Add global checkpoint assertion in index shard

We previously removed this assertion because it could be violated in
races. This commit adds this assertion back with sampling done more
carefully to avoid failures solely due to race conditions.

core/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -1521,6 +1521,8 @@ public void updateGlobalCheckpointOnPrimary() {
      */
     public void updateGlobalCheckpointOnReplica(final long globalCheckpoint) {
         verifyReplicationTarget();
+        // we sample the recovery stage before sampling the local checkpoint or we are subject to a race condition in the below assertion
+        final RecoveryState.Stage stage = recoveryState().getStage();
         final SequenceNumbersService seqNoService = getEngine().seqNoService();
         final long localCheckpoint = seqNoService.getLocalCheckpoint();
         if (globalCheckpoint > localCheckpoint) {
@@ -1530,10 +1532,10 @@ public void updateGlobalCheckpointOnReplica(final long globalCheckpoint) {
              * case that the global checkpoint update from the primary is ahead of the local checkpoint on this shard. In this case, we
              * ignore the global checkpoint update. This can happen if we are in the translog stage of recovery. Prior to this, the engine
              * is not opened and this shard will not receive global checkpoint updates, and after this the shard will be contributing to
-             * calculations of the the global checkpoint. However, we can not assert that we are in the translog stage of recovery here as
-             * while the global checkpoint update may have emanated from the primary when we were in that state, we could subsequently move
-             * to recovery finalization, or even finished recovery before the update arrives here.
+             * calculations of the the global checkpoint.
              */
+            assert stage == RecoveryState.Stage.TRANSLOG
+                    : "expected recovery stage [" + RecoveryState.Stage.TRANSLOG + "] but was [" + stage + "]";
             return;
         }
         seqNoService.updateGlobalCheckpointOnReplica(globalCheckpoint);