globalCheckpoint);
+        }
+
         @Override
         public long localCheckpoint() {
             return indexShard.getLocalCheckpoint();
@@ -1025,47 +1031,60 @@ public ReplicationGroup getReplicationGroup() {
 
     public static class ReplicaResponse extends ActionResponse implements ReplicationOperation.ReplicaResponse {
         private long localCheckpoint;
+        private long globalCheckpoint;
 
         ReplicaResponse() {
 
         }
 
-        public ReplicaResponse(long localCheckpoint) {
+        public ReplicaResponse(long localCheckpoint, long globalCheckpoint) {
             /*
-             * A replica should always know its own local checkpoint so this should always be a valid sequence number or the pre-6.0 local
+             * A replica should always know its own local checkpoints so this should always be a valid sequence number or the pre-6.0
              * checkpoint value when simulating responses to replication actions that pre-6.0 nodes are not aware of (e.g., the global
              * checkpoint background sync, and the primary/replica resync).
              */
             assert localCheckpoint != SequenceNumbers.UNASSIGNED_SEQ_NO;
             this.localCheckpoint = localCheckpoint;
+            this.globalCheckpoint = globalCheckpoint;
         }
 
         @Override
         public void readFrom(StreamInput in) throws IOException {
+            super.readFrom(in);
             if (in.getVersion().onOrAfter(Version.V_6_0_0_alpha1)) {
-                super.readFrom(in);
                 localCheckpoint = in.readZLong();
             } else {
                 // 5.x used to read empty responses, which don't really read anything off the stream, so just do nothing.
-                localCheckpoint = SequenceNumbersService.PRE_60_NODE_LOCAL_CHECKPOINT;
+                localCheckpoint = SequenceNumbersService.PRE_60_NODE_CHECKPOINT;
+            }
+            if (in.getVersion().onOrAfter(Version.V_6_0_0_rc1)) {
+                globalCheckpoint = in.readZLong();
+            } else {
+                globalCheckpoint = SequenceNumbersService.PRE_60_NODE_CHECKPOINT;
             }
         }
 
         @Override
         public void writeTo(StreamOutput out) throws IOException {
+            super.writeTo(out);
             if (out.getVersion().onOrAfter(Version.V_6_0_0_alpha1)) {
-                super.writeTo(out);
                 out.writeZLong(localCheckpoint);
-            } else {
-                // we use to write empty responses
-                Empty.INSTANCE.writeTo(out);
+            }
+            if (out.getVersion().onOrAfter(Version.V_6_0_0_rc1)) {
+                out.writeZLong(globalCheckpoint);
             }
         }
 
         @Override
         public long localCheckpoint() {
             return localCheckpoint;
         }
+
+        @Override
+        public long globalCheckpoint() {
+            return globalCheckpoint;
+        }
+
     }
 
     /**

core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointSyncAction.java

@@ -89,7 +89,8 @@ protected void sendReplicaRequest(
         if (node.getVersion().onOrAfter(Version.V_6_0_0_alpha1)) {
             super.sendReplicaRequest(replicaRequest, node, listener);
         } else {
-            listener.onResponse(new ReplicaResponse(SequenceNumbersService.PRE_60_NODE_LOCAL_CHECKPOINT));
+            final long pre60NodeCheckpoint = SequenceNumbersService.PRE_60_NODE_CHECKPOINT;
+            listener.onResponse(new ReplicaResponse(pre60NodeCheckpoint, pre60NodeCheckpoint));
         }
     }