From fa54be2dcd1dfecdf065f5bb74066f97bcf138ac Mon Sep 17 00:00:00 2001
From: Nhat Nguyen <nhat.nguyen@elastic.co>
Date: Thu, 31 May 2018 15:14:32 -0400
Subject: [PATCH] CCR: Do not minimization requesting range on leader (#30980)

Today before reading operations on the leading shard, we minimization
the requesting range with the global checkpoint. However, this might
make the request invalid if the following shard generates a requesting
range based on the global-checkpoint from a primary shard and sends
that request to a replica whose global checkpoint is lagged.

Another issue is that we are mutating the request when applying
minimization. If the request becomes invalid on a replica, we will
reroute the mutated request instead of the original one to the primary.

This commit removes the minimization and replaces it by a range check
with the local checkpoint.
---
 .../xpack/ccr/action/ShardChangesAction.java             | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/ShardChangesAction.java b/x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/ShardChangesAction.java
index 165625844c3..e59c51d2921 100644
--- a/x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/ShardChangesAction.java
+++ b/x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/ShardChangesAction.java
@@ -235,9 +235,14 @@ public class ShardChangesAction extends Action<ShardChangesAction.Request, Shard
         protected Response shardOperation(Request request, ShardId shardId) throws IOException {
             IndexService indexService = indicesService.indexServiceSafe(request.getShard().getIndex());
             IndexShard indexShard = indexService.getShard(request.getShard().id());
-
             final long indexMetaDataVersion = clusterService.state().metaData().index(shardId.getIndex()).getVersion();
-            request.maxSeqNo = Math.min(request.maxSeqNo, indexShard.getGlobalCheckpoint());
+            // The following shard generates the request based on the global checkpoint which may not be synced to all leading copies.
+            // However, this guarantees that the requesting range always be below the local-checkpoint of any leading copies.
+            final long localCheckpoint = indexShard.getLocalCheckpoint();
+            if (localCheckpoint < request.minSeqNo || localCheckpoint < request.maxSeqNo) {
+                throw new IllegalStateException("invalid request from_seqno=[" + request.minSeqNo + "], " +
+                    "to_seqno=[" + request.maxSeqNo + "], local_checkpoint=[" + localCheckpoint + "], shardId=[" + shardId + "]");
+            }
             final Translog.Operation[] operations =
                 getOperationsBetween(indexShard, request.minSeqNo, request.maxSeqNo, request.maxTranslogsBytes);
             return new Response(indexMetaDataVersion, operations);